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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 256  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3160 journals]
  • Spectroscopic properties and martensitic phase transition of Y4Al2O9:Ce
           single crystals under high pressure
    • Abstract: Publication date: 15 February 2019Source: Acta Materialia, Volume 165Author(s): Yongjie Wang, R. Hrubiak, S. Turczyński, D.A. Pawlak, M. Malinowski, D. Włodarczyk, K.M. Kosyl, W. Paszkowicz, H. Przybylińska, A. Wittlin, A. Kaminska, Ya Zhydachevskyy, M.G. Brik, Li Li, Chong-Geng Ma, A. Suchocki High pressure studies of monoclinic yttrium aluminum oxide single crystals (Y4Al2O9 -YAM) doped with Ce3+ ions grown by micro-pulling-down method are reported. The results of absorption measurements in the mid-infrared prove the existence of four different sites in which Ce3+ ions substitute Y ions, in accordance with the crystallographic structure of YAM. The lowest 5d level of Ce3+ is located very close to the bottom of the conduction band for two of four Ce3+ related centers at ambient pressure, which results in strong temperature quenching of their luminescence. Two other Ce3+ centers do not emit at ambient pressure since their 5d levels are resonant with the conduction band. Application of high pressure above 11 GPa restores their luminescence. Consequences of such energy structure of various Ce3+ sites in YAM for the Dorenbos theory are discussed. The bandgap energy of YAM is found to be 5.93 eV, which is the smallest out of the three yttrium aluminum oxides. High-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements identify a phase transition occurring at pressures between 8 and 11 GPa, which has martensitic character. The second phase transition to another structure (likely with hexagonal symmetry) occurs at a pressure around 16 GPa.Graphical abstractImage 1
  • The role of Ga addition on the thermodynamics, kinetics, and tarnishing
           properties of the Au-Ag-Pd-Cu-Si bulk metallic glass forming system
    • Abstract: Publication date: 15 February 2019Source: Acta Materialia, Volume 165Author(s): Nico Neuber, Oliver Gross, Miriam Eisenbart, Alexander Heiss, Ulrich E. Klotz, James P. Best, Mikhail N. Polyakov, Johann Michler, Ralf Busch, Isabella Gallino Effective strategies to improve the tarnishing resistance of the 18 K (karat) gold-based bulk glass-forming composition Au49Ag5.5Pd2.3Cu26.9Si16.3 were recently found, with the addition of Ga at the expense of Cu and a sufficient reduction of Si. However, the modification of the alloy is accompanied by a reduction of the glass-forming ability (GFA) from 5 to 2 mm in terms of the critical casting thickness, and eventually collapses for Ga contents higher than about 9 at%. In this work, thermodynamic and kinetic studies of the newly discovered 18 K Au-Ag-Pd-Cu-Ga-Si system shed light on the reason for the loss in GFA with increasing Ga content. Investigations of the liquid kinetics in terms of viscosity and transition time, assessed by three-point beam bending and the Tg-shift method, reveal an unexpected change of the fragility to stronger liquid kinetics with increasing Ga concentration, which is usually attributed to an increase in the GFA. Thermodynamic considerations based on specific heat capacity measurements reveal, in contrast, an ascending driving force for crystallization with an increasing Ga-content, accountable for the drop in GFA. Therefore, a Ga-rich melt beyond the threshold concentration of 9 at% is not desirable for the production of bulk metallic glasses (BMGs) from the liquid state. With the intention to outrun this barrier, while preserving the amorphous structure, Ga-ion implantation with a liquid metal focused ion beam source is used as a post-processing routine for a cast Ga-containing 18 K Au-BMG. Hence, the thermodynamic borderline concentration is successfully crossed with additional tremendous improvement of the tarnishing resistance in the Ga-enriched areas.Graphical abstractImage 1
  • Effect of carbon on strain-rate and temperature sensitivity of
           twinning-induced plasticity steels: Modeling and experiments
    • Abstract: Publication date: 15 February 2019Source: Acta Materialia, Volume 165Author(s): Y.Z. Li, Z.C. Luo, Z.Y. Liang, M.X. Huang The temperature- and rate-dependent yielding of twinning-induced plasticity (TWIP) steels containing various carbon contents are investigated in the present work. The activation volume and the activation energy have been determined. The magnitude of these thermal activation parameters for high-carbon TWIP steels are largely different from those of conventional fcc metals, implying the fundamental role of carbon on the thermally activated dislocation activities in carbon-added TWIP steels. A constitutive model, which rationalizes yielding as the thermally assisted bowing out of dislocations under the pinning effect of carbon solutes, is proposed, and for the first time quantitatively predicts the thermal activation parameters of TWIP steels as a function of carbon content. Based on the modeling results of thermal activation parameters, the overall temperature- and rate-dependent yield stresses of TWIP steels containing various carbon contents are predicted, showing good agreements with experimental results.Graphical abstractImage 1
  • Magnetic and magnetocaloric properties of the Co2-xMn x B system by
           experiment and density functional theory
    • Abstract: Publication date: 15 February 2019Source: Acta Materialia, Volume 165Author(s): Semih Ener, Maximilian Fries, Franziska Hammerath, Ingo Opahle, Eszter Simon, Patrizia Fritsch, Sabine Wurmehl, Hongbin Zhang, Oliver Gutfleisch The Co2B system shows a significant magnetovolume effect around its Curie temperature which makes it potentially attractive for magnetocaloric applications or thermomagnetic power generation, as a large coupling between the lattice and spin degrees of freedom is expected. We report on the synthesis of a series of Co2-xMnxB alloys and the investigation of their properties. The structural analysis indicates a single phase behavior up to x = 0.8 with no structural symmetry changes throughout the series. Measurements of both, macroscopic and local magnetic properties, reveal an anomalous behavior of the spontaneous magnetization, Curie temperature, and element-specific magnetic moments as a function of manganese concentration. The elemental contributions to the magnetization are analyzed using nuclear magnetic resonance (NMR) studies. Density functional theory (DFT) calculations guide us in the understanding of the origin of the observed anomaly, which is due to a complex magnetic coupling behavior between Mn atoms, which significantly affects the corresponding exchange interactions. The magnetocaloric properties of the Co2-xMnxB alloys show that the maximum entropy change peak temperature can be shifted between room temperature and 450 K upon variation of the manganese concentration without significant impact on the magnetocaloric response. The highest entropy change of −1.37 Jkg−1K−1 at 442 K is obtained for x = 0.1 for a field change of 2 T. This value is, however, quite low for any possible magnetocaloric or thermomagnetic power generation applications. Nevertheless, the good agreement between the advanced characterization and theory gives a deeper understanding of the Co2-xMnxB material system which can in the future be extended to other systems.Graphical abstractImage 1
  • Multiscale mechanical fatigue damage of stainless steel investigated by
           neutron diffraction and X-ray microdiffraction
    • Abstract: Publication date: 15 February 2019Source: Acta Materialia, Volume 165Author(s): Runguang Li, Yan-Dong Wang, Wenjun Liu, Chang Geng, Qingge Xie, Dennis E. Brown, Ke An Mechanical fatigue behavior of AL6XN stainless steel as a typical type of planar slip alloy was investigated by in situ neutron diffraction and synchrotron-based X-ray microdiffraction methods. Under cyclic loading at a high strain amplitude (±0.8%), the fatigue damage originated mainly from the accumulation of statistical stored dislocations, as clearly evidenced from a continuous increase in diffraction peak width with increasing the number of load cycles. However, under cyclic loading at a low strain amplitude (±0.3%), the density of statistical stored dislocations became saturated just after a hundred loading cycles and the fatigue damage was mainly dominated by the accumulation of persistent Lüders bands (PLBs) and the complex interactions among various PLBs as evidenced through X-ray microdiffraction measurements. It was further found that there exists obvious grain-orientation-dependent local damage in the low-strain-amplitude fatigued sample. In particular, fatigued grains orientated with [001] paralleling the loading direction are subjected to compressive stress and contain a large number of broad PLBs in boundaries arraying the edge dislocation pile-ups, which generate a large stress gradient leading to local plastic instability. The highly localized stress field at PLBs in the cyclically-deformed sample at a low strain amplitude may explain the obvious cyclic stress softening.Graphical abstractImage 1
  • Σ 5 ( 310 ) / [ 001 ] +tilt+Grain+Boundary+on+Oxygen-ion+movement+In+Yttria-Stabilized+Zirconia:+Insights+from+molecular+dynamics&rft.title=Acta+Materialia&rft.issn=1359-6454&">Effect of the Σ 5 ( 310 ) / [ 001 ] tilt Grain Boundary on Oxygen-ion
           movement In Yttria-Stabilized Zirconia: Insights from molecular dynamics
    • Abstract: Publication date: Available online 3 December 2018Source: Acta MaterialiaAuthor(s): Methary Jaipal, Abhijit Chatterjee We present a new methodology for investigating the combined effect of the Σ5(310)/[001] symmetric tilt grain boundary (GB) and the local cation environment in polycrystalline yttria stabilized zirconia (YSZ) on two important quantities that determine the ionic conductivity, namely, (i) the local hopping rate of O2- ion and (ii) the probability of O2--ion -vacancy pairs within the YSZ structure. How these quantities vary with distance to the GB core are estimated for the first time using waiting time distributions associated with O2- ion hop events in molecular dynamics simulations. We conclude that indeed fewer hop events occur in the presence of a GB. However, the GB effect can be felt at a far greater distance than previously believed. Most importantly, interactions between the O2- ions, nearby cations and the GB results in a hopping behavior that is different from one observed in single-crystal YSZ. Anisotropy in O2- ion movement in the vicinity of the GB is also studied. These results and the use of our novel technique have a direct implication on the development of improved models for ionic conduction in solid state electrolytes.Graphical abstractImage 1
  • A paradigm shift towards compositionally zero-sum binderless 3D printing
           of magnesium alloys via capillary-mediated bridging
    • Abstract: Publication date: Available online 3 December 2018Source: Acta MaterialiaAuthor(s): Mojtaba Salehi, Saeed Maleksaeedi, Sharon Mui Ling Nai, Ganesh Kumar Meenashisundaram, Min Hao Goh, Manoj Gupta Several metallurgical issues arise during melting and solidification-based additive manufacturing (AM) methods which limit their use to only a few alloys. This study shows how capillarity-driven bridging can serve as a new and rapid tool of assembling powder particles into 3D structures providing the least metallurgical complexity. According to the established conceptual framework, in-situ binding agent can be derived autogenously from selective interactions of a liquid solvent with superficial layer of powder particles to form in-situ solid interparticle bridges which enables AM at ambient conditions. Magnesium (Mg) is the most difficult engineering metal to handle in all 3D AM processes in view of its intrinsic properties. Supported by transmission and scanning electron microscopies together with Fourier transform infrared and Raman spectroscopy, the introduced 3D printing concept is readily accomplished for Mg alloys by conversion of MgO film, inevitably existing on the outermost layer of Mg powder, into interparticle bridges. In the absence of pyrolysis-adapted sintering profile, these bridges fully decompose during the following sintering step which result in functional Mg parts with chemical composition same as the starting Mg powder with zero contamination, according to chemical and thermal analysis results. The introduced capillary-mediated binding of particles is simple and generic that could be extended to additively manufacture other metals, ceramics and found applications in other traditional ex-situ binder-based processes.Graphical abstractImage 1
  • Real-time observation of the temperature-induced phase transformation in
           GeTe and its thermal expansion properties
    • Abstract: Publication date: Available online 3 December 2018Source: Acta MaterialiaAuthor(s): Xuan Quy Tran, Min Hong, Hiroshi Maeno, Youichirou Kawami, Takaaki Toriyama, Kevin Jack, Zhi-Gang Chen, Jin Zou, Syo Matsumura, Matthew S. Dargusch The GeTe-based system has long been considered as a promising candidate system for various functional applications; many of which are directly related to the polymorphic phase transformation in their crystalline forms. Consequently, the microstructure underlying their intriguing phase transition has been the subject of numerous studies. Here we provide real-time observation of the microstructural changes associated with the reversible pseudo-cubic (or rhombohedral) to cubic GeTe phase transition using high-voltage transmission electron microscopy (HV-TEM) operating at 1,250kV and complementary high-temperature X-ray diffraction (XRD). As a result of the phase transition, the pseudo-cubic GeTe domain’s configuration significantly changes from its original band-like to a spike-like morphology with a different orientation after a heating/cooling cycle. The coefficients of thermal expansion (CTE) properties as a function of temperature are also explored in relation to the GeTe phase transition.Graphical abstractImage 1
  • Short crack propagation from cracked non-metallic inclusions in a Ni-based
           polycrystalline superalloy
    • Abstract: Publication date: Available online 28 November 2018Source: Acta MaterialiaAuthor(s): Damien Texier, Jean-Charles Stinville, McLean P. Echlin, Stéphane Pierret, Patrick Villechaise, Tresa M. Pollock, Jonathan Cormier Fatigue cracks initiating from surface and sub-surface non-metallic inclusions (NMIs) have recently been demonstrated to be a necessary but not sufficient explanation for atypically short low-cycle fatigue life in Inconel 718 alloy at intermediate temperature. Therefore, the early stages of short crack propagation from surface NMIs were investigated in a crystallographic and two-dimensional versus three-dimensional morphological manner after room temperature low cycle fatigue (LCF) testing. In the present investigation, NMIs were purposely pre-cracked using different techniques to suppress the natural crack initiation period and thus the incubation period prior to the early stages of crack propagation. Under such fatigue testing conditions, different mechanisms of crack transmission from pre-cracked NMIs were identified: (i) no propagation, (ii) NMI/adjacent metallic grain interfacial debonding, (iii) transgranular crack propagation within the adjacent metallic grain. Focused-ion-beam cross-section observations of numerous fatigue tested NMIs aimed to define a morphological criterion for non-propagating carbides. Large cracked NMIs at the surface (2c) with limited extension into the depth (a) did not propagate under such fatigue conditions for 2c/a ratio higher than 3. Furthermore, specific crystallographic relationships between NMIs and the adjacent metallic grain explained different crack propagation configurations from pre-cracked NMIs, i.e. interfacial debonding and transgranular crack propagation involving single or multiple-slip activity.Graphical abstractImage 1
  • Outstanding tensile properties of a precipitation-strengthened
           FeCoNiCrTi0.2 high-entropy alloy at room and cryogenic temperatures
    • Abstract: Publication date: Available online 26 November 2018Source: Acta MaterialiaAuthor(s): Y. Tong, D. Chen, B. Han, J. Wang, R. Feng, T. Yang, C. Zhao, Y.L. Zhao, W. Guo, Y. Shimizu, C.T. Liu, P.K. Liaw, K. Inoue, Y. Nagai, A. Hu, J.J. Kai A FeCoNiCrTi0.2 high-entropy alloy strengthened by two types of coherent nano-precipitates but with the same composition was fabricated, and its tensile properties at room (293 K) and cryogenic temperatures (77 K) and the corresponding defect-structure evolution were investigated. Compared with the single-phase FeCoNiCr parent alloy, the precipitation-strengthened FeCoNiCrTi0.2 high-entropy alloy exhibits a significant increase in yield strength and ultimate tensile strength but with little sacrifice in ductility. Similar to the single-phase FeCoNiCr high-entropy alloy, the deformation behavior of this precipitation-strengthened FeCoNiCrTi0.2 high-entropy alloy shows strong temperature dependence. When the temperature decreases from 293 K to 77 K, its yield strength and ultimate tensile strength are increased from 700 MPa to 860 MPa and from 1.24 to 1.58 GPa, respectively, associated with a ductility improvement from 36% to 46%. However, different from the single-phase FeCoNiCr high-entropy alloy with a twinning-dominant deformation mode at 77 K, multiple-layered stacking faults with a hierarchical substructure prevail in the precipitation-strengthened FeCoNiCrTi0.2 high-entropy alloy when deformed at 77 K. The mechanism of twinning inhibition in this precipitation-strengthened high-entropy alloy is the high energy barrier for twin nucleation in the ordered γ′ nano-particles. Our results may provide a guide for the design of tough high-entropy alloys for applications at cryogenic temperatures through combining precipitation strengthening and twinning/stacking faults.Graphical abstractImage 1
  • Effect of Machining on Stress Corrosion Crack Initiation in Warm-Forged
           Type 304L Stainless Steel in High Temperature Water
    • Abstract: Publication date: Available online 26 November 2018Source: Acta MaterialiaAuthor(s): Litao Chang, Liberato Volpe, YongLiang Wang, M. Grace Burke, Agostino Maurotto, David Tice, Sergio Lozano-Perez, Fabio Scenini Warm-forged Type 304L stainless steel specimens have been tested in high temperature hydrogenated water under slow strain rate tensile test conditions to investigate the effect of machining on stress corrosion crack initiation. Roughness, residual stress and cross-section microstructures of the as-machined surfaces were characterized prior to the tests, and both plan-view and cross-section examinations were performed post-test. The results indicated that machining produced a deformation layer characterized by an ultrafine-grained outer layer and a highly deformed inner layer. The ultrafine-grained layer promoted a more uniform oxidation and enhanced SCC initiation resistance of the material compared to the highly polished deformation-free surfaces, which initiated significantly more intergranular cracks. The mechanisms for SCC initiation in the material have been discussed.Graphical abstractImage 1
  • Quantitative prediction of the aged state of Ni-base superalloys using PCA
           and tensor regression
    • Abstract: Publication date: Available online 26 November 2018Source: Acta MaterialiaAuthor(s): S. Gorgannejad, M. Reisi Gahrooei, K. Paynabar, R.W. Neu The microstructure of Ni-base superalloy components evolves and degrades during the operation of gas turbines. Since the remaining life depends on the degradation, it is highly desirable to have a quantitative descriptor of the aged state of the microstructure that can be linked to the operating conditions. In this paper, data analytics algorithms are used to develop such relationships. High-throughput aging experiments were performed to generate a dataset comprising multiple aged microstructure images. The digital images of the γ/γ' phase are used as an indicator of the aged state and statistically evaluated using 2-point spatial correlation functions. To reduce the high-dimensional structural information so that a quantitative linkage can be made between aging conditions and the aged state, two algorithms were considered. The first algorithm involves two steps, first using conventional principal component analysis (PCA) to provide a lower dimension descriptor of the microstructure and then regression analysis to generate the linkage. The second algorithm, called tensor regression (TR), is a novel algorithm that merges the dimensionality reduction and model construction step into a single step. The output of the TR model is directly the statistical descriptors of the microstructure rather than the PC scores, thereby reducing the amount of information loss. Even though PCA provides an effective tool for visualization and classification of data, the model built based on the TR algorithm is shown to have stronger prediction capability.Graphical abstractImage 1
