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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 319  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3182 journals]
  • Intrinsic Toughness of the Bulk-Metallic Glass Vitreloy 105 Measured Using
           Micro-Cantilever Beams
    • Abstract: Publication date: Available online 11 November 2019Source: Acta MaterialiaAuthor(s): Daniel Sorensen, Eric Hintsala, Joseph Stevick, Jesse Pischlar, Bernard Li, Daniel Kiener, Jason C. Myers, Hui Jin, Jia Liu, Douglas Stauffer, Antonio. J. Ramirez, Robert O. Ritchie Bulk-metallic glasses (BMGs) are a class of structural materials with many attractive processing featuers such as the ability to be processed into parts with fine features, dimensional precision, and repeatability; however, their fracture behavior is complex and size-dependent. Previous work has shown that BMGs can display strong size effects on toughness, where multiple mechanisms on different length-scales, e.g., crack bridging and bifurication, shear band spacing and length, can significantly affect the properies. This length-scale dependence on the fracture toughness has importance not only for advancing the understanding of fracture processes in these materials, but also for the potential future applications of BMGs, such as for microdevices. Here, using in situ scanning electron microscopy (SEM), we report on notched micro-cantilever bending experiments to address the lack of data regarding fracture properties of BMGs at the microscale. Sudden catastrophic propagation of shear bands resulted in failure for these specimens at stress intensities much lower than the bulk material, which may be due to a lack of extrinsic toughening mechanisms at these dimensions. This is explored further with post mortem SEM and transmission electron microscopy (TEM) analysis of the fractured beams while the fracture toughness results are verified using finite element modeling. The excellent agreement between model and micro cantilever beam bending experiments suggests that the intrinsic fracture toughness of Vitreloy 105, 9.03±0.59 MPa.m½, is being reported for the first time.Graphical Image, graphical abstract
  • Distinct driven steady states emerge from diverse initial textures in
           rolled nanocomposites
    • Abstract: Publication date: Available online 11 November 2019Source: Acta MaterialiaAuthor(s): Ian Chesser, Elizabeth A. Holm, Michael J. Demkowicz Severe plastic deformation is a widespread method of making high-performance metallic materials. Single-phase polycrystalline metals undergoing severe plastic deformation develop steady-state textures that are characteristic of the mode of deformation. By contrast, we show that two-phase, Cu-Nb nano-laminate composites reach a variety of different steady-state textures under a single mode of deformation. Using molecular statics simulations and a novel algorithm for crystal rotation analysis, we observe that the final, steady state texture and interface character in these materials depends on the initial texture of the composite. This finding suggests that the range of bulk Cu-Nb nano-composite textures that may be made by severe plastic deformation is larger than previously demonstrated, with multiple plastically-driven steady states accessible, depending on initial texture. We propose a modification of accumulative roll bonding with highly textured seed layers as a means of accessing different driven steady states in layered composites.Graphical abstractImage, graphical abstract
  • Effect of sputter pressure on microstructure and properties of
           β-Ta thin films
    • Abstract: Publication date: Available online 9 November 2019Source: Acta MaterialiaAuthor(s): Elizabeth A.I. Ellis, Markus Chmielus, Shangchen Han, Shefford P. Baker Tantalum thin films may be deposited in two phases. The stable bulk alpha phase is well known, but the metastable tetragonal beta phase is relatively poorly understood. We reported previously on a series of 100% β-Ta films deposited under varying sputter pressures in a low-oxygen environment, and discussed texture, stresses, and phase selection. Here, we discuss microstructure, morphology, and properties of these same β-Ta films. Grain size increases with sputter pressure, which can be explained by the energies of incident species at the growing film. Mechanical properties were measured by nanoindentation. Hardness decreases with grain size in accordance with the Hall-Petch relation while comparison of indentation modulus with biaxial modulus measurements indicates that the β phase is elastically anisotropic, and much stiffer in the [001] direction than in others. Finally, a canonical resistivity value for virtually oxygen-free, 100% β-Ta films of 169 ± 5 μΩcm is reported for the first time.Graphical abstractImage, graphical abstract
  • Atomic scale configuration of planar defects in the Nb-rich C14 laves
           phase NbFe2
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): M. Šlapáková, A. Zendegani, C.H. Liebscher, T. Hickel, J. Neugebauer, T. Hammerschmidt, A. Ormeci, J. Grin, G. Dehm, K.S. Kumar, F. Stein Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ∼1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable.Graphical Image, graphical abstract
  • Reconstructing the decomposed ferrite phase to achieve toughness
           regeneration in a duplex stainless steel