  • In situ x-ray diffraction analysis of 2D crack patterning in thin films
    • Abstract: Publication date: Available online 26 November 2018Source: Acta MaterialiaAuthor(s): D. Faurie, F. Zighem, P. Godard, G. Parry, T. Sadat, D. Thiaudière, P.-O. Renault In this work, the effect of the loading path on the multicracking of Nickel thin films on Kapton® substrate was studied thanks to an experimental set-up combining controlled biaxial deformation, x-ray diffraction and digital image correlation. Samples were biaxially stretched up to 10% strain following either a single equibiaxial path or a complex one consisting of loading successively along each of the axes of the cruciform specimen. While the first path leads to a mud-crack pattern (random), the second leads to a roman-bricks one (square). Moreover, the in situ x-ray diffraction experiments show that the stress field developed in the thin film during the multicracking is clearly dependent on the loading path. By combining the study of stresses and x-ray diffraction peaks linewidth, we evidenced mechanical domains related to initiation of cracks and their multiplication for each loading path. Moreover, stress evolution in the thin film during mud-crack pattern formation is significantly smoother than in the case of roman-bricks one as represented in the plane stress space.Graphical abstractImage 1
  • Unraveling the structural mechanism of Li insertion in γ’-V2O5 and its
           effect on cycling properties
    • Abstract: Publication date: Available online 23 November 2018Source: Acta MaterialiaAuthor(s): R. Baddour-Hadjean, M. Safrany Renard, J.P. Pereira-Ramos We report the first electrochemical and structural study of the γ'-V2O5 polymorph toward Li insertion, with cycling properties evaluated in the 4 V-2.4 V, 3.6 V-2.4 V and 4 V-2.15 V potential ranges. This cathode material is synthesized through topotactic lithium removal from the ternary γ-LiV2O5 bronze, using a strong oxidizing agent. It exhibits two pairs of well-defined reversible steps at 3.58/3.47 V and 2.42/2.36 V separated by a sharp potential drop of about 1 V. A high capacity of 285 mAh g-1 is involved in the 4.00 V- 2.15 V voltage window corresponding to the insertion of nearly 2 Li/mole of oxide. The γ'-V2O5 material can deliver stable capacities of 120-185 mAh g-1 over 45 cycles at C/10 when the lower cut-off voltage is limited to 2.4 V. Cycling experiments in the widest 4.00 V- 2.15 V potential range induce however a significant capacity decline. A detailed XRD and Raman spectroscopy study delivers a detailed picture of the structural changes occurring in γ’-V2O5 as a function of Li content. A complete phase diagram of the γ’-V2O5/Li system is provided during the first discharge-charge cycle in the extended 4.00 V – 2.15 V voltage range, i.e. for 0 ≤ x ≤ 1.94 in LixV2O5. It is demonstrated the existence of a wide solid solution domain in the 0.4 ≤ x < 1.4 composition range (3.6 V – 2.4 V), leading to a remarkable enhancement of the cycle performance in the corresponding controlled voltage window. The Raman fingerprint of the fully lithiated ζ-Li1.94V2O5 phase is newly identified, indicating that deep structural rearrangements at the atomic scale take place during the γ-Li1.4V2O5 → ζ-Li1.94V2O5 transition. The resulting phase diagram accounts for the potential-composition profile and sheds light on the nature of the cycling properties in the different voltage windows.Graphical abstractImage 1
  • Origin of the low precipitation hardening in magnesium alloys
    • Abstract: Publication date: Available online 23 November 2018Source: Acta MaterialiaAuthor(s): C.M. Cepeda-Jiménez, M. Castillo-Rodríguez, M.T. Pérez-Prado In this work electron backscattered diffraction (EBSD)-assisted slip trace analysis and transmission electron microscopy have been utilized to investigate the interaction of basal dislocations with precipitates in the Mg alloys Mg-1%wt.Mn-0.7%wt.Nd (MN11) and Mg-9%wt.Al-1%wt.Zn (AZ91), with the ultimate aim of determining the origin of their poor precipitation hardening. Precipitates in these alloys have a plate-shaped morphology, with plates being, respectively, perpendicular (MgxNdy) and parallel (Mg17Al12) to the basal plane of the magnesium matrix. Mechanical tests were carried out in solid solution and peak-aged samples, in tension and compression, both at RT and at moderate temperature (250ºC). EBSD-assisted slip trace analysis revealed a clear dominance of basal slip under a wide range of testing conditions in the peak-aged MN11 and AZ91 alloys. At room temperature, the origin of the low precipitation hardening lies at the easiness with which precipitates are sheared by basal dislocations, which is promoted by the excellent lattice matching at the precipitate-matrix interface. At high temperature, dislocation-precipitate interactions are highly dependent on the deformation mode. In tension, enhanced basal slip localization gives rise to high stress concentrations at the intersection between coarse slip traces and particle interfaces, leading to precipitate fracture; in compression, a more homogenous distribution of basal slip leads to the dominance of particle shearing. Our study demonstrates experimentally that basal dislocations are able to shear, and even fracture, the MgxNdy and Mg17Al12 plates when, for appropriate testing conditions, the local stress due to dislocation accumulation at particle interfaces exceeds the precipitate strength.Graphical abstractImage 1
  • Formation and temporal evolution of modulated structure in high
           Nb-containing lamellar γ-TiAl alloy
    • Abstract: Publication date: Available online 23 November 2018Source: Acta MaterialiaAuthor(s): Guo-dong Ren, Cheng-ren Dai, Wei Mei, Jian Sun, Song Lu, Levente Vitos The formation and temporal evolution of the modulated structure in a lamellar γ-based Ti−45Al−8.5Nb alloy have been investigated by transmission electron microscopy (TEM) in combination with first-principles theory in this work. The results show that the Nb-rich O phase as a constituent of the modulated structure is thermodynamically stable below 650 °C in the α2 lamellae. The morphology of the O phase variants changes from thin plate-like shape with a low volume fraction at initial annealing to rectangle/square shape with a high volume fraction after a prolonged annealing, and the retransformed α2, named as α2-ІІ hereafter, emerges at intersections of the variants with two orthogonal habit planes due to their elastic interactions. The partitioning coefficient of Nb between the O phase and α2 is about 2 at 600 °C. The diffusion coefficient of Nb derived from growth kinetics of the O phase is about (1.3 ± 0.2) × 10−22 m2s−1 in the α2 lamellae. Significant precipitation hardening effect of the O phase has been revealed for the α2 lamellae and γ/(α2+O) lamellar microstructure, which is supposed to be attributed to refining the α2 lamellae associated with elastic strain energy from the α2→O phase transformation and introducing the interface between the modulated lamella and adjacent γ phase.Graphical abstractImage 1
  • Role of the slow diffusion species in the dewetting of compounds: the case
           of NiSi on a Si isotope multilayer studied by atom probe tomography
    • Abstract: Publication date: Available online 23 November 2018Source: Acta MaterialiaAuthor(s): T. Luo, C. Girardeaux, H. Bracht, D. Mangelinck Dewetting or agglomeration is a crucial process in material science since it controls the stability of thin films or can be used for film nanostructuration by formation of islands. The models developed for dewetting usually assume diffusion at the interface and/or at the surface but no direct evidence of such diffusion was demonstrated. Moreover, these models are usually dealing with elemental materials and not with compounds in which several elements can diffuse. The mechanisms behind agglomeration of polycrystalline compounds thin film are still not fully understood. In this work, Si isotope multilayers coupled with atom probe tomography (APT) are used to reveal the agglomeration mechanism of NiSi, a binary compound. The diffusion of Si, the less mobile species in NiSi, at the NiSi/Si interface is demonstrated through comparison between the three dimension redistribution of the Si isotopes determined by APT and models taking into account grooving and agglomeration. The implication for the understanding and control of agglomeration in poly-crystalline compound thin films are highlighted.Graphical abstractImage 1
  • Mechanistic understanding of the temperature dependence of crack growth
           rate in Alloy 600 and 316 stainless steel through high-resolution
    • Abstract: Publication date: Available online 22 November 2018Source: Acta MaterialiaAuthor(s): Zhao Shen, Martina Meisnar, Koji Arioka, Sergio Lozano-Perez Stress corrosion cracking of Alloy 600 (A600) and 316 stainless steel (316SS) exposed to simulated pressurized water reactor primary water at temperatures of 320-360ºC has been investigated and compared. Intergranular oxidation developed ahead of all crack tips prepared from these two alloys. High-resolution characterization reveals that there are two main rate-controlling mechanisms contributing to crack propagation: a diffusion-based and a mechanical deformation-based. The different temperature dependence of crack growth rate (CGR) in A600 and 316SS can be explained after confirming that the two rate-controlling mechanisms exhibit different “weights” in the two alloys. The diffusion-based mechanism plays a dominant role in accelerating the CGR of A600, while the mechanical deformation-based mechanism is responsible for the observed CGR decrease of 316SS at higher temperatures.Graphical abstractImage 1
  • Transmutation-induced Precipitation in Tungsten Irradiated with A Mixed
           Energy Neutron Spectrum
    • Abstract: Publication date: Available online 21 November 2018Source: Acta MaterialiaAuthor(s): Xunxiang Hu, Chad Parish, Kun Wang, Takaaki Koyanagi, Benjamin P. Eftink, Yutai Katoh Transmutation-induced precipitation in neutron-irradiated tungsten is an important performance concern for its application as plasma facing material in fusion reactors. In this study, segregation and precipitation of transmutant elements in single crystal and polycrystal tungsten irradiated at 460∼1100°C to 0.02∼2.4 displacements per atom (dpa) in the High Flux Isotope Reactor were investigated using transmission electron microscopy. The results indicated that nanoscale W-Re-Os clusters were identified in the low dose regime from 0.02 to 0.44 dpa with irradiation temperature lower than 800°C while acicular-shape precipitates formed when irradiation dose is higher than 1.5 dpa. A tentative roadmap of the kinetics process of the transmutation-induced precipitation in neutron-irradiated tungsten is presented characterizing the defect features (i.e., W-Re-Os clusters and precipitates) consisting of transmutant elements in tungsten irradiated to various doses. All TEM-visible voids were associated with the acicular-shape precipitates. Voids were formed prior to the formation of acicular-shape precipitates and act as strong trapping sites for mobile species involved in the precipitation together with dislocations. Thermal stability of W-Re-Os clusters was assessed by performing a 2-h anneal at 1200°C on tungsten irradiated to 0.44 dpa at 705°C. The kinetics process of transmutant elements and radiation defects are discussed to reveal the underlying mechanisms controlling the formation of precipitates in tungsten.Graphical abstractThe general process of transmutation-induced precipitation in neuron-irradiated tungsten was revealed by capturing the microstructures of tungsten irradiated to various conditions. Dislocations and voids are nucleation sites for segregation and precipitation of transmutant elements, controlled by the high mobility of W-Re and W-Os dumbbells.Image 1
  • Superior energy absorption in porous magnesium: contribution of texture
           development triggered by intra-granular misorientations
    • Abstract: Publication date: Available online 21 November 2018Source: Acta MaterialiaAuthor(s): Tsuyoshi Mayama, Masakazu Tane, Yuichi Tadano The effect of the initial crystallographic texture and its development on the energy absorption capability of porous magnesium (Mg) with parallel cylindrical pores and a fiber texture was studied. The fiber texture, whose symmetrical axis is oriented along the longitudinal axis of pores, was modeled using crystal plasticity finite element method (FEM). Subsequently, the effect of the preferred orientation angle of the basal planes in Mg grains, denoted as the angle α, on the compressive deformation along the longitudinal axis of pores was analyzed. The analyses revealed that the orientation angle α of the basal planes strongly affects the development of the intra-granular crystallographic misorientations, which dominates the compressive deformation and energy absorption capability. When the angle α is low, a strong deformation constraint, originating from inter-granular interaction, occurs at the grain boundaries. Owing to the constraint, large intra-granular crystallographic misorientations occur during deformation. Consequently, the axial symmetry of the fiber texture becomes asymmetric, which causes the appearance of a plateau stress region, where compressive deformation proceeds with a small increase in stress. As a result, superior energy absorption is achieved for a low angle α. The crystal plasticity analyses also indicate that an ideal plateau stress region, where deformation proceeds with almost no increase in stress, appears by controlling the initial fiber texture using a compressive preloading on a porous Mg sample prepared via the present directional solidification process. Owing to the superior plateau stress region, the porous Mg achieves high energy absorption efficiency and capability simultaneously.Graphical abstractImage 1
  • Mechanical properties of the magnetocaloric intermetallic LaFe11.2Si1.8
           alloy at different length scales
    • Abstract: Publication date: Available online 20 November 2018Source: Acta MaterialiaAuthor(s): Oleksandr Glushko, Alexander Funk, Verena Maier-Kiener, Philipp Kraker, Maria Krautz, Jürgen Eckert, Anja Waske In this work the global and local mechanical properties of the magnetocaloric intermetallic LaFe11.2Si1.8 alloy are investigated by a combination of different testing and characterization techniques in order to shed light on the partly contradictory data in recent literature. Macroscale compression tests were performed to illuminate the global fracture behavior and evaluate it statistically. LaFe11.2Si1.8 demonstrates a brittle behavior with fracture strains below 0.6 % and widely distributed fracture stresses of 180-620 MPa leading to a Weibull modulus of m = 2 to 6. The local mechanical properties, such as hardness and Young’s modulus, of the main and secondary phases are examined by nanoindentation and Vickers microhardness tests. An intrinsic strength of the main magnetocaloric phase of at least 2 GPa is estimated. The significantly lower values obtained by compression tests are attributed to the detrimental effect of pores, microcracks, and secondary phases. Microscopic examination of indentation-induced cracks reveals that ductile α-Fe precipitates act as crack arrestors whereas pre-existing cracks at La-rich precipitates provide numerous ‘weak links’ for the initiation of catastrophic fracture. The presented systematic study extends the understanding of the mechanical reliability of La(Fe, Si)13 alloys by revealing the correlations between the mechanical behavior of macroscopic multi-phase samples and the local mechanical properties of the single phases.Graphical abstractImage 1
  • Elastocaloric effect at ultra-low temperatures in nanocrystalline shape
           memory alloys
    • Abstract: Publication date: Available online 20 November 2018Source: Acta MaterialiaAuthor(s): Aslan Ahadi, Takuro Kawasaki, Stefanus Harjo, Won-Seok Ko, QingPing Sun, Koichi Tsuchiya Superelastic shape memory alloys (SMAs) exhibit a reversible elastocaloric effect that originates from a release/absorption of latent heat associated with a stress-induced martensitic phase transformation. In typical SMAs, the conventional elastocaloric effect will vanish when the operating temperature falls below the temperature range in which martensitic phase transformation can be triggered by stress. We report emergence of an unprecedented elastocaloric effect with a decrease of temperature, well below the temperature range of martensitic phase transformation, in a model nanocrystalline NiTi that preserves slim-hysteresis superelasticity at ultra-low temperatures. The new elastocaloric effect emerges at a temperature of ∼90 K, exhibits an opposite sign than the conventional elastocaloric effect, and intensifies gradually with a decrease of temperature to 18 K. At 18 K, a large adiabatic temperature change ΔTad of +3.4 K is measured upon rapid release of tensile stress. The measured ΔTad are larger and extend over a wider temperature span compared with electrocaloric, piezocaloric, and barocaloric cryo-refrigeration materials. We show that such low temperature elastocaloric effect originates from an entropic elasticity associated with large non-linear elastic deformations of the nanocrystalline microstructure at ultra-low temperatures. Our study suggests a new avenue to cool ultra-low temperature ambients.Graphical abstractImage 1
  • Dewetted nanostructures of gold, silver, copper, and palladium with
           enhanced faceting
    • Abstract: Publication date: Available online 20 November 2018Source: Acta MaterialiaAuthor(s): Arin S. Preston, Robert A. Hughes, Trevor B. Demille, Victor M. Rey Davila, Svetlana Neretina At the foundation of nanoscience and nanotechnology is the ability to shape-engineer nanometric objects so as to exert control over their physical and chemical properties. Architectural control is achieved by manipulating thermodynamic and kinetic factors that are able to guide reactions along pathways that lead to the formation or elimination of particular crystal facets. While the dewetting of ultrathin metal films provides a straightforward method for generating substrate-based metallic nanostructures, the ability to shape-engineer these structures is limited to such an extent that even the formation of highly faceted equiaxed structures often proves challenging. This, however, is not the case for colloidal syntheses where the exquisite chemical controls and synthetic ease offered by liquid-phase chemistry has led to the generation of a diverse library of nanostructure architectures. Here, it is demonstrated that the faceting of dewetted structures of gold, silver, copper, and palladium can be enhanced by subjecting them to a liquid-phase chemical environment in which metal ions are reduced to a neutral state and deposited on the nanostructure surface in manner that leads to facet formation. The faceting procedure, which can be carried out in minutes, is also shown to be amenable to a templated dewetting approach in which lithographically-defined metal discs formed in an array each agglomerate to form a single nanostructure. The work has the potential to increase the functionality of dewetted nanostructures by enabling facet-dependent chemical reactivity and plasmonic hot spots.Graphical abstractImage 1
  • Deformation compatibility between nanotwinned and recrystallized grains