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Xuebing Liu, Wenjun Lu, Xinfang Zhang Duplex stainless steels suffer from thermal aging embrittlement that results from severe phase decomposition in ferrite phase after a long-term service at temperatures of 550–700 K, leading to the severe performance deterioration of duplex stainless steels. To ensure reliability and extend the service life of fabricated components made of duplex stainless steels, the development of techniques to efficiently and completely regenerate the deteriorated performance induced by spinodal decomposition and precipitation are extremely important. In this study, a novel pathway–an external electric field, is developed to eliminate the emerging Cr-rich (α′) phase and Fe-rich (α) phase resulting from spinodal decomposition as well as to dissolve the precipitates of G-phase in ferrite by introducing extra electrical free energy. The investigation is evidenced by microstructural and mechanical analyses using atom probe tomography, transmission electron microscopy, and nanoindentation. This high-efficiency (performance recovery above 90 %), low-energy consumption, online repair at the service temperature (700 K) is considerably superior to the traditional heat treatment process, which requires off-site repair at high temperatures (> 823 K). This new concept of manipulating precipitates using electric current to reconstruct the decomposed microstructure and achieve performance regeneration is expected to further stimulate the interest of researchers to extend the service life of materials by this means.Graphical abstractImage, graphical abstract
  • Microstructural evolution and strain-hardening in TWIP Ti alloys
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Guo-Hua Zhao, Xin Xu, David Dye, Pedro E.J. Rivera-Díaz-del-Castillo A multiscale dislocation-based model was built to describe, for the first time, the microstructural evolution and strain-hardening of {332}⟨113⟩ TWIP (twinning-induced plasticity) Ti alloys. This model not only incorporates the reduced dislocation mean free path by emerging twin obstacles, but also quantifies the internal stress fields present at β-matrix/twin interfaces. The model was validated with the novel Ti-11Mo-5Sn-5Nb alloy (wt.%), as well as an extensive series of alloys undergoing {332}⟨113⟩ twinning at various deformation conditions. The quantitative model revealed that solid solution hardening is main contributor to the yield stress, where multicomponent alloys or alloys containing eutectoid β-stabilisers exhibited higher yield strength. The evolution of twinning volume fraction, intertwin spacing, dislocation density and flow stress were successfully described. Particular attention was devoted to investigate the effect of strain rate on the twinning kinetics and dislocation annihilation. The modelling results clarified the role of each strengthening mechanism and established the influence of phase stability on twinning enhanced strain-hardening. The origin of strain-hardening is owing to the formation of twin obstacles in early stages, whereas the internal stress fields provide a long-lasting strengthening effect throughout the plastic deformation. A tool for alloy design by controlling TWIP is presented.Graphical abstractGraphical abstract for this article
  • Modeling the interface structure of Type II twin boundary in B19′ NiTi
           from an atomistic and topological standpoint
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Ahmed Sameer Khan Mohammed, Huseyin Sehitoglu This study addresses fundamental quandaries in the understanding of Type II twin interface in B19′ NiTi. A combined atomistic-topological approach is proposed to resolve a longstanding debate on the interface structure, affirming the hypothesis of a semi-coherent ledged geometry comprising of disconnected terraces. Atomic registry across the terrace is shown to require interface coherence strains. The twinning plane is shown to be a non-crystallographic virtual boundary separating the strained twin variants. Consequently, the issue of lattice offset arises and is addressed by an atomistic evaluation of interface energetics upon parametric variation of an offset parameter. Required atomic movements for migration of the terrace are established from a crystallographic analysis of the strained interface structure, and validated by a Molecular Statics (MS) simulation of the twin migration segment in the Generalized Planar Fault Energy (GPFE) curve. The GPFE calculation estimates a twinning partial magnitude consistent with an earlier ab initio prediction. This twinning partial serves as a “perfect” interface dislocation which, along with the coherence strain, feed into a topological model causally explaining the known irrational indices of the effective Twin Boundary (TB). A complete mechanistic picture of diffusionless TB migration is presented, the importance of which is discussed.Graphical abstractImage, graphical abstract
  • Experimental and crystal plasticity study on deformation bands in single
           crystal and multi-crystal pure aluminium