           enhances resistance to interface cracking in cyclic loaded stainless steel
    • Abstract: Publication date: Available online 20 November 2018Source: Acta MaterialiaAuthor(s): Q. Li, F.K. Yan, N.R. Tao, D. Ponge, D. Raabe, K. Lu Cracks often initiate at phase boundaries in conventional second phase reinforced alloys during cyclic loading, which limits their fatigue properties. Here, we prepared a nanotwin strengthened 316L stainless steel consisting of nanotwinned and recrystallized grains by using plastic deformation and subsequent partial recrystallization annealing. Fatigue tests revealed that interfaces separating hard nanotwinned grains from soft recrystallized ones exhibited excellent resistance to crack initiation. More than half of the cracks (57% in number fraction) are found in recrystallized grains while a small fraction (11%) is observed at the interfaces between nanotwinned and recrystallized grains. This is ascribed to the elastic homogeneity and cyclic deformation compatibility between nanotwinned and recrystallized grains. At small cumulative cyclic strains (below 4000 cycles at σa = 450 MPa), nanotwinned grains deform compatibly with the recrystallized grains without noticeable strain localization at their interfaces. Nanotwins can accommodate cyclic plastic strains by interaction of dislocations with twin boundaries, especially through the motion of the well-ordered threading dislocations inside the twin lamellae. At large cumulative strains, a moderate strain gradient is developed in recrystallized grains surrounding nanotwinned grains as a function of distance from the interfaces due to the occurrence of localized deformation in nanotwinned grains. The nanotwinned grains show high microstructural stability without notable de-twinnning, thus retarding crack initiation and propagation. Therefore, improved fatigue property with high fatigue limit of ∼350 MPa and high fatigue ratio of ∼0.45 is achieved in the nanotwin strengthened stainless steel, which is better than that of conventional second phase reinforced steels with comparable strength.Graphical abstractImage 1
  • Nano-scale microstructure damage by neutron irradiations in a novel
           Boron-11 enriched TiB2 ultra-high temperature ceramic
    • Abstract: Publication date: Available online 19 November 2018Source: Acta MaterialiaAuthor(s): A. Bhattacharya, C.M. Parish, T. Koyanagi, C.M. Petrie, D. King, G. Hilmas, W.G. Fahrenholtz, S.J. Zinkle, Y. Katoh Ultra-high temperature transition-metal ceramics are potential candidates for fusion reactor structural/plasma-facing components. We reveal the irradiation damage microstructural phenomena in Boron-11 enriched titanium diboride (TiB2) using mixed-spectrum neutron irradiations, combined with state-of-art characterization using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Irradiations were performed using High Flux Isotope Reactor at ∼220 and 620 °C up to 2.4x1025 n.m-2 (E>0.1 MeV). Total dose including contribution from residual Boron-10 (10B) transmutation recoils, was ∼4.2 displacements per atom. TiB2 is susceptible to irradiation damage in terms of dislocation loop formation, cavities and anisotropic swelling induced micro-cracking. At both 220 and 620 °C, TEM revealed dislocation loops on basal and prism planes, with nearly two orders of magnitude higher number density of prism-plane loops. HRTEM, electron diffraction and relrod imaging revealed additional defects on {101¯0} prism planes, identified as faulted dislocation loops. High defect cluster density on prism planes explains anisotropic a-lattice parameter swelling of TiB2 reported in literature which caused grain boundary micro-cracking, the extent of which decreased with increasing irradiation temperature. Dominance of irradiation-induced defect clusters on prism planes in TiB2 is different than typical hexagonal ceramics where dislocation loops predominantly form on basal planes causing c-lattice parameter swelling, thereby revealing a potential role of c/a ratio on defect formation/aggregation. Helium generation and temperature rise from residual 10B transmutation caused matrix and grain boundary cavities for the irradiation at 620 °C. The study additionally signifies isotopic enrichment as a viable approach to produce transition-metal diborides for potential nuclear structural applications.Graphical abstractImage 1
  • High temperature thermal and mechanical stability of high-strength
           nanotwinned Al alloys
    • Abstract: Publication date: Available online 19 November 2018Source: Acta MaterialiaAuthor(s): Qiang Li, Jaehun Cho, S. Xue, X. Sun, Y.F. Zhang, Z. Shang, H. Wang, X. Zhang Al alloys have widespread applications but often suffer from low yield strength. Recently, nanotwinned Al-Fe solid solution alloys have shown high flow stress (>1.5 GPa), ascribed to nanograins with abundant incoherent twin boundaries and solute-stabilized 9R phase. However, the high temperature mechanical behaviors of high-strength twinned Al-Fe alloys remain unknown. In this study, we show that nanotwinned microstructures are stable up to 280 ˚C, followed by recrystallization at 300 ˚C. In-situ uniaxial compression tests in a scanning electron microscope show that the nanotwinned Al alloys retain their high flow stress of ∼1.3 GPa when tested up to 200 ˚C, and substantial softening occurs at a test temperature of 300 ˚C. This work reveals the superb thermal stability and high temperature mechanical behaviors of the nanotwinned Al-Fe alloys, and offers a new perspective to design future strong and thermally stable nanostructured Al alloys.Graphical abstractImage 1
  • Effects of Mo and Mn microadditions on strengthening and over-aging
           resistance of nanoprecipitation-strengthened Al-Zr-Sc-Er-Si alloys
    • Abstract: Publication date: Available online 18 November 2018Source: Acta MaterialiaAuthor(s): Anthony De Luca, David N. Seidman, David C. Dunand Combined microadditions of 0.09 at.% Mo and 0.4 at.% Mn to a dilute Al-0.10Zr-0.01Sc-0.007Er-0.10Si (at.%) alloy lead to increases in strength upon peak-aging, and improved over-aging resistance at 400, 425 and 450 °C for at least 6 months. These improvements are related to four cumulative effects. Firstly, Mn and Mo provide, in the as-cast state, a solid-solution-strengthening contribution of ∼90 MPa. The solid-solution contribution from Mo (∼80 MPa) remains essentially unchanged during aging at 400-450 °C, due to its extremely small diffusivity and precipitation. Secondly, Mn and Mo partition to the cores and shells, respectively, of the nano-size coherent, L12 (Al,Si)3(Zr,Sc,Er) nanoprecipitates, which nucleate in
  • The response of different tungsten material grades to 1 MeV Kr+2 heavy ion
           irradiation at different conditions Unprecedented irradiation resistance
           of nanocrystalline tungsten with equiaxed nanocrystalline grains to
           dislocation loop accumulation
    • Abstract: Publication date: Available online 15 November 2018Source: Acta MaterialiaAuthor(s): O. El-Atwani, E. Esquivel, E. Aydogan, E. Martinez, K. Baldwin, M. Li, B.P. Uberuaga, S.A. Maloy Nanocrystalline metals are often postulated as irradiation tolerant materials due to higher grain boundary densities. The efficiency of these materials in mitigating irradiation damage is still under investigation. Here, we present an in-situ transmission electron microscopy with ion irradiation study on equiaxed 35 nm grained tungsten (NCW-35 nm) and compare its radiation tolerance, in terms of dislocation loop damage, to several other grades of tungsten with different grain sizes at two temperatures (RT and 1073 K). The NCW-35 nm was shown to possess significant higher radiation tolerance in terms of loop damage. As demonstrated by Kinetic Monte Carlo simulations, at least part of the higher radiation tolerance of the small grains is due to higher interstitial storage (at the grain boundaries) and defect recombination (in the grain interiors) in the small grain material. In addition, experimental observations reveal rapid and efficient dislocation loop absorption by the grain boundaries and this is considered the dominant factor for mass transport to the boundaries during irradiation, enabling the remarkable radiation tolerance of 35 nm grained tungsten. This study demonstrates the possibility of attaining high radiation tolerant materials, in terms of dislocation loop damage, by minimizing grain sizes in the nanocrystalline regime.Graphical abstractImage 1
  • An Abnormal Meta-stable Nanoscale Eutectic Reaction Revealed by
           in-situ Observations
    • Abstract: Publication date: Available online 14 November 2018Source: Acta MaterialiaAuthor(s): Lin Zhou, Fanqiang Meng, Shihuai Zhou, Kewei Sun, TaeHoon Kim, Ryan Ott, Ralph Napolitano, Matthew J. Kramer Phase selection and growth of materials far from equilibrium provides fertile ground for novel phases and morphologies since a multitude of different pathways may be energetically accessible. In this study, a complex metastable devitrification of Al60Sm11 (ε-phase) from its amorphous precursor is discovered using a combination of in-situ high-energy X-ray diffraction (HEXRD), providing insight into the average bulk behavior, and in-situ aberration corrected scanning transmission electron microscopy, revealing the atomic scale mechanisms of growth and their dynamics. We have found that non-equilibrium chemical partitioning disrupts the nominal planer growth by formation of nanoscale Al enriched regions inhomogeneously segregated at the ε/glass interface, to locally balance the compositionally dependent driving force and the associated diffusional burden imposed on its grain growth. These Al-rich regions form fcc-Al-rich nanocrystallites epitaxially with the ε-phase, modifying ε/glass interface mobility and creating a crenulated growth front. This new mechanism offers a pathway for fabricating alloy structures with nanoprecipitate dispersions through a meta-stable phase transition.Graphical abstractImage 1
  • Revealing fracture mechanisms of medium manganese steels with and without
    • Abstract: Publication date: Available online 14 November 2018Source: Acta MaterialiaAuthor(s): Binhan Sun, Dhanalakshmi Palanisamy, Dirk Ponge, Baptiste Gault, Fateh Fazeli, Colin Scott, Stephen Yue, Dierk Raabe Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, δ-ferrite and α'-martensite with a wide range of fractions for each constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced α′-martensite (α′) interfaces. In contrast, the fracture surface of δ-ferrite containing steels shows a combination of cleavage in δ-ferrite and dimple/quasi-cleavage zones in the γ-α (or γ/α′-α) constituent. The embrittlement of δ-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to δ-ferrite and crack-arresting ability of the γ-α layers. Austenite stability is critical for the alloys’ fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing δ-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of δ-ferrite containing steels increases from ∼13% to ∼33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and δ-ferrite.Graphical abstractImage 1
  • On pinning-depinning and microkink-flow in solid state dewetting: Insights
           by in-situ ESEM on Al thin films
    • Abstract: Publication date: Available online 14 November 2018Source: Acta MaterialiaAuthor(s): Stefan Werner Hieke, Marc-Georg Willinger, Zhu-Jun Wang, Gunther Richter, Dominique Chatain, Gerhard Dehm, Christina Scheu The dynamics of solid state dewetting phenomena of a 50 nm thick, mazed bicrystalline Al film on single crystalline α-Al2O3 (sapphire) substrates was studied in-situ using an environmental scanning electron microscope (ESEM). The bicrystalline Al thin films served as a model system where the influence of grain boundaries and texture effects are well determined compared to polycrystalline films. The experiments were performed in controlled oxidizing and reducing atmospheres at 773 K and 823 K, respectively, to shed light on the differences in dewetting mechanisms and dynamics.While the reducing atmosphere led to spontaneous dewetting at 823 K after an incubation time of a few minutes, a hierarchical dewetting process was observed for the sluggish dewetting under oxidizing conditions. Voids initiated at (substrate or surface) defects and expanded trying to maintain a hexagonal shape. Pinning and depinning processes led to a discontinuous void growth and irregular void shapes including finger instabilities. As a consequence, the void growth followed a variety of power law exponents between 0.10 and 0.55. A new microkink-flow mechanism was discovered at the terminating Al planes at the void.Graphical abstractImage 1
  • Time-resolved atomic-scale observations of deformation and fracture of
           nanoporous gold under tension
    • Abstract: Publication date: Available online 13 November 2018Source: Acta MaterialiaAuthor(s): Pan Liu, Xiao Wei, Shuangxi Song, Lihua Wang, Akihiko Hirata, Takeshi Fujita, Xiaodong Han, Ze Zhang, Mingwei Chen It has been known for decades that nanopores produced by selective leaching during galvanic corrosion can lead to dramatic loss of materials ductility and strength under tension. However, the underlying atomic mechanisms of the nanopore induced embrittlement remain to be poorly known. Here we report in situ observations of the deformation and failure of dealloyed nanoporous gold by utilizing the state-of-the art aberration-corrected transmission electron microscopy and fast direct electron detection camera. Our time-resolved atomic observations reveal that the brittle failure of the nanoporous gold originates from plastic instability of individual gold ligaments by the interplay between dislocation plasticity and stress-driving surface diffusion.Graphical abstractWe report in situ observations of the deformation and failure of dealloyed nanoporous gold by utilizing the state-of-the art aberration-corrected transmission electron microscopy and fast direct electron detection camera. Time-resolved atomic observations reveal that the brittle failure of the nanoporous gold originates from plastic instability of individual gold ligaments by the interplay between dislocation plasticity and stress-driving surface diffusion.Image 1
  • { 11 2 ¯ 1 } +secondary+twinning+associated+with+ { 11 2 ¯ 2 } +twin+in+titanium&rft.title=Acta+Materialia&rft.issn=1359-6454&">Steps and { 11 2 ¯ 1 } secondary twinning associated with
           { 11 2 ¯ 2 } twin in titanium
    • Abstract: Publication date: Available online 13 November 2018Source: Acta MaterialiaAuthor(s): Mingyu Gong, Shun Xu, Dongyue Xie, Shujuan Wang, Jian Wang, Christophe Schuman, Jean-Sébastien Lecomte {112¯2} twinning commonly takes place in α-titanium (α-Ti). High-resolution transmission electron microscopies (HRTEM) explored various Steps along {112¯2} coherent twin boundary. Topological model of {112¯2} twin revealed twinning disconnections (TDs) that are represented by (bi, ih{112¯2}) corresponding to a step height ih{112¯2} and a shear vector bi. Atomistic simulations were conducted to study the energies and kinetics of TDs. Combining microscopies and atomistic simulations, we concluded that (b3, 3h{112¯2}) is the elementary TD and (b1, h{112¯2}) is the reassemble TD. Steps observed in HRTEM thus can be treated as a reassembly of (b3, 3h{112¯2}) TDs and (b1, h{112¯2}) TDs. In addition, Electron Backscatter Diffraction (EBSD) maps revealed {112¯2}→{112¯1} double twins in α-Ti. Using two-dimensional and three-dimensional atomistic simulations, we demonstrated the nucleation of (b1, h{112¯2}) TD and {112¯2}→{112¯1} double twin through the interaction between basal dislocation and {112¯2}
  • Predicting grain boundary structure and energy in BCC metals by integrated
           atomistic and phase-field modeling
    • Abstract: Publication date: Available online 13 November 2018Source: Acta MaterialiaAuthor(s): Di Qiu, Pengyang Zhao, Chen Shen, Weijie Lu, Di Zhang, Matous Mrovec, Yunzhi Wang We predict structure and energy of low-angle (11¯0)pure twist grain boundaries (GBs) in five BCC transition metals (β-titanium, molybdenum, niobium, tungsten, and tantalum) using a combination of atomistic and microscopic phase-field (MPF) modeling. The MPF model takes as inputs solely the generalized stacking fault (GSF) energy surfaces (i.e., the γ-surface) and elastic constants obtained from the atomistic simulations. Being an energy-based method, the MPF model lifts the degeneracy of the geometric models in predicting GB structures. For example, the multiple indefinite solutions offered by the Frank-Bilby equation are shown to converge to exactly the same equilibrium structure. It predicts a transition of the equilibrium GB structure from a pure screw hexagonal network (Mo and W) to mixed hexagonal networks (Nb and Ta) to a rhombus network (β-Ti) of dislocations. Parametric simulation studies and detailed analyses of the underlying dislocation reactions that are responsible for the formation of the rhombus and hexagonal structures reveal a close correlation between material properties (including the elastic anisotropic ratio and the local curvature on the γ-surface) and the GB structure and energy in BCC metals. This integrated approach allows one to explore, through high throughput calculations, the potential to tailor the structure and energy of special GBs in BCC metals by alloying.Graphical abstractImage 1
  • Strengthening Mechanisms in Directed Energy Deposited Austenitic Stainless
    • Abstract: Publication date: Available online 13 November 2018Source: Acta MaterialiaAuthor(s): Thale R. Smith, Joshua D. Sugar, Chris San Marchi, Julie M. Schoenung Microstructures and mechanical properties are evaluated in austenitic stainless steel structures fabricated by directed energy deposition (DED) considering the effects of applied loading orientation, build geometry, and distance from the deposition baseplate. Locations within an as-deposited build with different thermomechanical history display different yield strength, while those locations with similar history have approximately the same yield strength, regardless of test specimen orientation. Thermal expansion of deposited material near the baseplate is inhibited by the mechanical constraint imposed by the baseplate, promoting plastic deformation and producing a high density of dislocations. Concurrently, high initial cooling rates decrease away from the baseplate as the build is heated, causing an increased spacing of cellular solidification features. An analysis of strengthening mechanisms quantitatively established for the first time the important strengthening contribution of high dislocation densities in the materials (166-191 MPa) to yield strength that ranged from 438-553 MPa in the present DED fabricated structures. A newly adopted mechanistic relationship for microsegregation strengthening from the literature indicated an additional important contribution to strengthening (123-135 MPa) due to the cellular solidification features. These findings are corroborated by the measured evolution of microstructure and hardness caused by annealing the DED material. These results suggest that the mechanical properties of deposited austenitic stainless steels can be influenced by controlling thermomechanical history during the manufacturing process to alter the character of compositional microsegregation and the amount of induced plastic deformation.Graphical abstractImage 1
  • A mechanism-based homogenization of a dislocation source model for bending
    • Abstract: Publication date: Available online 13 November 2018Source: Acta MaterialiaAuthor(s): Severin Schmitt, Markus Stricker, Peter Gumbsch, Katrin Schulz The homogenization of dislocation dynamics including the mechanisms of dislocation nucleation is a great challenge in dislocation based continuum formulations. Due to the loss of the local and temporal resolution in a continuum model, physical nucleation mechanisms have to be incorporated in an average sense. Consequences can be the over- or underestimation of the macroscopic production rate of dislocation density which results in artificial softening or hardening phenomena. In this paper, we derive a mechanism-based homogenization of a dislocation source model based on the theory of critical thickness, which accounts for the relation between the external loading condition and the resulting dislocation density production rate. The formulation is applied to pure and cantilever bending problems, validated in comparison to discrete dislocation dynamics simulations, and discussed for the discrete-continuum transition regime.Graphical abstractImage 1
  • Two modes of screw dislocation glide in fcc single-phase concentrated
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): Yuri N. Osetsky, George M. Pharr, James R. Morris Concentrated solid solution alloys (CSSAs), including medium- and high-entropy alloys, are currently being considered as prospective materials in many applications. The behavior of CSSAs under different conditions, including mechanical loading, differs from that of conventional alloys and has been the subject of intensive study by different techniques. In many cases, their behavior is treated by modifying solid solution hardening models, which, in principle, does not reflect many important features of CSSAs where the distinction between solute and solvent atoms is not clear. In this work, we report the results of an atomic-scale study of ½{111} screw dislocation motion in an fcc equiatomic Ni-Fe alloy. Molecular dynamics simulations demonstrate that the dislocation has two distinctive modes for glide. At lower stress, dislocations move in a very rough manner that cannot be described as continuous glide but rather as jerky motion through a set of obstacles. At high stress, they glide in a manner similar to lattice friction-controlled conditions in single component systems. The stress for the transition between modes depends on the dislocation segment length and temperature. At 300K, the flow stress saturates at ∼130 MPa for lengths above ∼140 b (b is the Burgers vector).Graphical abstractImage 1
  • a -axis+Mg+single+crystals+during+early+compression&rft.title=Acta+Materialia&rft.issn=1359-6454&">Dislocation distribution and patterning in 〈 a 〉 -axis Mg single
           crystals during early compression
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): M. Niewczas, A. Kula, H. Kitahara, S. Ando Dislocation distribution and patterning during the onset of uniaxial compression of 〈a〉-axis Mg single crystals was investigated by transmission electron microscopy (TEM) and etch-pitting technique. The crystals deform by dislocation glide using basal and nonbasal slip systems. The high hardening rate Θ≈E/50−E/30 indicates the effective forest hardening by nonbasal dislocations observed in the microstructure. Distribution of dislocation etch-pits shows the polygonized arrangement of dislocations along the basal plane. TEM observations reveal the heterogeneous structure of dislocations clustered in dense arrays surrounded by dislocation free areas. Dislocation arrangements that are building blocks of the microstructure include monopoles, dipoles, multipoles, ribbons, walls, bundles, and braids. The patterns are characterized with respect to their dislocation content and spatial distribution. They fall into the category of low energy structures, which favorite the configurations reducing the elastic energy of the arrays. The degree of interactions between basal and 〈c+a〉 slip systems is attributed to playing a key role in the formation of observed dislocation arrangements.Graphical abstractImage 1
  • Structural mechanism behind piezoelectric enhancement in
           off-stoichiometric Na0.5Bi0.5TiO3 based lead-free piezoceramics
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): Anupam Mishra, Dipak Kumar Khatua, Arnab De, Bhaskar Majumdar, Till Frömling, Rajeev Ranjan While studies in the past have shown that certain kinds of off-stoichiometry enhance the piezoelectric response of the lead-free piezoceramic Na0.5Bi0.5TiO3 (NBT), there is a lack of clarity regarding the mechanism associated with this interesting phenomenon from the fundamental structural perspective. In this paper, we have investigated this issue comprehensively and succeeded in establishing a mutual correspondence between off-stoichiometry, grain size, crystal structure, dielectric and piezoelectric properties in Na0.5Bi0.5TiO3 (NBT). Of the four different types of off-stoichiometric samples synthesized as per nominal formulae namely Na0.5+xBi0.5TiO3 (Na-excess Na-series), Na0.5-xBi0.5TiO3 (Na-deficient Na-series), Na0.5Bi0.5+xTiO3 (Bi-excess Bi-series), and Na0.5Bi0.5-xTiO3 (Bi-deficient Bi-series), the best piezoelectric response (d33 ∼ 100 pC/N) was obtained in the Na-deficient series with x=0.04. We succeeded in establishing the structural link between off-stoichiometry and piezoelectricity of this series by examining the structural state of the specimens in their poled state. We show that the off-stoichiometric compositions exhibiting higher piezoelectric response contain a higher fraction of the disordered ferroelectric phase coexisting with the field stabilized long-range ferroelectric (R3c) order. Beyond the critical off-stoichiometry (x>0.04), the dominance of the structural disorder collapses the piezoelectric response of the system. We also show that what can be achieved by off-stoichiometry can as well be achieved by reducing the grain size of stoichiometric NBT. Our results suggest that the enhanced piezoelectric response of the off-stoichiometric compositions is due to their reduced grain size as compared to the stoichiometric composition, and that the nature of the defect species has a secondary role, if any. We found the same phenomenon/mechanism to be operative in the off-stoichiometric morphotropic phase boundary composition 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (NBT-6BT). While our experiments confirm the role of the surviving structural heterogeneity (after poling) as an important contributing factor which enhances the piezoelectric response of NBT-based lead-free piezoceramics, we also use dielectric dispersion as a tool to show that the off-stoichiometric composition exhibiting highest piezoelectric response is characterized by maximum suppression of the disordered phase by the poling field.Graphical abstractImage 1
  • Unravelling uniaxial strain effects on electronic correlations,
           hybridization and bonding in transition metal oxides
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): Zhihua Yong, Jiajun Linghu, Shibo Xi, Xinmao Yin, Meng Lee Leek, Lei Shen, Rainer Timm, Andrew T.S. Wee, Yuan Ping Feng, Jisheng Pan The interplay among spin, lattice, charge and orbit is of central importance for several rich and fascinating properties of oxides, and is the subject of intense research at present. Here, we present an approach to manipulate this interplay by Sn doping to effectively apply uniaxial strain on the TiO2 lattice. The evolution of this interplay in pseudo-homoepitaxial Ti1-xSnxO2 films is measured using a combination of X-ray absorption near edge spectroscopy at the O K and Ti L3,2-edges. Supported by various theoretical calculations, we find that the multiplet-type electronic correlations, long-range bonding and hybridization in the system can be controlled by independently modifying uniaxial strain, thereby allowing us to establish the correlations among these effects, doping concentration, and strain. This significantly widens the phase space for experimental exploration of predictive models and leads to new possibilities for manipulation over materials’ functional properties. The methodology presented here can be applied in general to study the nature of the multiplet-type electronic correlations and bonding properties in octahedral-coordinated 3dN transition metal oxides.Graphical abstractImage 1
  • Growth, interfacial microstructure and optical properties of NiO thin
           films with various types of texture
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): Y. Wang, J. Ghanbaja, P. Boulet, D. Horwat, J.F. Pierson NiO thin films with random, fiber and in-plane textures have been successfully deposited at near room temperature by reactive magnetron sputtering on glass, silicon and Al2O3 (0001) substrates. Self-texture related with the deposition conditions and crystallographic characters competes with the driving force from the matched substrate. Such a competition can be used to control the texture of thin films on matched substrates, especially when the promoting orientations from self-texture and substrate are different. Enhancing this competition tends to suppress the self-texture of NiO thin films on Al2O3 (0001) substrates, whereas restricting the competition is beneficial to produce the in-plane textured NiO thin films. In addition, it is found that the optical transmittance of NiO thin films on Al2O3 (0001) substrates can also be tuned by such competition. Interfacial microstructure analyses of NiO thin films on amorphous substrates clearly evidence the existence a nanocomposite layer at the initial growth, which is composed of NiO nanocrystals surrounded by amorphous matrix. In contrast, in-plane textured NiO thin films on Al2O3 (0001) substrates exhibit sharp interface without nanocrystals or amorphous matrix. We believe these results provide a general framework of tuning the textures and properties of thin films on matched substrates.Graphical abstractImage 1
  • Bayesian Uncertainty Quantification and Information Fusion in
           CALPHAD-based Thermodynamic Modeling
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): P. Honarmandi, T.C. Duong, S.F. Ghoreishi, D. Allaire, R. Arroyave Calculation of phase diagrams is one of the fundamental tools in alloy design—more specifically under the framework of Integrated Computational Materials Engineering. Uncertainty quantification of phase diagrams is the first step required to provide confidence for decision making in property- or performance-based design. As a manner of illustration, a thorough probabilistic assessment of the CALPHAD model parameters is performed against the available data for a Hf-Si binary case study using a Markov Chain Monte Carlo sampling approach. The plausible optimum values and uncertainties of the parameters are thus obtained, which can be propagated to the resulting phase diagram. Using the parameter values obtained from deterministic optimization in a computational thermodynamic assessment tool (in this case Thermo-Calc) as the prior information for the parameter values and ranges in the sampling process is often necessary to achieve a reasonable cost for uncertainty quantification. This brings up the problem of finding an appropriate CALPHAD model with high-level of confidence which is a very hard and costly task that requires considerable expert skill. A Bayesian hypothesis testing based on Bayes’ factors is proposed to fulfill the need of model selection in this case, which is applied to compare four recommended models for the Hf-Si system. However, it is demonstrated that information fusion approaches, i.e., Bayesian model averaging and an error correlation-based model fusion, can be used to combine the useful information existing in all the given models rather than just using the best selected model, which may lack some information about the system being modelled.Graphical abstractImage 1
  • Long-period structural modulation on the global length scale as the
           characteristic feature of the morphotropic phase boundaries in the
           Na0.5Bi0.5TiO3 based lead-free piezoelectrics
    • Abstract: Publication date: Available online 12 November 2018Source: Acta MaterialiaAuthor(s): Gobinda Das Adhikary, Dipak Kumar Khatua, Anatoliy Senyshyn, Rajeev Ranjan The inherent structural disorder has a profound effect on the dielectric, ferroelectric and the electromechanical response of the Na0.5Bi0.5TiO3 (NBT) based lead-free piezoelectrics. While analogous to the lead-based classical morphotropic phase boundary (MPB) systems the existence of MPB has been recognized in some derivatives of NBT displaying enhanced electromechanical response, there is a lack of clarity on the strucural state of the MPB compositions on NBT-based systems on the global length scale. We have examined this issue on the well known MPB system (1-x)Na0.5Bi0.5TiO3-(x)K0.5Bi0.5TiO3(NBT-KBT) by carrying out structural investigations on local and global length scales using Eu+3 photoluminiscence and high-resolution neutron powder diffraction techniques, respectively. Our study reveals that the MPB of this system is characterized by the onset of a long-period modulated structure with a periodicity of ∼40 Å on the global scale. Temperature depedent neutron diffraction study revealed that the intermediate temperature P4bm phase which appears in NBT is suppressed for the MPB composition. The MPB composition rather develops a long-period modulated phase on cooling from the cubic phase. The ergodic-nonergodic relaxor ferroelectric transition occurs within this long-period modulated phase. In the non-ergodic regime, however, strong electric field irreversibly transforms the long-period modulated phase to the rhombohedral ferroelectric (R3c). We demonstrate that thermal depolarization of this system is a distinct structural event characterized by the system losing its field-induced long range rhombohedral (R3c) coherence and transforming back to the long-period modulated phase. Our study suggests that the long-period modulated phase is the primary structural feature of the MPB compositions in NBT-based piezoelectrics.Graphical abstractImage 1
  • Interfacial-dislocation-controlled deformation and fracture in nanolayered
           composites: toward higher ductility of drawn pearlite
    • Abstract: Publication date: Available online 10 November 2018Source: Acta MaterialiaAuthor(s): Tomotsugu Shimokawa, Tomoaki Niiyama, Masashi Okabe, Jun Sawakoshi The excellent combination of high strength and high ductility of drawn pearlitic steels is likely derived from the synergistic effects between the ferrite and cementite phases. However, the detailed mechanism, especially the mechanism responsible for the improvement in ductility, has not yet been fully elucidated. In this study, to achieve improved ductility of drawn pearlitic steels, interfacial-dislocation-controlled deformation and fracture in nanolayered composites of ferrite and cementite phases with the Bagaryatsky relationship are investigated via uniaxial tensile and compressive deformation tests using molecular dynamics simulations. Various modes of inelastic deformation are observed at the yield point according to the spacing of interfacial dislocations on the interface between the ferrite and cementite phases in the nanolayered-composite models. Spacing of the interfacial dislocations, which accommodates misfit strains between the ferrite and cementite phases, determines the phase stress and the interfacial dislocation structure in the nanolayered-composite models. This phase stress and interfacial dislocation structure influences the resolved shear/normal stress and the critical resolved shear/normal stress for each inelastic-deformation mode, respectively. Thus, interfacial dislocation spacings can control which inelastic deformation mode is activated at the yield point. We find specific interfacial dislocation structures on the ferrite–cementite interface that nucleate lattice dislocations with lower Schmid factors at the first plastic event. This interfacial dislocation structure can improve the ductility of drawn pearlitic steels because the high strain-hardening rate in the ferrite phases, resulting from the nucleation of dislocations with lower Schmid factors, is clearly expected to suppress the concentration of plastic deformation in the cementite phase [T. Ohashi et al., Mater. Sci. Eng. A 588 (2013) 214–220]. The possibility of the interfacial-dislocation-controlled deformation and fracture enabling higher ductility of drawn pearlitic steels is discussed.Graphical abstractImage 1
  • γ’-(L12) Precipitate Evolution during Isothermal Aging of a Co-Al-W-Ni
    • Abstract: Publication date: Available online 10 November 2018Source: Acta MaterialiaAuthor(s): Daniel J. Sauza, David C. Dunand, Ronald D. Noebe, David N. Seidman The coarsening kinetics and elemental partitioning behavior of γ’-(L12) precipitates in a γ(f.c.c.)-matrix for a model quaternary Co-8.8Al-8.9W-9.9Ni at.% superalloy are investigated utilizing isothermal aging conditions at 650, 800 and 900 °C. The γ’-precipitate’s mean radius, number density, and volume fraction, at 800 and 900 °C, were studied using scanning electron microscopy; the calculated temporal exponents associated with coarsening of γ’-precipitates display good agreement with model predictions for quasi-stationary coarsening. An atom probe tomographic (APT) investigation of the aged γ/γ’ microstructure at 650 °C demonstrates that the compositions and volume fractions of both phases vary continuously up to 4096 h. The aged microstructure at 650 °C consists of interconnected nanoscale γ’-precipitates, corroborated utilizing SEM for the 4096 h aged-specimen. The activation energy for coarsening is estimated for the temperature range 650 – 900 °C to be 283 kJ mol-1, in reasonable agreement with activation energies for diffusion of Al, W, and Ni in Co, suggesting that coarsening of γ’-precipitates is limited by bulk-diffusion. APT measurements of specimens aged for 1024 h at 800 and 900 °C demonstrate that the isothermal aging temperature has a significant effect on the compositions and partitioning behavior of Co, Al, W and Ni between the γ- and γ’-phases. The partitioning ratio of the concentrations between the γ’- and γ-phases is largest for W, decreasing linearly from 5.3 ± 0.1 at 650 °C to 2.1 ± 1.2 at 900 °C, and smallest for Co, decreasing from 0.86 ± 0.01 at 650 °C to 0.73 ± 0.01 at 900 °C.Graphical abstractImage 1
  • Ductile deformation of core-shell Si–SiC nanoparticles controlled by
           shell thickness
    • Abstract: Publication date: Available online 9 November 2018Source: Acta MaterialiaAuthor(s): D. Kilymis, C. Gérard, L. Pizzagalli Although the literature on mechanical properties of nanostructures is extensive, there are still few studies focusing on core–shell nanoparticles. In these systems, which are interesting in a broad range of applications, one could genuinely assume that the softest part, be it the core or the shell, will first yield when submitted to compression. To test this view, we have carried out large scale molecular dynamics simulations of uniaxially compressed core–shell Si–SiC nanoparticles. Our first conclusion is that for the investigated size range (diameters equal or below 50 nm), the nanoparticles yield plastically with no signs of fracture, in agreement with experiments on single material systems. Furthermore, our investigations also reveal that depending on the shell thickness, plastic deformation is confined either in the core or in the shell. We propose a model, based on the theory of contact mechanics and geometrical arguments, to explain this surprising result. Furthermore, we find that for a specific shell to diameter ratio, corresponding to the transition between core and shell, the stress concentration in the nanoparticles is apparently hindered, leading to a delayed plastic deformation.Graphical abstractImage 1
  • Atomistic Simulation of the Formation and Fracture of Oxide Bifilms in
           Cast Aluminum
    • Abstract: Publication date: Available online 8 November 2018Source: Acta MaterialiaAuthor(s): Jialin Liu, Qigui Wang, Yue Qi Formation and entrainment of double layer oxides (bifilm) in aluminum casting is inevitable due to the high oxidation rate of liquid aluminum and particularly turbulence during the mold filling process. The final mechanical properties of the aluminum castings suffer from these inclusions but neither the formation process nor fracture mechanism is fully understood due to the difficulty of in-situ observation on nano-scale aluminum oxide thin film. To understand the impact of bifilms on the fracture mechanism at different bifilm formation stages and the aging processes, atomic level bifilm slab models were built according to their formation history. ReaxFF reactive forcefield-based molecular dynamics (MD) method was used to simulate the formation and deformation of different types of bifilms. The MD simulations showed that an incomplete “healing” process happened at the oxide/oxide interface during bifilm formation and the fracture occurred at the Al/oxide interface instead of the oxide/oxide interface. When the oxide transformed from amorphous to α-Al2O3 due to aging, the fracture energy increased from 0.43 J/m2 to 0.53 J/m2. With 30% coverage of hydroxyl group surface contamination, the –OH-terminated oxide bifilm fractured at the oxide/oxide interface and the corresponding fracture energy dropped to 0.30 J/m2. This is most likely due to the H2 bubbles being trapped in the aluminum oxide bifilm interface. To facilitate multiscale modeling, the MD predicted oxide bifilms fracture energy and fracture strength were converted to cohesive zone parameters, via a simple size bridging relationship, for future finite element modeling.Graphical abstractImage 1
  • Nickel incorporation in perovskite-type metal oxides – implications
           on reducibility
    • Abstract: Publication date: Available online 7 November 2018Source: Acta MaterialiaAuthor(s): Patrick Steiger, Ivo Alxneit, Davide Ferri Nickel is often applied in heterogeneous catalysis for its high catalytic activity towards a large variety of reactions at affordable price. Nickel reduction from perovskite-type mixed oxides is increasingly exploited to generate active and stable Ni catalysts. To investigate implications of the host perovskite structure on Ni reducibility, Ni was incorporated on the B-site of three perovskite-type mixed metal oxides of different lattice symmetries (LaFeO3 – orthorhombic, LaCoO3 – rhombohedral and La0.3Sr0.55TiO3 – cubic). Structural parameters of the phase pure undoped and Ni-doped perovskites were determined using synchrotron X-ray diffraction (XRD) and Ni K-edge X-ray absorption (XAS). Rietveld refinement and extended X-ray absorption fine structure (EXAFS) data fitting were used to verify that at this substitution level Ni enters the B-site and adopts its coordination environment. Expansion of the unit cell was found in LaCoO3 and La0.3Sr0.55TiO3, whereas contraction was observed in the case of LaFeO3. X-ray absorption spectra showed that Ni-containing unit cells exhibit the same symmetry as the host perovskite-type mixed oxides. The mean oxidation state of Ni was found to be equal in all three cases irrespective of the host lattice (+2.5). Lattice symmetry had significant effect on lowering the Ni reduction temperature determined by hydrogen temperature programmed reduction and a correlation between reduction temperature and crystal tolerance factor was found.Graphical abstractImage 1
  • In-situ observation of evolving microstructural damage and associated
           effective electro-mechanical properties of PZT during bipolar electrical
    • Abstract: Publication date: Available online 7 November 2018Source: Acta MaterialiaAuthor(s): Wei Lin Tan, Katherine T. Faber, Dennis M. Kochmann We investigate the fatigue behavior of bulk polycrystalline lead zirconate titanate (PZT) during bipolar electric field cycling. We characterize the frequency- and cycle-dependent degradation in both the effective electro-mechanical properties (specifically, the electrical hysteresis and the macroscopic viscoelastic stiffness and damping measured by Broadband Electromechanical Spectroscopy, BES) and the microstructural damage evolution (quantified via scanning electron microscopy). The BES setup enables the mechanical characterization while performing electrical cycling so as to measure the evolving viscoelasticity without remounting the sample; particularly measuring the viscoelastic damping allows us to gain insight into the ferroelectric domain wall activity across the full electric hysteresis and over the full range of cycles. A clear dependence on the electric cycling frequency is observed in the rates of degradation of all measured properties including an up to 10% increase in dynamic compliance and a 70% decrease in electric displacement magnitude. We quantify the evolving micro-crack density across wide ranges of numbers of cycles and compare with changes in the effective compliance. Interestingly, the observed strong degradation in the ferroelectric hysteresis is contrasted by relatively mild changes in the effective viscoelastic moduli, while samples clearly indicate increasing levels of micro-damage.Graphical abstractImage 1
  • Analysis of austenite-martensite phase boundary and twinned microstructure
           in shape memory alloys: The role of twinning disconnections
    • Abstract: Publication date: Available online 5 November 2018Source: Acta MaterialiaAuthor(s): Emil Bronstein, Eilon Faran, Doron Shilo An austenite-martensite phase boundary in shape memory alloys (SMA) is associated with a periodic microstructure of martensite twin lamellas. Microscopy studies show that the period, which represents the thickness of the twin lamellas, increases with the distance from the habit plane. This observation is often overlooked when the microstructure and energy of the austenite-martensite interface are evaluated. In this paper we introduce a model that reproduces the variation in the twin lamella period. For this purpose, the overall energy of the phase boundary and the accompanied twinned microstructure is formulated and minimized. In particular, the effect of twinning disconnections, via which twins are tapered or broaden, and the additional energy due to the disconnections, are considered. Fittings of model predictions with measurements based on microscopy images provide evaluations of the twin boundary and twinning disconnection energies. Comparison of the results with expressions based on the theory of dislocations indicates that interactions between disconnections play a dominant role in determining the overall energy of twinning disconnections.Graphical abstractImage 1
  • Influencing mechanisms of atomic diffusion and compositional distribution
           on the magnetic anisotropy of Cr/SmCo/(Cu)/Cr thin films
    • Abstract: Publication date: Available online 5 November 2018Source: Acta MaterialiaAuthor(s): Y. Hong, Z.G. Qiu, Z.G. Zheng, G. Wang, H.Y. Yu, D.Y. Chen, G.B. Han, D.C. Zeng, J.P. Liu Rare earth (RE) permanent magnetic thin films, such as SmCo-based films, are promising candidates for future thermal-assisted magnetic recording media because of their supreme thermal stability and fine superparamagnetic critical size. Phase compositional distributions and crystallographic orientations can directly influence the magnetic domain evolutions and magnetic performance of SmCo films. However, these films often exist in multi-phases without well-defined distinct magnetic anisotropy, thereby causing difficulties in analyzing their functions. The effects of atomic diffusions induced by annealing on crystallization and magnetic anisotropy energy are poorly understood. In this work, the influence of doping Cu atoms via introducing Cu layer on magnetic anisotropy of Cr/SmCo/(Cu)/Cr films is investigated. Introducing a Cu layer and post-annealing lead to tunable magnetic anisotropy from isotropy to anisotropy and domain structure formation. Micromagnetic simulation is used to further analyze the effects of crystallization, anisotropy field and compositional distribution on the magnetization reversal behavior and coercivity variations in SmCo films. It reveals that the differences of anisotropy field of Co and amorphous phases could affect the domain wall motions and coercivity, which shows the analogous trend as an experimental result. Moreover, the elevated anisotropy constant of Sm(Co,Cu) alloy and the small fractions of SmCo5 boundary phases are also beneficial to the enhancement of coercivity derived by calculating domain wall energy. In-plane magnetic anisotropy is improved by the SmCo5 phase, which has an in-plane preferred orientation of the c axis. This work provides theoretical and experimental bases for the future fabrication of SmCo-based films by controlling atomic diffusion with optimized grain boundary phase distribution.Graphical abstractImage 1
  • Strong shear-flow modulation of instabilities in rapid directional
    • Abstract: Publication date: Available online 5 November 2018Source: Acta MaterialiaAuthor(s): Katarzyna N. Kowal, Stephen H. Davis We examine the effect of a strong shear flow on morphological instabilities that occur in the directional solidification of a dilute, binary alloy when the interface departs from local thermal equilibrium in a frozen-temperature, one-sided model. In particular, the flow velocity U∞ is much larger than the rate of solidification V and the Schmidt number is arbitrary. In contrast to solidification processes under small or no flow, for which both a cellular and an oscillatory mode of instability appear, the liquid-solid interface under flows of large magnitude is susceptible to a single mode of instability. All experiments on banding occur in the overlap region between these modes under no flow and since these two primary modes coalesce into one time-dependent, spatially-dependent mode, there is no mechanism for the production of bands anymore in the large-flow regime. No experiments have been conducted to date on this rapid solidification under flow and this suggests that such experiments would show no bands. The flow significantly stabilizes and selects higher wavenumbers for the preferred form of instability in comparison to systems involving small or no flow. The introduction of a strong flow provides an effective mechanism to eliminate instabilities at high solidification rates. However, stability thresholds at low solidification rates remain mainly indifferent to the presence of flow.Graphical abstractImage 1
  • Modelling recrystallization textures driven by intragranular fluctuations
           implemented in the viscoplastic self-consistent formulation
    • Abstract: Publication date: Available online 3 November 2018Source: Acta MaterialiaAuthor(s): Miroslav Zecevic, Ricardo A. Lebensohn, Rodney J. McCabe, Marko Knezevic This paper presents a recrystallization model driven by intragranular orientation gradients and strain energy fields calculated by means of the viscoplastic self-consistent (VPSC) formulation. The VPSC model is extended for calculation of the coupling between intragranular stress fluctuations with corresponding second moments of lattice spin and misorientation fields in the grains. Access to these quantities allows modelling of transition bands and nucleation kinetics. In the proposed recrystallization model, grain growth is assumed to be proportional to the difference between the stored energy of each grain and that of the effective medium. Recrystallization textures for several cubic metals are simulated, showing good agreement with corresponding experiments. The model reveals the importance of considering appropriate, microstructurally-based and orientation-dependent recrystallization nucleation mechanisms. The recrystallization texture of heavily rolled copper with a strong cube texture component is found to be a consequence of nucleation at transition bands, which is also the cause of the recrystallization textures in compressed iron and drawn copper wire. In contrast, the recrystallization texture of rolled interstitial-free steel is found to be caused by grain boundary nucleation occurring in grains with the highest strain energy.Graphical abstractImage 1
  • Stress-enhanced dynamic grain growth during high-pressure spark plasma
           sintering of alumina
    • Abstract: Publication date: Available online 3 November 2018Source: Acta MaterialiaAuthor(s): Barak Ratzker, Avital Wagner, Maxim Sokol, Sergey Kalabukhov, Nachum Frage Applying high pressure during the sintering of ceramic materials is a common practice that allows for a reduction of the sintering temperature and the obtaining of fine-grained microstructures. In this work, we show that the final grain size of submicron alumina increased consistently with applied pressure during low temperature (1000-1050°C), high pressure (500-800 MPa) spark plasma sintering. Grain size trajectories and microstructural observations indicated that stress-enhanced grain growth occurred during the final stage of the sintering process, whereas thermally controlled grain boundary migration was negligible. We suggest that this dynamic, stress-enhanced grain growth is controlled by grain-boundary sliding, grain rotation and coalescence. A strong correlation was found between calculated creep strain rates and grain growth rates, such as during superplastic deformation.Graphical abstractImage 1
  • { 10 1 ¯ 2 } +twin+boundaries+in+deformation+of+magnesium&rft.title=Acta+Materialia&rft.issn=1359-6454&">Dislocation absorption and transmutation at { 10 1 ¯ 2 } twin
           boundaries in deformation of magnesium
    • Abstract: Publication date: Available online 3 November 2018Source: Acta MaterialiaAuthor(s): Peng Chen, Fangxi Wang, Bin Li How matrix dislocations, i.e. basal, prismatic and pyramidal, interact with {101¯2}101¯1¯ twin boundaries in hexagonal close-packed metals has been discussed extensively in the literature. However, so far no systematic investigation has been reported. In this work, we performed atomistic simulations to study interaction between matrix dislocations in pure Mg with a {101¯2} twin boundary. Our results show that for the basal and the prismatic slip, when the Burgers vector is parallel to the zone axis of the twins, a matrix basal dislocation can be transmuted to a twin prismatic dislocation and vice versa. However, when the Burgers vector of the matrix dislocation is non-parallel to the zone axis, no transmutation occurs and the dislocation is absorbed by the twin boundary which acts as a dislocation sink. For a matrix pyramidal dislocation, the dislocation is absorbed by the twin boundary and no transmutation occurs either. It appears that if the product dislocation is a real slip system, transmutation may occur during twin-slip interaction. Otherwise the matrix dislocation will be shredded by atomic shuffling and then absorbed by the twin boundary. If the core structure of the product dislocation is complex, transmutation may not occur as well and dislocation absorption will occur. Lattice correspondence in deformation twinning was applied in explaining the interaction mechanisms. Our results can be well correlated to macroscopic experimental observations which show twin-slip interaction only contributes negligibly to work hardening in deformation of hcp metals.Graphical abstractImage 1
  • Flux effects in precipitation under irradiation – Simulation of
           Fe-Cr alloys
    • Abstract: Publication date: Available online 3 November 2018Source: Acta MaterialiaAuthor(s): Jia-Hong Ke, Elaina R. Reese, Emmanuelle A. Marquis, G. Robert Odette, Dane Morgan Radiation-enhanced precipitation of Cr-rich α′ in irradiated Fe-Cr alloys, which results in hardening and embrittlement, depends on the irradiating particle and the displacement per atom (dpa) rate. Here, we utilize a Cahn-Hilliard phase-field based approach, that includes simple models for nucleation, irradiating particle and rate dependent radiation-enhanced diffusion and cascade mixing to simulate α′ evolution under neutrons, heavy ions, and electron irradiations. Different irradiating particles manifest very different cascade mixing efficiencies. The model was calibrated using neutron data. For cascade inducing neutron/heavy-ion dpa rates at 300 °C between 10−8 and 10−6 dpa/s the model predicts approximately constant number density, decreasing radius, decreasing α′ Cr composition, and lower α′ volume fraction. The model then predicts a dramatic transition to no α’ formation above approximately 10−5 dpa/s, while electron irradiation, with weak mixing, had little effect at dpa rates up to 10−3 dpa/s. These model predictions are consistent with experiments. We explain the results in terms of the flux dependence of the radiation enhanced diffusion, cascade mixing, and their ratio, which all vary significantly in relevant flux ranges for neutron and cascade inducing ion irradiations. These results show that both cascade mixing and radiation enhanced diffusion must be accounted for when attempting to emulate neutron-irradiation effects using accelerated ion irradiations.Graphical abstractImage 1
  • Dynamic Characterization of Shock Response in Crystalline-Metallic Glass
    • Abstract: Publication date: Available online 2 November 2018Source: Acta MaterialiaAuthor(s): K. Vijay Reddy, Chuang Deng, Snehanshu Pal The dynamic response of crystalline Cu-amorphous Cu63Zr37 nanolaminates under shock loading has been investigated in the present study by atomistic simulations to provide an insight of their overall deformation behavior with respect to different grain structure in the crystalline region. The dynamic characterization of the structural evolution of the nanolaminates during shock loading has been carried out based on various techniques including common neighbor analysis, dislocation analysis, Voronoi cluster analysis, pressure profile, and kinetic energy maps. Pressure profiles of single crystalline Cu-Cu63Zr37 metallic glass (SC/MG) nanolaminate at relatively low shock velocity show the presence of an elastic precursor in the crystalline region owing to the plane-plane collision phenomenon. Increasing the shock velocities in the SC/MG specimen results in FCC to BCC phase transition in the crystalline region. In particular, the crystalline/amorphous interface causes the generation of reflected rarefaction wave back into the crystalline region which aids inthe evolution and stabilization of the BCC phase. In the NC/MG specimen, the misalignment of planes across different grains reduces the intensity of elastic precursor at low shock velocity due to disruption in the plane-plane collision, whereas the grain boundaries act as nucleating region for the BCC phase during the high-velocity shock propagation. The coordination number of the Cu63Zr37 glass region has been found to increase during high-velocity shock loading which can be accounted by the formation of and indexed Voronoi polyhedra.Graphical abstractImage 1
  • Interstitial equiatomic CoCrFeMnNi high-entropy alloys: carbon content,
           microstructure, and compositional homogeneity effects on deformation
    • Abstract: Publication date: Available online 2 November 2018Source: Acta MaterialiaAuthor(s): Zhiming Li While interstitial alloying has been utilized to improve mechanical properties of multi-component high-entropy alloys (HEAs), its effectiveness depends on the interstitial content, microstructure and compositional homogeneity states. Here we present and discuss the influences of these factors on the mechanical behavior of interstitial equiatomic CoCrFeMnNi HEAs at room temperature. Interstitial HEAs containing carbon of 0.2, 0.5 and 0.8 at. % were processed to different compositional homogeneity states and grain sizes. We found that deformation of the various interstitial HEAs at early deformation stages is accommodated by dislocation slip whereas twinning occurs at the later stages. Upon an identical local strain at the later stages of deformation, nano-twin density decreases as the increase of carbon content due to the increased stacking fault energy. Also, the increase of C content leads to significantly higher energy barrier to recrystallization during annealing. Partially recrystallized (∼20 vol. %) interstitial HEA with C content of 0.8 at. % shows more than five times higher yield strength compared to the as-homogenized coarse-grained (∼200 μm) reference material, suggesting the significant beneficial effect of interstitials enabled microstructural adjustment on performance of the interstitial HEAs. Further, the compositionally inhomogeneous coarse-grained (∼200 μm) interstitial HEAs exhibit lower work-hardening ability and ultimate strength compared to the homogenized reference material due to that the compositional inhomogeneity promotes the localized plasticity. Some more insights for the design and processing of interstitial HEAs are generalized and discussed.Graphical abstractImage 1
  • The fabrication of graphene-reinforced Al-based nanocomposites using
           high-pressure torsion
    • Abstract: Publication date: Available online 1 November 2018Source: Acta MaterialiaAuthor(s): Yi Huang, Piotr Bazarnik, Diqing Wan, Dan Luo, Pedro Henrique R. Pereira, Malgorzata Lewandowska, Jin Yao, Brian E. Hayden, Terence G. Langdon Metal matrix nanocomposites were fabricated by high-pressure torsion (HPT) using 5% graphene nanoplates as a reinforcement contained within an Al matrix. Powders were mixed and compacted at room temperature and then processed by HPT at three different temperatures of 298, 373 and 473 K. After processing, microstructural observations were undertaken to reveal the distributions of graphene in the matrix, the grain refinement in the aluminium and the nature of the graphene-aluminium interfaces. Tests were performed to measure the microhardness, the tensile stress-strain curves and the electrical conductivity. The results show that processing by HPT is advantageous because it avoids the sintering and high temperature deformation associated with other processing routes.Graphical abstractImage 1
  • Strengthening mechanisms in Al-Ni-Sc alloys containing Al3Ni microfibers
           and Al3Sc nanoprecipitates
    • Abstract: Publication date: Available online 1 November 2018Source: Acta MaterialiaAuthor(s): C. Suwanpreecha, J. Perrin Toinin, R.A. Michi, P. Pandee, D.C. Dunand, C. Limmaneevichitr Dilute Al-Sc alloys (
  • Topology optimization of photonic crystals with exotic properties
           resulting from Dirac-like cones
    • Abstract: Publication date: Available online 1 November 2018Source: Acta MaterialiaAuthor(s): Yafeng Chen, Fei Meng, Guangyao Li, Xiaodong Huang The Dirac-like cones underlie many unique properties of photonic crystals (PhCs). This paper aims to design fabrication-friendly PhCs with Dirac-like cones for transverse magnetic (TM) modes and transverse electric (TE) modes at different specific frequencies. By maximizing the minimum of a collection of the local density of states corresponding to different judiciously selected sources, this paper demonstrates that Dirac-like cones formed by the degeneracy of a doubly degenerate mode and a single mode at different desired frequencies are successfully obtained. The exotic wave manipulation properties associated with Dirac-like cones, such as cloaking, wavefront shaping and tunneling through bent channels, are exhibited based on the optimized structures. This paper also demonstrates that the proposed method could be used for the design of PhCs, with one Dirac-like cone at ω, and one monopolar band at 2ω at the Γ point and PhCs with third order Dirac-like cones, which have potential application in nonlinear optics. All topological patterns of the optimized PhCs are reported and have regular and smooth features, meaning they can be readily fabricated.Graphical abstractImage 1
  • Mechanism for Zr poisoning of Al-Ti-B based grain refiners
    • Abstract: Publication date: Available online 1 November 2018Source: Acta MaterialiaAuthor(s): Y. Wang, C.M. Fang, L. Zhou, T. Hashimoto, X. Zhou, Q.M. Ramasse, Z. Fan Al-Ti-B based master alloys have been widely used for grain refining of Al-alloys in industry for many decades. However, the effectiveness of such grain refiners is severely compromised when a few hundred ppm of Zr is present in the Al melt, and this phenomenon is referred to as Zr poisoning in the literature. So far the exact mechanisms for Zr poisoning are not clear albeit significant research effort on the subject in the last few decades. In this work we investigated the mechanism for Zr poisoning through extensive examinations of the Al/TiB2 interface using the state-of-the-art electron microscopy and ab initio molecular dynamics simulations. We found that the presence of Zr in Al melts leads to (i) the dissolution of the Al3Ti 2-dimensional compound (2DC) formed on the (0 0 0 1) TiB2 surface during the grain refiner production process; and (ii) the formation of an atomic monolayer of Ti2Zr 2DC on the (0 0 0 1) TiB2 surface, which replaces the original Ti-terminated TiB2 basal surface. This monolayer of Ti2Zr not only has large lattice misfit (4.2%) with α-Al, but also is atomically rough, rendering the TiB2 particles impotent for heterogeneous nucleation of α-Al. This work, in combination of our previous work, demonstrates that heterogeneous nucleation can be effectively manipulated, either enhanced or impeded, by chemical segregation of selected alloying/impurity elements at the liquid/substrate interface.Graphical abstract(a, b) High resolution STEM HAADF images showing an atomic monolayer of Ti2Zr 2-dimensional compound (2DC) on (0 0 0 1) surface of TiB2 being viewed along (a) [1 1 -2 0]TiB2 and (b) [1 0 -1 0]TiB2 direction respectively, and (c) 3D construction of the Ti2Zr 2DC on top of TiB2.Image 1
  • Self-Assembled Porous Metal-Intermetallic Nanocomposites via Liquid Metal
    • Abstract: Publication date: Available online 31 October 2018Source: Acta MaterialiaAuthor(s): Bernard Gaskey, Ian McCue, Alyssa Chuang, Jonah Erlebacher A major challenge in the synthesis of high surface area metals via subtractive processes such as dealloying is maintaining the mechanical integrity of the resulting porous materials. This problem is especially apparent in liquid metal dealloying, in which high-temperature selective dissolution in a molten metal bath leads to bicontinuous porosity formation. In liquid metal dealloying of polycrystalline alloys, grain boundary separation leads to the detachment of individual grains.In this work, we show that addition of small amounts of silicon to Nb-Ti or Ta-Ti parent alloys leads to the generation of self-assembled arrays of intermetallic (niobium silicide or tantalum silicide) plates that are structurally merged with the usual bicontinuous porosity seen in dealloying. These silicide plates pass through grain boundaries and hold the niobium or tantalum network intact without strongly affecting the microstructural evolution during dealloying. Our approach yields a mechanically robust porous metal-intermetallic composite, which can be further processed to form tertiary materials via re-impregnation by a new third phase. The materials design strategy introduced here can be generalized to serve as a platform to form dense multiphase nanocomposites.Graphical abstractImage 1
  • Intragranular nucleation of tetrahedral precipitates and discontinuous
           precipitation in Cu-5wt%Ag
    • Abstract: Publication date: Available online 30 October 2018Source: Acta MaterialiaAuthor(s): M. Bonvalet, X. Sauvage, D. Blavette Both continuous and discontinuous precipitation is known to occur in CuAg alloys. The precipitation of Ag-rich phase has been experimentally investigated by atom probe tomography and transmission electron microscopy after ageing treatment of Cu-5%wtAg at 440°C during 30’. Both continuously and discontinuously formed precipitates have been observed. The precipitates located inside the grains exhibit two different faceted shapes: tetrahedral and platelet-shaped precipitates. Dislocations accommodating the high misfit at the interface between the two phases have also been evidenced. Based on these experimental observations, we examine the thermodynamic effect of these dislocations on the nucleation barrier and show that the peculiar shapes are due to the interfacial anisotropy. The appropriate number of misfit dislocations relaxes the elastic stress and lead to energetically favorable precipitates. However, due to the large misfit between the parent and precipitate phases, discontinuous precipitation that is often reported for CuAg alloys can be a lower energetic path to transform the supersaturated solid solution. We suggest that the presence of vacancy clusters may assist intragranular nucleation and decrease the continuous nucleation barrier. We eventually propose qualitative thermodynamic and kinetic justifications accounting for the relative importance of homogeneous and discontinuous precipitation modes as a function of temperature.Graphical abstractImage 1
  • Tailoring electronic and thermal transport properties of
           CaO(CaMnO3)m-based (m=1 and m=∞) composites for thermoelectric power
    • Abstract: Publication date: Available online 30 October 2018Source: Acta MaterialiaAuthor(s): Amram Azulay, Yaron Amouyal Oxide thermoelectric (TE) materials are promising for waste heat recovery at high temperatures thanks to their good chemical stability at elevated temperatures and low cost. We study Nb-doped n-type TE oxides of the CaO(CaMnO3)m-series. The CaMnO3 (m=∞) and Ca2MnO4 (m=1) derivatives feature extremely opposite transport coefficients, where the m=∞ structure exhibits high electrical and thermal conductivity, and the m=1 one exhibits the opposite combination. We synthesize composite materials based on these two phases of different ratios to draw correlations between the TE properties, microstructure evolution, and composition of the material. We determine the optimum sintering temperature to be 1373 K, and measure both thermal and electronic transport coefficients, then perform a thorough general effective medium (GEM) analysis. Interestingly, we find that most ratios obey to the GEM behavior, where deviations are elucidated in terms of interfacial effects. This study provides us with tools for identifying the significance of bulk vs. interfacial effects in design of composite materials with controllable transport properties.Graphical abstractImage 1
  • Switchable polar spirals in tricolor oxide superlattices
    • Abstract: Publication date: Available online 30 October 2018Source: Acta MaterialiaAuthor(s): Zijian Hong, Long-Qing ChenABSTRACTThere are increasing evidences that ferroelectric states at the nanoscale can exhibit fascinating topological structures including polar vortices and skyrmions, akin to those observed in the ferromagnetic systems. Here we report the discovery of a new type of polar topological structure, an ordered array of nanoscale spirals, in the PbTiO3/BiFeO3/SrTiO3 tricolor ferroelectric superlattice system obtained via phase-field simulations. This polar spiral structure is composed of fine ordered semi-vortex arrays with vortex cores forming a wavy distribution. It is demonstrated that the tricolor system has an ultrahigh Curie temperature of ∼1000 K and a temperature of ∼800 K for the phase transformation from spiral structure to in-plane orthorhombic domain structure, demonstrating a great thermal stability. The periodicity phase-diagram is constructed, showing multiple phases from inplane domain, polar spiral, and polar vortex to flux-closure with increasing ferroelectric layer thickness. Moreover, the spiral structure has a net in-plane polarization that could be switched by an experimentally-feasible irrotational in-plane field. The switching process involves a metastable vortex state and is fully reversible. This discovery could open up a new routine to design novel multiferroic topological structures with enhanced stability and tunability towards with potential future applications in next-generation electronics.Graphical abstractImage 1
  • Intermediate Crystallization Kinetics in Germanium-Tellurides
    • Abstract: Publication date: Available online 30 October 2018Source: Acta MaterialiaAuthor(s): Yimin Chen, Hongbo Pan, Sen Mu, Guoxiang Wang, Rongping Wang, Xiang Shen, Junqiang Wang, Shixun Dai, Tiefeng Xu Germanium-Telluride has been widely studied as a phase-change material due to its fast crystallization speed. The understanding of the crystallization kinetics is important to evaluate the potential applications of the material, but this is limited by the conventional calorimetry with low heating rate and narrow temperature range. We here employed an ultrafast calorimetry method, named flash differential scanning calorimetry, to investigate the crystallization kinetics of GexTe100-x in a wide compositional range (15 ≤ x ≤ 55). By means of the X-ray diffraction, we found the complicated competition between crystalline GeTe and Te (or Ge) phases in these binary alloys. The crystallization kinetics of first crystalline phase were estimated and it was found that, GexTe100-x generally has intermediate crystal growth speed and fragility, which is ascribed to the border between covalent and metallic properties. Component dependences of maximum crystal growth rate (Umax) and fragility were investigated, revealing the component in x = 20.4 has the lowest Umax of 1.22 × 10-3 m s-1 with the smallest fragility of 42.2, and the component in x ≈ 50 possesses the largest Umax of 3 m s-1. It confirms that, GeTe is the most suitable phase-change material for information storage and GeTe4 is the best media for information transparency in Ge-Te binary. Moreover, a tri-counter pattern was carried out for obtaining the crystal growth rate directly in studied supercooled GexTe100-x liquids (15 ≤ x ≤ 55). In addition, we first found a peculiar component Ge22Te78 with terrible thermal properties, i.e., phase separation, low crystallization temperature, ultrahigh fragility and anomalous crystallization kinetics. More importantly, together with the crystallization kinetics parameters of other glass formers, it was found a specific relation between reduced glass temperature (Trg) and Umax for which can be benefit to simplify material screenings and performance optimizations.Graphical abstractImage 1
  • In-situ Irradiation Tolerance Investigation of High Strength Ultrafine
           Tungsten-Titanium Carbide Alloy
    • Abstract: Publication date: Available online 29 October 2018Source: Acta MaterialiaAuthor(s): O. El-Atwani, W.S. Cunningham, E. Esquivel, M. Li, J.R. Trelewicz, B.P. Uberuaga, S.A. Maloy Refining grain size and adding alloying elements are two complementary approaches for enhancing the radiation tolerance of existing nuclear materials. Here, we present detailed in-situ irradiation research on defect evolution behavior and irradiation tolerance of ultrafine W-TiC alloys (thin foils) irradiated with 1 MeV Kr+2 at RT and 1073 K, and compare their overall performance to pure coarse grained tungsten. Loop Burgers vector was studied confirming the presence of loops whose population increased at high temperature. Loop density, average loop area, and overall damage are reported as a function of irradiation dose revealing distinct defect evolution behavior from pure materials. The overall damage generally followed the average loop size trend, which decreased with time for both temperatures, but was higher at 1073 K and attributed to biased vacancy sink behavior of the TiC dispersoids evidenced by large vacancy clusters on their interfaces. By comparison, the overall loop and void damage in pure tungsten was larger by a factor of six and two, respectively. The improved irradiation damage resistance in the alloys is thus attributed to the effect of dispersoids in 1) the enhancement in annihilating defects and mutual defect recombination due to both dispersoids and a higher grain boundary density; 2) decreasing the loop mobility, causing shrinkage and annihilation of loop density, which was confirmed via in-situ video. Several mechanisms are illustrated to describe the performance of the complex alloy system. The results motivate further experimental and modeling research that aims to understand the many different phenomena occurring at different time scales.Graphical abstractImage 1Damage evolution behavior for pure W and ultrafine W-TiC alloy irradiated with 1 MeV Kr+2 at 1073 K.
  • Potential benefit of amorphization in the retention of gaseous species in
           irradiated pyrochlores
    • Abstract: Publication date: Available online 29 October 2018Source: Acta MaterialiaAuthor(s): Terry G. Holesinger, James A. Valdez, Matthew T. Janish, Yongqiang Wang, Blas P. Uberuaga Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with B=Ti amorphize much more readily than B=Zr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr++ ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.Graphical abstractImage 1
  • Experimental and DFT characterization of η′ nano-phase and its
           interfaces in Al-Zn-Mg-Cu alloys
    • Abstract: Publication date: Available online 29 October 2018Source: Acta MaterialiaAuthor(s): Fuhua Cao, Jingxu Zheng, Yong Jiang, Bin Chen, Yiren Wang, Tao Hu The structures and energetics of η′ nano-phase and its interfaces in a peak-aged Al-Zn-Mg-Cu alloy were thoroughly investigated, using the combination of aberration corrected HAADF-STEM imaging and first-principles calculations. The most feasible atomic structure of η′, along with the solute substitution in η′, were calculated and compared with the atom resolution Z-contrast images. The interface phase diagram of η′/Al was constructed as a function of the excess chemical potential of Zn, to determine the equilibrium interface structures. Solute segregation to these interfaces was further calculated, and the results were adopted to interpret the experimental Z-contrast images. Finally, all the bulk and interface results were integrated to predict the solute partition in the matrix and further its potential impacts on interface properties, and based on which, a new strategy was proposed for future optimal design of Al-Zn-Mg-Cu alloys.Graphical abstractImage 1
  • Phase coexistence near the polymorphic phase boundary
    • Abstract: Publication date: Available online 27 October 2018Source: Acta MaterialiaAuthor(s): Oscar A. Torres Matheus, R. Edwin García, Catherine M. Bishop A novel multiphase field theory for ferroelectric systems in the vicinity of a polymorphic phase boundary (PPB) is developed by coupling the Landau-Devonshire thermodynamic potentials of the individual phases. The model naturally predicts metastable coexistence of the rhombohedral (R) and tetragonal (T) phases near the PPB temperature, TPPB=43C∘, for the BZT-40BCT system, and provides a maximum temperature of coexistence, TC,0=49.9C∘, in agreement with experiments. For T>TPPB, results show that metastable coexistence of two ferroelectric phases is a result of a phase transformation-induced polarization rotation plus switching mechanism. Metastable domains of the low-temperature R phase coexist with the high-temperature, thermodynamically stable T phase for long periods of time, from minutes to hours. For T
  • Novel insight into the formation of α″-martensite and ω-phase with
           cluster structure in metastable Ti-Mo alloys
    • Abstract: Publication date: Available online 26 October 2018Source: Acta MaterialiaAuthor(s): Mingjia Li, Xiaohua Min, Kai Yao, Fei Ye On the basis of the “-Mo-Ti-Mo-” linear unit along the specific β, β, and β directions, the cluster structures of α″-martensite and ω-phase were constructed in metastable Ti-Mo alloys to examine phase stability, elastic property, and crystal structure evolution by first-principles calculations combined with experimental analyses. With the increase in Mo content, the orthorhombicity and shuffle magnitude of {110}β plane along β direction decreased, leading to change in the crystal structure of martensite from hexagonal close-packed to orthorhombic structure; the displacive collapse degree of {112}β plane along β direction decreased, indicating that the crystal structure of ω-phase transited from hexagonal to trigonal structure. The softening effect of tetragonal shear elastic constant (C′) and Young’s modulus (E100) was favorable for the shuffle and shear components of α″-martensite, respectively, whereas that of shear modulus (G111) was beneficial to the collapse component of ω-phase. The competition among C′, E100, and G111 affected the phase transformation following the sequence of hexagonal close-packed α′-martensite, orthorhombic α″-martensite, hexagonal ω-phase, and trigonal ω-phase in metastable titanium alloys.Graphical abstractImage 1
  • Attractive-domain-wall-pinning controlled Sm-Co magnets overcome the
           coercivity-remanence trade-off
    • Abstract: Publication date: Available online 26 October 2018Source: Acta MaterialiaAuthor(s): Hansheng Chen, Yunqiao Wang, Yin Yao, Jiangtao Qu, Fan Yun, Yuqing Li, Simon P. Ringer, Ming Yue, Rongkun Zheng Traditional approaches for increasing the intrinsic coercivity of magnets typically come at the expense of remanence, a dilemma known as intrinsic coercivity-remanence trade-off, leading to a substantial reduction of maximum energy product. New metallurgical processing might offer the possibility of overcoming this trade-off. Here, we achieve a combination of an intrinsic coercivity of 26.9 kOe, a remanence of 11.2 kG, and a maximum energy product up to 26.6 MGOe, which surpasses most of conventional Sm-Co based permanent magnets, by manipulating the gradient of domain wall energy landscape of constituent phases to realize the attractive domain wall pinning in Sm(Co,Fe,Cu,Zr)z permanent magnets. Using powerful atomic-scale analysis technique known as atom probe tomography and micromagnetic simulations, we reveal that an enlarged attractive domain wall pinning strength results in the substantial coercivity enhancement with little sacrifice of remanence and maximum energy product in the Cu-particle-alloyed magnet. These results provide atomic-level insights into the coercivity mechanism of rare earth permanent magnets, with the methodology offering exciting possibilities for quantitative analyses and prediction between compositions and magnetic properties of other magnetic materials.Graphical abstractImage 1
  • Permeability prediction for flow normal to columnar solidification
           structures by large–scale simulations of phase–field and lattice
           Boltzmann methods
    • Abstract: Publication date: Available online 26 October 2018Source: Acta MaterialiaAuthor(s): Tomohiro Takaki, Shinji Sakane, Munekazu Ohno, Yasushi Shibuta, Takayuki Aoki Computer simulation is the most promising approach for the systematical permeability prediction of liquid flow in the mushy zone for various solidification conditions. In this study, we propose a permeability prediction method by using large–scale simulation of phase–field and lattice Boltzmann methods. Using the proposed method, permeability predictions are performed for the flow normal to the columnar solidification structures with multiple dendrites/cells. In addition, the prediction is also performed for columnar solidification structures with periodically arranged dendrites/cells to evaluate the permeability in the full range of solidification fraction and investigate the effect of primary arm array on the permeability. As a result, it is concluded that the dimensionless permeability using a specific interface area of columnar solidification structures can be well approximated by that of a regular hexagonal array of cylinders. Moreover, it is confirmed that the columnar dendrite/cell structure with a periodic regular hexagonal array provides realistic permeability predictions for columnar solidification structures.Graphical abstractImage 1
  • Large-scale dislocation dynamics simulations of strain hardening of Ni
           microcrystals under tensile loading
    • Abstract: Publication date: Available online 26 October 2018Source: Acta MaterialiaAuthor(s): S.I. Rao, C. Woodward, B. Akdim, E. Antillon, T.A. Parthasarathy, J. El-Awady, D.M. Dimiduk The strain hardening in FCC Ni was studied along low index directions using 3-dimensional discrete dislocation dynamics. Large (20 x 20 x 50 μm) Ni microcrystals were simulated using rectangular parallelepiped-cells loaded in tension along four low-index directions ([111], [001], [110] and [112]) to shear strains of ∼0.01 – 0.02. Loading was at a constant strain rate of 10/sec, and all surfaces of the cell are treated as free surfaces. The FCC dislocation mobility routines were modified to include thermally activated cross-slip processes, as a function of three different stress components, using the results of previous atomistic simulations. These include bulk cross slip (cross-slip at atomic jogs), intersection cross slip (attractive and repulsive) as well as surface cross slip. One of these, repulsive intersection cross-slip, has zero activation energy and is present at all simulated deformation temperatures. The simulations were performed for three different temperatures, 5, 150 and 300 K. The strain-hardening rate is independent of temperature and of the order of μ/200 – μ/400, in agreement with experimental data for the and orientations of deformation. For the [001] orientation, at 5 and 150K, the strain hardening rate decreases considerably when repulsive intersection cross-slip is removed from the simulations. The [110] and [112] orientations exhibit single-slip glide and a very low strain-hardening rate (∼μ/3000). Heterogeneity of dislocation microstructure develops spontaneously at the higher temperatures as a result of increased cross slip. Even though the strain-hardening rate is independent of temperature, the increase in dislocation density with shear strain is larger at higher temperatures. It is proposed that higher temperatures deformation produces larger dislocation-microstructure heterogeneities, providing for higher average dislocation densities and regions of low density where deformation can proceed. Also, the strain hardening rate at 300K is controlled by the rate of increase of forest dislocation density in these lean regions.Graphical abstractThe dislocation microstructure at the end of the simulations of 20 x 20 x 50 μm Ni microcrystals deformed along the (a) [001]; (b) [111]; and (c) [110] directions. The simulations were performed at 300K. Dislocations are colored according to their slip system. Associated strain hardening rates for the three orientations are also shown.Image 1
  • Quantification of irradiation-induced defects IN UO2 using Raman and
           positron annihilation spectroscopies
    • Abstract: Publication date: Available online 25 October 2018Source: Acta MaterialiaAuthor(s): R. Mohun, L. Desgranges, C. Jégou, B. Boizot, O. Cavani, A. Canizarès, F. Duval, C. He, P. Desgardin, M.-F. Barthe, P. Simon “In UO2, Raman spectroscopy has recently put into evidence the existence of a specific signature, referred to as the triplet defect bands, which is characteristic to irradiation damages. In this work, we perform a detailed experimental analysis to investigate how this Raman signature can be used to characterize irradiated nuclear fuels. For this purpose, an electron irradiation experiment of sintered UO2 disks coupled with ex situ Raman and positron annihilation spectroscopy measurements were carried out. The obtained findings showed that the Raman defect bands take their origin from the ballistic collisions of the incident electrons with the U and O atoms and are due to the formation of point defects. These defects induce the re-arrangement of UO2 lattice atoms giving rise to domains with symmetry lower than Fm-3m with the loss of one or more symmetry elements, such as translational symmetry, centering F, mirror or rotational symmetry operations”.Graphical abstractImage 1
  • In situ and atomic-scale investigations of the early stages of γ
           precipitate growth in a supersaturated intermetallic Ti-44Al-7Mo (at.%)
           solid solution
    • Abstract: Publication date: Available online 24 October 2018Source: Acta MaterialiaAuthor(s): Petra Erdely, Peter Staron, Andreas Stark, Thomas Klein, Helmut Clemens, Svea Mayer Intermetallic β-stabilised γ-TiAl based alloys offer novel opportunities for microstructural design. This paper investigates the growth behaviour of γ precipitates from a supersaturated βo matrix in a β-homogenised Ti-44Al-7Mo (at.%) alloy. Combining in situ high-energy X-ray diffraction and small-angle scattering at a synchrotron radiation source with atom probe tomography as a direct imaging technique, the early stages of γ precipitate growth are characterised for the first time. The results show that the βo → γ phase transformation occurs without the formation of an intermediate phase. At a heating rate of 10 K·min-1, first diffusional processes that can be ascribed to the βo → γ phase transformation commence at about 450 °C. Elemental redistribution controls the growth of the γ precipitates, which is connected with the introduction of misfit-induced strain fields around the initially coherent γ particles. Further heating results in the loss of coherency between the disc-shaped γ precipitates and the βo matrix. The presented findings advance the fundamental understanding of the βo → γ phase transformation in γ-TiAl based alloys and provide quantitative data for the design of refined microstructures in the course of technological heat treatments.Graphical abstractImage 1
  • Crucial microstructural feature to determine the impact toughness of
           intercritically annealed medium-Mn steel with triplex-phase microstructure
    • Abstract: Publication date: Available online 24 October 2018Source: Acta MaterialiaAuthor(s): Min Tae Kim, Tak Min Park, Kyeong-Ho Baik, Won Seok Choi, Pyuck-Pa Choi, Jeongho Han We investigated the correlation between the impact toughness and microstructures of annealed Fe-8Mn-0.2C-3Al-1.3Si (wt.%) steel to identify the key microstructural feature determining the impact toughness of medium-Mn steel. The microstructural constituents were varied by changing the hot-rolling temperature in the range of 1000–1200 °C before intercritical annealing. The annealed steels exhibited a triplex-phase microstructure consisting of δ ferrite with coarse grains and an elongated structure along the rolling and transverse directions and nanolaminate α martensite plus γR retained austenite with ultrafine size. While the volume fraction of γR remained almost constant regardless of the hot-rolling temperature, the volume fraction of δ increased and that of α decreased with increase in the hot-rolling temperature. The average grain size for all phases increased with the hot-rolling temperature. The stability of γR decreased with the increase of the hot-rolling temperature owing to grain coarsening and a reduction in the Mn and C concentrations. A lower hot-rolling temperature resulted in improved impact toughness. We observed that deep parallel cracks formed and propagated along the δ interface decorated with Mn, ultimately causing a fracture. This result indicates that δ ferrite was the crucial factor determining the toughness among the existing phases, and the steels with a higher fraction of δ exhibited a lower impact toughness. The decrease of the retained austenite stability and the increase of the size of prior γ grains with increasing hot-rolling temperature were identified as other microstructural factors determining the impact toughness.Graphical abstractImage 1
  • Smooth-shell metamaterials of cubic symmetry: Anisotropic elasticity,
           yield strength and specific energy absorption
    • Abstract: Publication date: Available online 23 October 2018Source: Acta MaterialiaAuthor(s): Colin Bonatti, Dirk Mohr Shell-lattices consist of a single, periodic, non-intersecting shell of uniform wall thickness that separates two intertwined void phases. To obtain a comprehensive overview on their small and large strain response, three families of shell-lattices are derived from Simple-Cubic (SC), Face-Centered Cubic (FCC) and Body-Centered Cubic (BCC) tube-lattices using a parameterized surface-smoothening functional. Each family's central element is an approximation of a Triply Periodic Minimal Surface (TPMS). Detailed finite element simulations are carried out for more than 800 shell-lattices covering relative densities ranging from 1% to 80%. It is found that the TMPS-like structures exhibit highly anisotropic elastic and plastic properties that depend on the type of cubic symmetry. However, when averaging the mechanical properties over all possible directions of loading, the performance of the SC, FCC and BCC shell-lattices turns out to be similar, with all structures providing substantially higher stiffness and strength than optimal truss-lattices of equal mass. They also exhibit high specific energy absorption for large strain compression. It is found that the macroscopic deformation mode changes from foam-like crushing (for relative densities below 10%) to bulk-like positive strain hardening (for relative densities above 20%). The spectrum of anisotropic structures obtained through varying the bias parameter of the surface-defining functional also includes elastically-isotropic shell-lattices. The Young's modulus of the isotropic shell-lattices of FCC and BCC symmetry is slightly higher than the average modulus of their TPMS-like counterparts, while the opposite holds true for SC structures. Compression experiments are performed on additively-manufactured stainless steel 316L specimens to validate the conclusions drawn from numerical simulations.Graphical abstractImage 1
  • Deformation-induced phase transformation in a Co-Cr-W-Mo alloy studied by
           high-energy X-ray diffraction during in-situ compression tests
    • Abstract: Publication date: Available online 23 October 2018Source: Acta MaterialiaAuthor(s): Irmgard Weißensteiner, Manuel Petersmann, Petra Erdely, Andreas Stark, Thomas Antretter, Helmut Clemens, Verena Maier-Kiener Nickel-free Co-Cr-W-Mo alloys exhibit a very low or even negative stacking fault energy, and therefore a pronounced tendency towards a deformation-induced phase transformation of the metastable face centered cubic (fcc) γ-phase to the hexagonal close-packed (hcp) low-temperature ε-phase. In order to analyze the phase transformation in-situ and to correlate it to an external strain, compression tests between 30 °C and 400 °C were performed in a deformation dilatometer simultaneously to high-energy X-ray diffraction. Hence, the elastic strains of the fcc unit cell during compression, the external loads for the onset of the phase transformation and the temperature-dependency could be determined. In the parent fcc γ-phase, the evolution of an 101 fiber texture as well as texture inheritance effects and a distinct variant selection could be observed. Further, for the investigated alloy composition it is demonstrated that the continuum concepts of i) a structural stretch tensor and ii) an invariant plane strain perfectly agree with the widely-accepted nucleation theory of ε-martensite formation in Co-Cr alloys via Shockley partial dislocations on every second {111}γ plane. Both, the observed transformation texture as well as crystallographic transformation strains reveal the importance of shear stresses in this system.Graphical abstractImage 1
  • Hot deformation behaviour of Mo-TZM and understanding the restoration
           processes involved
    • Abstract: Publication date: Available online 23 October 2018Source: Acta MaterialiaAuthor(s): Atanu Chaudhuri, A.N. Behera, A. Sarkar, R. Kapoor, R.K. Ray, Satyam Suwas Hot deformation behaviour of Mo-TZM alloy over a temperature range of 1400 to 1700 °C and strain rate range of 0.001 to 10.0 s-1 was investigated. The microstructure after deformation was characterized at each deformation condition using electron back scatter diffraction technique. The high strain rate sensitivity domain was found to be in the strain rate range of 10-2 - 10-3 s-1 and in temperature range of 1480-1650 °C. The flow stress behaviour of the material indicated dynamic recovery as well as recrystallization of the material during deformation. Microstructural investigation confirmed the occurrence of continuous dynamic recrystallization from 1400 to 1500 °C. At higher temperature (1600 to 1700 °C) and low strain rates (10-2 - 10-3 s-1) grain growth was dominant. At high strain rates (0.1 to 10 s-1) and high temperature (1600 to 1700 °C) dynamic recrystallization was not observed. Based on the experimental observations a schematic model of the microstructure evolution of TZM during deformation at high temperatures was proposed.Graphical abstractImage 1
  • Microscopic characterization of structural relaxation and cryogenic
           rejuvenation in metallic glasses
    • Abstract: Publication date: Available online 23 October 2018Source: Acta MaterialiaAuthor(s): T.J. Lei, L. Rangel DaCosta, M. Liu, W.H. Wang, Y.H. Sun, A.L. Greer, M. Atzmon Plasticity improvement in metallic glasses resulting from cycling treatment between room and liquid nitrogen temperature has been reported and attributed to rejuvenation due to non-uniform thermal expansion coefficient. However, the detailed microscopic effect is still unclear. The present study focuses on the microscopic effect of room-temperature ageing and cryogenic cycling. La70Cu15Al15 and La70Ni15Al15 metallic glasses were subjected to varying ageing times, after which some were also cryogenically cycled. Quasi-static anelastic relaxation measurements were then employed to characterize the time-constant spectra over seven orders of magnitude. The overall anelastic strain decreases with increasing ageing time, but is not noticeably affected by cryogenic cycling. On the other hand, while ageing also causes an increase in characteristic time constants, cycling reverses this effect. The new details shed light on the effect of structural relaxation and rejuvenation on the properties of shear transformation zones.Graphical abstractImage 1
  • Helium irradiated cavity formation and defect energetics in Ni-based
           binary single-phase concentrated solid solution alloys
    • Abstract: Publication date: Available online 23 October 2018Source: Acta MaterialiaAuthor(s): Zhe Fan, Shijun Zhao, Ke Jin, Di Chen, Yury N. Osetskiy, Yongqiang Wang, Hongbin Bei, Karren L. More, Yanwen Zhang Binary single-phase concentrated solid solution alloys (SP-CSAs), including Ni80Co20, Ni80Fe20, Ni80Cr20, Ni80Pd20, and Ni80Mn20 (in atomic percentage), were irradiated with 200 keV He+ ions at 500 oC. He cavity size and density distribution were systematically investigated using transmission electron microscope. Here we show that alloying elements have a clear impact on He cavity formation. Cavity size is the smallest in Ni80Mn20 but the largest in Ni80Co20. Alloying elements could also substantially affect cavity density profile. In-depth examination of cavities at peak damage region (∼500 nm) and at low damage region (∼300 nm) demonstrates that cavity size is depth (damage) dependent. Competition between consumption and production of vacancies and He atoms could lead to varied cavity size. Density functional theory (DFT) calculations were performed to obtain the formation and migration energies of interstitials and vacancies. Combined experimental and simulation results show that smaller energy gap between interstitial and vacancy migration energies may lead to smaller cavity size and narrower size distribution observed in Ni80Mn20, comparing with Ni80Co20. The results of this study call attention to alloying effects of specific element on cavity formation and defect energetics in SP-CSAs, and could provide fundamental understanding to predict radiation effects in more complexed SP-CSAs, such as high entropy alloys.Graphical abstractImage 1
  • Interaction between Al and Atomic Layer Deposited (ALD) ZrN under
           High-Energy Heavy Ion Irradiation
    • Abstract: Publication date: Available online 22 October 2018Source: Acta MaterialiaAuthor(s): Sumit Bhattacharya, Xiang Liu, Yinbin Miao, Kun Mo, Zhi-Gang Mei, Laura Jamison, Walid Mohamed, Aaron Oaks, Ruqing Xu, Shaofei Zhu, James F. Stubbins, Abdellatif M. Yacout Uranium-molybdenum (U-Mo) particles dispersed in an aluminum matrix is the most promising candidate fuel to convert high-power research and test reactors in Europe from using high-enriched to using low-enriched fuel. However, chemical interaction between the U-Mo and the Al matrix leads to undesirable fuel behavior. Zirconium nitride (ZrN) is used as a diffusion barrier between the U-Mo fuel particles and the Al matrix. To understand the potential microstructural evolution of ZrN during irradiation, a high-energy heavy ion (84 MeV Xe) irradiation experiment was performed on atomic layer deposited (ALD) nanocrystalline ZrN deposited on an Al plate. A fluence of 1.86×1017 ions/cm2, or 90.3 dpa was reached during this experiment. Both analytic transmission electron microscopy (TEM) and synchrotron microbeam X-ray diffraction (μXRD) techniques were utilized to investigate the kinetics of radiation-induced grain growth of ZrN at various radiation doses based on the Williamson-Hall analyses. The grain growth kinetics can be described by a power law expression, Dn−D0n=Kφ, with n=5.1. The Al-ZrN interaction products (Al3Zr and AlN) created by radiation-induced ballistic mixing/radiation-enhanced diffusion and their corresponding formation mechanism were determined from electron diffraction and elemental composition analysis. These experimental results were confirmed by first principle thermodynamic density functional theory (DFT) calculations. The results from this ion irradiation study were also compared to in-pile irradiation data from physical vapor deposited (PVD) ZrN samples for a comprehensive evaluation of the interaction between Al and ZrN and its influence on diffusion barrier performance.
  • sssPassivation of a Corrosion Resistant High Entropy Alloy in
           Non-oxidizing Sulfate Solutions
    • Abstract: Publication date: Available online 22 October 2018Source: Acta MaterialiaAuthor(s): Kathleen F. Quiambao, Stephen J. McDonnell, Daniel K. Schreiber, Angela Y. Gerard, Keren M. Freedy, Pin Lu, James E. Saal, Gerald S. Frankel, John R. Scully The passivation behavior of a novel high entropy alloy (HEA) of composition 38Ni-21Cr-20Fe-13Ru-6Mo-2W at.% (33Ni-16Cr-17Fe-19Ru-9Mo-6W wt.%) was investigated in sulfate solutions of various pH levels. The HEA was compared to a commercially available Ni-Cr-Mo-W-Fe alloy, C-22. Experiments were conducted using in-situ electrochemical and ex-situ surface-sensitive methods to probe the growth and dissolution of the passive layer. The HEA exhibited excellent corrosion resistance in highly acidic and highly alkaline solutions, maintaining passivity through a broad range of potentials below the Cr transpassivity range. Extremely rapid and efficient oxide formation and lower quasi-steady passive current densities were observed during oxide growth in the passive range compared to commercial alloy C-22. Ex-situ characterization of the passive film by atom probe tomography and X-ray photoelectron spectroscopy provided insight into the oxide composition, thickness, and elemental valence states of potentiostatically-grown and air-formed oxides. All elements in the alloy were oxidized following potentiostatic oxide growth. Instead of distinct stoichiometric compounds (i.e. with integer cation ratios) and oxide phases, a non-stoichiometric oxide solid solution was observed with significant enrichment in Cr and Ru, as well as depletion of Fe and Ni. The behavior of Cr was modeled with a modified surface enrichment model. The oxide thickness was estimated to be 3-4 nm thick. The connection between the solid solution oxide, enrichment of Cr in the passive layer, the presence of Mo and W, and excellent passivity is discussed.Graphical abstractImage 1
  • In situ tension-tension strain path changes of cold-rolled Mg AZ31B
    • Abstract: Publication date: Available online 22 October 2018Source: Acta MaterialiaAuthor(s): K. Sofinowski, T. Panzner, M. Kubenova, J. Čapek, S. Van Petegem, H. Van Swygenhoven The mechanical behavior of cold-rolled Mg AZ31B is studied during in-plane multiaxial loading and tension-tension strain path changes performed on cruciform samples using in situ neutron diffraction and EBSD. The results are compared with uniaxial tension loading of dogbone-shaped samples measured with in situ neutron diffraction and acoustic emission. The activity of slip and twinning mechanisms and the active twin variants are discussed for the different strain paths. It is shown that initial strains of 4–5% cause a strengthened yield stress during reload for strain path change angles of 90 and 135°. The strengthening is primarily due to dislocation accumulation during the initial load impeding dislocation motion during the reload. The twinning observed during the prestrain activates complex multivariant secondary twinning which may also contribute to the strengthening in the reload.Graphical abstractImage 1
  • Concentration dependent properties lead to plastic ratcheting in thin
           island electrodes on substrate under cyclic charging and discharging
    • Abstract: Publication date: Available online 19 October 2018Source: Acta MaterialiaAuthor(s): Kai Guo, Wei Zhang, Brian W. Sheldon, Huajian Gao It is known that the mechanical properties of electrodes in lithium-ion batteries, such as modulus, yield stress, and interfacial strength, can depend strongly on lithium concentration. Here we show that a thin film island electrode with properties dependent on lithium concentration naturally undergoes plastic ratcheting with accumulative deformation and failure during cyclic charging and discharging. Some key predictions from numerical simulations are validated by galvanostatic tests. Strategies to avoid ratcheting include limiting the electrode size and/or selecting a balanced combination of concentration dependent materials properties.Graphical abstractImage 1
  • The influence of Laves phases on the room temperature tensile properties
           of Inconel 718 fabricated by powder feeding laser additive manufacturing
    • Abstract: Publication date: Available online 19 October 2018Source: Acta MaterialiaAuthor(s): Shang Sui, Hua Tan, Jing Chen, Chongliang Zhong, Zuo Li, Wei Fan, Andres Gasser, Weidong Huang In this paper, a powder feeding laser additive manufacturing technology has been used for fabricating the Inconel 718 super-alloy. Laves phases of different sizes and morphologies have been obtained by using three types of heat treatments. The influence of Laves phases on the room temperature tensile properties of laser additive manufactured Inconel 718 has been investigated. The results show that small and granular Laves phase can be gained after heat treatment at 1050 °C for 15 min (S-15 sample). When the holding time extends to 45 min, the morphologies of Laves phases basically remain unchanged while its volume fraction further decreases (S-45 sample). Nevertheless, irregular and long-striped Laves phases still exist in the samples only after direct aging heat treatment (DA sample). The room temperature tensile results reveal that the S-15 samples have better tensile strength and ductility than that of S-45 samples. Besides, the DA samples with irregular and long-striped Laves phases show the lowest tensile strength and ductility. Hence, a certain amount of small and granular Laves phases are presumably beneficial for the room temperature tensile properties of Inconel 718. Moreover, a model has been established to describe the fracture of the Laves phase. On the basis of the fracture model, the critical stress needed for the fracture of long-striped Laves phases is lower than that needed for the fracture of granular Laves phases. Therefore, the former generally suffered internal fracture while the latter often fail by interfacial decohesion. Through influencing the volume fraction, the size and the distribution of γ" phase, the effect of the Laves phases on the room temperature tensile property is achieved. Furthermore, a yield strength model has been developed to reveal this influence in terms of numbers. The yield strength increments caused by grains, solution elements and γ' phase are almost the same for the three kinds of samples. The differences of the yield strength are mainly caused by γ" phase. In addition, in terms of ductility, granular Laves phases are more favorable than long-striped Laves phases.Graphical abstractImage 1
  • γ ' / γ ' ' +Coprecipitates+in+Ni-Base+Superalloys&rft.title=Acta+Materialia&rft.issn=1359-6454&">Growth Behavior of γ ' / γ ' ' Coprecipitates in Ni-Base Superalloys
    • Abstract: Publication date: Available online 19 October 2018Source: Acta MaterialiaAuthor(s): Rongpei Shi, Donald P. McAllister, Ning Zhou, Andrew J. Detor, Richard DiDomizio, Michael J. Mills, Yunzhi Wang Precipitation of the γ'' phase on {100} facets of preceding γ' precipitates is found to prevent the latter from overaging upon slow cooling from solution treatment in Ni-base superalloys based on the composition of alloy 718. By computer simulation using a multi-phase-field model, we find that the growth of a coprecipitate involves several concurrent and closely coupled processes, including thickening and lengthening of γ'' shells, growth of the γ' core along the 001γ', 011γ' and 111γ' directions, hard impingement between γ' and γ'' precipitates, and soft impingement among γ'' precipitates of different variants. These processes at different stages of growth are analyzed systematically as a function of coprecipitate size and configuration, and the results show that the growth kinetics of the γ' core in a coprecipitate is controlled by the interplay among: (1) partial removal of supersaturated γ matrix surrounding the γ' core by coprecipitation of γ'' shells, (2) cooperative growth of γ' and γ'' in the coprecipitates and (3) atomic mobility of γ'-formers in the γ'' phase. To maximize the effect of coprecipitation on preventing γ' from overaging upon slow cooling, the alloy composition and heat treatment schedule should be optimized to minimize the size of γ' cores at which coprecipitation of γ'' shells occurs and to reduce diffusion of γ'-formers through γ''.Graphical abstractImage 1
  • Phase field simulations of martensitic transformation in pre-strained
           nanocomposite shape memory alloys
    • Abstract: Publication date: Available online 17 October 2018Source: Acta MaterialiaAuthor(s): Dong Wang, Qianglong Liang, Shuangshuang Zhao, Pengyang Zhao, Tianlong Zhang, Lishan Cui, Yunzhi Wang We show in this paper how strain engineering alters the fundamental characteristic of a martensitic transformation (MT) and gives it a new set of properties including large quasi-linear elastic strain response with nearly vanishing hysteresis and low elastic modulus. The work is motivated and inspired by a recent experimental study on elastic and inelastic (transformation) strain matching in a pre-strained nano-composite with Nb nanowires embedded in a NiTi shape memory alloy matrix. In particular, we demonstrate by computer simulation that dislocations at Nb/NiTi interfaces produced by the pre-straining are responsible for the unprecedented properties. Microstructural evolution captured in the simulations reveals that local stress fields associated with the dislocations regulate the nucleation and growth of martensite, turning the otherwise sharp, strong first-order transition into a continuous, high-order like transition. The simulations predict that the stress-strain hysteresis and modulus of the composite decrease with increasing amount of pre-strain, which agrees well with the experimental measurement. This study suggests a design strategy by introducing non-uniform stress fields for enhanced properties of shape memory alloys.Graphical abstractImage 1
  • Grain size dependent physical properties in lead-free multifunctional
           piezoceramics: a case study of NBT-xST system
    • Abstract: Publication date: Available online 17 October 2018Source: Acta MaterialiaAuthor(s): Xing Liu, Saidong Xue, Feifei Wang, Jiwei Zhai, Bo Shen Grain size effect is one of the most important issues to develop next-generation functional devices. In this work, we firstly provide a systematic investigation on the grain size dependent physical properties based on a flexible (Na0.5Bi0.5)TiO3-xSrTiO3 (NBT-xST) system with multifunctionality. The NBT-20ST, -26ST and -35ST with multiple phase boundaries/structures were chosen as the studied compositions. The densified ceramics with a series of grain sizes were successfully fabricated by normal and two-step sintering method. For NBT-20ST and -26ST compositions, the coarse grain size is more favorable for improving the direct (small-signal d33) and converse (large-signal d33∗) piezoelectricity. The critical grain size of NBT-20ST and -26ST compositions for improving d33 and d33∗ is both around 1 μm. Rayleigh analysis and local PFM mapping indicate that the high d33 in coarse-grained NBT-20ST samples originates from the increased extrinsic contribution and easier domain wall motion, while the large d33∗ in coarse-grained NBT-26ST samples stems from the polarization enhancement through a linear electrostrictive effect. For NBT-35ST composition, an improved energy storage performance with high recoverable energy density over 1 J/cm3 was achieved in a 0.56 μm-sized sample owing to a large fraction of low-polarizability grain boundary layer. This study opens up a new way for designing novel lead-free multifunctional piezoceramics with superior electromechanical properties.Graphical abstractImage 1
  • Phase-field simulation of solid state sintering
    • Abstract: Publication date: Available online 15 October 2018Source: Acta MaterialiaAuthor(s): Johannes Hötzer, Marco Seiz, Michael Kellner, Wolfgang Rheinheimer, Britta Nestler Manufacturing materials for high performance applications with tailored properties requires a deep knowledge about the sintering process and especially the underlying microstructure evolution. Due to the complex interplay of the material and process parameters as well as complex geometries it is challenging to predict the microstructure evolution during sintering with analytical models. A phase-field model based on the grand potential approach considering volume, surface and grain boundary diffusion is presented to describe the microstructural evolution during solid state sintering. To efficiently investigate realistic green bodies with multiple thousand particles in three dimensions, the model is implemented in a highly optimized manner in the massive parallel phase-field solver framework Pace3D. By comparing the neck growth rates and the particle approach in a two particle system for the different diffusion mechanisms a good agreement to analytic solutions is found. Based on a three dimensional green body of 24897 Al2O3-grains the densification is investigated with respect to the dominant diffusion mechanisms and compared with the analytic Coble model. Finally, the appearance of isolated pores in the microstructure is discussed.Graphical abstractImage 1
  • Micromechanical modeling of non-linear stress-strain behavior of
           polycrystalline microcracked materials under tension
    • Abstract: Publication date: Available online 13 October 2018Source: Acta MaterialiaAuthor(s): Giovanni Bruno, Mark Kachanov, Igor Sevostianov, Amit Shyam The stress-strain behavior of microcracked polycrystalline materials (such as ceramics or rocks) under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with stable microcrack propagation and “roughness” of intergranular cracks playing critical roles. Experiments involving complex loading histories were done on large- and medium grain size β-eucryptite ceramic. The model is shown to reproduce the basic features of the observed stress-strain curves.Graphical abstractThe stress-strain behavior of microcracked polycrystalline materials under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with “roughness” of intergranular cracks playing critical roles: in forward loading, roughness profiles of crack faces get mismatched when nonlinearity starts due to crack propagation (point 2); at unloading (point 4) the faces get “stuck” (their displacement at peak load C is locked).Image 1
  • Point defect structure of La-doped SrTiO3 ceramics with
           colossal permittivity
    • Abstract: Publication date: Available online 13 October 2018Source: Acta MaterialiaAuthor(s): Mengjie Qin, Feng Gao, Jakub Cizek, Shengjie Yang, Xiaoli Fan, Lili Zhao, Jie Xu, Gaogao Dong, Mike Reece, Haixue Yan Sr1-xLaxTiO3 (SLTO) ceramics with colossal permittivity were fabricated by conventional solid-state reaction method. The point defects of pure STO and SLTO ceramics were analyzed by Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening (CDB). The charge compensation mechanisms and dielectric properties of ceramics were investigated. The results indicated that the intrinsic defects in pure STO ceramics were mainly VTi″″. The charge compensation mechanism of SLTO ceramics was predominantly formation of VSr″. With increasing La content, εr of SLTO ceramics increased up to ∼70000 at room temperature. The results of first-principle calculations indicated that the colossal permittivity came from a sharp polarization increase caused by dipole structure of defects. tanδ of SLTO ceramics showed obvious Debye relaxation at high temperatures and the relaxation showed a multiple relaxation times derived from different kinds of polarization mechanism. The main polarization mechanism of SLTO ceramics gradually changed from ion displacement polarization to defect dipole polarization influenced by the concentration of La dopants.Graphical abstractImage 1
  • Effects of the stacking fault energy fluctuations on the strengthening of
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Yifei Zeng, Xiaorong Cai, Marisol Koslowski In alloys and high entropy alloys the stacking fault energy varies with the local composition. The effects of these energy fluctuations on the strengthening are studied using dislocation dynamics simulations that track the evolution of partial dislocations in FCC metals at zero temperature. Different values of the intrinsic stacking fault energy are assigned to regions with size in the range of 0.5 nm–12 nm in the slip plane, while the other mechanical properties are left constant. A theoretical model is derived and compared to the simulation results. In the model and the simulations the predicted value of the yield stress grows with larger fluctuations of the stacking fault energy. Furthermore, a strong size dependency is observed, with a maximum in the strength attained when the mean region size approaches the average equilibrium stacking fault width. In summary, the strength of high entropy alloys can be improved by introducing disorder in the chemical misfit with a characteristic length scale of the order of the average stacking fault width.Graphical abstractImage 1
  • Dislocation evolution at a crack-tip in a hexagonal close packed metal
           under plane-stress conditions
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Zhouyao Wang, Christopher Cochrane, Travis Skippon, Qingshan Dong, Mark R. Daymond Understanding the stress state and microstructural features at a growing crack-tip is critical to understanding the failure mechanisms of engineering structures. To investigate the strain and dislocation evolution at a crack-tip, electron backscatter diffraction and geometrically necessary dislocation analysis were performed on fully annealed zirconium foils at room temperature. Different levels of macroscopic plastic strain were applied: 0.0%, 0.22%, 0.84%, 1.2%. Based on their different Burgers vectors and line vectors, prismatic , basal , screw , screw and pyramidal geometrically necessary dislocation densities were estimated during crack blunting and subsequent propagation. Most of the plastic deformation was accommodated by screw and pyramidal dislocations. Screw dislocations were found to be dominant over the as might be expected. Instead of twinning, pyramidal slip accommodated the strain along the c-axis caused by contraction at the crack-tip. Dislocation densities at the crack-tip were plotted according to the angle relative to the applied tension direction and the distance from the tip, and were compared with plastic strains simulated from a 3D static finite element model. Crack-tip singularity was observed and total geometrically necessary dislocation densities were in qualitatively good agreement with the equivalent plastic strain distribution predicted by the finite element method (FEM).Graphical abstractImage 1
  • The influence of stacking fault energy on plasticity mechanisms in
           triode-plasma nitrided austenitic stainless steels: implications for the
           structure and stability of nitrogen-expanded austenite
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Xiao Tao, Xingguang Liu, Allan Matthews, Adrian Leyland Austenitic stainless steels (ASSs), especially AISI type 304 and 316 ASSs, have been extensively studied after thermochemical diffusion treatments (e.g. nitriding, carburising) to resolve the anomalous lattice expansion after supersaturation of interstitial elements under paraequilibium conditions. The known issues are i) plastic deformation of surfaces under nitrogen-introduced strain at low treatment temperatures and ii) degradation in surface corrosion performance in association with chromium nitride formation at elevated treatment temperatures (and/or longer treatment times). In this study, a nitrogen-containing high-manganese ASS and a high-nickel ASS (i.e. Fe-17Cr-20Mn-0.5N and Fe-19Cr-35Ni, in wt.%) were triode-plasma nitrided under a high nitrogen gas volume fraction and low (and close to monoenergetic) ion energy of ∼200 eV at 400°C, 425°C and 450°C for 4hrs and 20hrs, respectively. Auxiliary radiant heating was used to facilitate different treatment temperatures at a deliberately controlled and constant substrate current density of ∼0.13 mA/cm2, under which material surface crystallographic structure was mainly influenced by the different treatment temperatures and times applied during nitriding. With respect to stacking fault energy (SFE), we illustrate and discuss i) the analogy of composition-induced plastic deformation phenomena to mechanical deformation processes, ii) two possible types of dislocation-mediated plasticity mechanism in γN, iii) two possible types of diffusional decomposition mechanism for γN, and iv) the lattice structures formed at low to moderate nitriding temperatures.Graphical abstractImage 1
  • Characterization of (Ti,Mo,Cr)C nanoprecipitates in an austenitic
           stainless steel on the atomic scale
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): N. Cautaerts, R. Delville, E. Stergar, D. Schryvers, M. Verwerft Nanometer sized (Ti,Mo,Cr)C (MX-type) precipitates that grew in a 24% cold worked Ti-stabilized austenitic stainless steel (grade DIN 1.4970, member of the 15-15Ti alloys) after heat treatment were fully characterized with transmission electron microscopy (TEM), probe corrected high angle annular dark field scanning transmission electron microscopy (HR-HAADF STEM), and atom probe tomography (APT). The precipitates shared the cube-on-cube orientation with the matrix and were facetted on {111} planes, yielding octahedral and elongated octahedral shapes. The misfit dislocations were believed to have burgers vectors a/6 which was verified by geometrical phase analysis (GPA) strain mapping of a matrix-precipitate interface. The dislocations were spaced five to seven atomic planes apart, on average slightly wider than expected for the lattice parameters of steel and TiC. Quantitative atom probe tomography analysis of the precipitates showed that precipitates were significantly enriched in Mo, Cr and V, and that they were hypostoichiometric with respect to C. These findings were consistent with a reduced lattice parameter. The precipitates were found primarily on Shockley partial dislocations originating from the original perfect dislocation network. These novel findings could contribute to the understanding of how TiC nanoprecipitates interact with point defects and matrix dislocations. This is essential for the application of these Ti-stabilized steels in high temperature environments or nuclear fast reactors.Graphical abstractImage 1
  • Elucidation of Cold-Spray Deposition Mechanism by Auger Electron
           Spectroscopic Evaluation of Bonding Interface Oxide Film
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Yuji Ichikawa, Ryotaro Tokoro, Masatoshi Tanno, Kazuhiro Ogawa The relationship between the cold spray deposition mechanism, microstructure, and strength of the resulting film must be understood for this innovative process to be practical. Previous studies have suggested that the coating mechanism is reliant on breaking the natural oxide film such that metallic bonding occurs through direct contact between the metal surfaces. In this study, the proposed model was experimentally verified by a small tensile adhesion test and auger electron spectroscopy analysis of the bonding interface. Since shear deformation does not occur at the tip (south pole) of the incoming particle, the oxide film is not broken, such that the bonding strength is weak. In contrast, at the outer edge of the particle, metallic bonding occurs, attaining a level of strength that exceeds that of the base material due to the huge plastic deformation. This phenomenon is known as the “south-pole problem,” and can lead to a decrease in the overall adhesion strength despite the local adhesion being strong. However, detailed observations revealed, in parts of the deposits, particles that had adhered across their entire surface. This suggests that, provided the collision state can be controlled, it is possible to overcome the south-pole problem and improve the adhesion strength.Graphical abstractImage 1
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