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Qinmeng Luan, Hui Xing, Jiao Zhang, Jun Jiang Deformation bands (DBs) formed in metals even in single crystals are known to give rise to the microstructural heterogeneities, thus contributing to some long-standing microstructure formation problems, such as the occurrence of recrystallization on the basis of deformed microstructure. Previous experimental transmission electron microscope (TEM) work has identified two types of DBs in the microscopic scale, i.e. kink bands and bands of secondary slips, showing the importance of understanding the slip activation for DBs. To extend the theory in mesoscale, single crystal and multi-crystal pure aluminium, as well as their corresponding crystal plasticity finite element (CPFE) models, are used in this paper to explore the effect of grain orientation, strain level and neighbouring grains on the formation of DBs. It is demonstrated that slip band intersection of primary and secondary slips is predicted to constrain the lattice sliding but facilitate the lattice rotation for the formation of DBs regarding the wall of DBs and its orientation. It is found that the impact of the above factors on the formation of DBs is caused by the slip field of primary slips. A sufficient amount of primary slips activated inside grains would be the key to the formation of distinct DBs with high area fraction and aspect ratio.Graphical abstractImage, graphical abstract
  • Complex phase transitions and associated electrocaloric effects in
           different oriented PMN-30PT single crystals under multi-fields of electric
           field and temperature
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Jianting Li, Ruowei Yin, Xiaopo Su, Hong-Hui Wu, Junjie Li, Shiqiang Qin, Shengdong Sun, Jun Chen, Yanjing Su, Lijie Qiao, Dong Guo, Yang Bai The phase composition and their evolution in ferroelectrics are very complex near the morphotropic phase boundary (MPB), especially under multiple fields of electric field and temperature, which results in versatile behaviors of electrocaloric effect (ECE). This paper systematically studied the phase transitions and associated ECEs in the , and oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-30PT) single crystals with the chemical composition in the MPB area. Based on the dielectric and polarization responses under multi-fields, the refined electric field-temperature phase diagrams are established, wherein the ECE properties are closely related to the electric-field-induced phase transitions and exhibit a complex evolution across positive and negative values. The negative ECE originates from the transition from a monoclinic phase to a tetragonal or orthogonal phase under the noncollinear electric field, so it only appears in the or oriented crystals. Conversely, the transition from the tetragonal phase to the rhombohedral or orthogonal phase in the or oriented crystals induces a positive ECE peak with ∆Tmax=0.40K (@10kV/cm). Just above Tm, the positive ECE reaches ∆Tmax=0.61K, 0.63K and 0.68K in the , and oriented crystals, respectively. In addition, there is an abnormal net endothermic phenomenon around the monoclinic-tetragonal or monoclinic-orthogonal phase boundaries due to the electric-field-induced irreversible transition.Graphical Image, graphical abstract
  • Half-Heusler alloys: Enhancement of ZT after Severe Plastic Deformation
           (ultra-low Thermal Conductivity)
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Gerda Rogl, Sanyukta Ghosh, Lei Wang, Jiri Bursik, Andriy Grytsiv, Michael Kerber, Ernst Bauer, Ramesh C. Mallik, Xingqiu Chen, Michael Zehetbauer, Peter Rogl Several n- and p-type Half-Heusler (HH) thermoelectric materials (Ti0.5Zr0.5NiSn-based and NbFeSb-based) have been processed by high-pressure torsion (HPT) to improve their thermoelectric performance via a drastic reduction towards ultra-low thermal conductivity. This reduction occurs due to grain refinement and a high concentration of deformation-induced defects, i.e. vacancies and dislocations as inferred by this severe plastic deformation and documented via SEM and TEM investigations. In most cases the figure of merit, ZT, and the thermo-electric conversion efficiency were enhanced up to η ∼ 10% for the thermally stable HPT-processed sample. Raman spectroscopy, backed by DFT calculations, proves that HPT induces a stiffening of the lattice and as a consequence, a blue-shift of the lattice vibrations occurs.Furthermore for all investigated specimens Vickers hardness values after HPT were significantly higher, whereas the change in the elastic moduli was less than 5% in comparison to the HP reference sample.Graphical abstractImage, graphical abstract
  • Presence of a purely tetragonal phase in ultrathin BiFeO3 films:
           thermodynamics and phase-field simulations
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Yang Zhang, Fei Xue, Zuhuang Chen, Jun-Ming Liu, Long-Qing Chen The stability of a purely tetragonal phase relative to the nominal rhombohedral phase in ultrathin BiFeO3 films is investigated using thermodynamics and phase-field simulations. The thermodynamic analysis demonstrates the possible presence of a purely tetragonal state primarily due to the interfacial effect from the constraint of the adjacent layer although the built-in potential and compressive in-plane strain also play a role. Phase-field simulations of the corresponding ultrathin films reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation. It is shown that the piezoelectric coefficient d33 of the two-phase mixture is up to 200% higher than that of the rhombohedral single phase.Graphical abstractA purely tetragonal phase can be stabilized in ultrathin BiFeO3 films primarily due to the interfacial constraint of bottom layers according to the thermodynamic analysis. The phase-field simulations further reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation, leading to the enhancement of piezoelectric performances.Image, graphical abstract
  • Editors for Acta Materialia
    • Abstract: Publication date: December 2019Source: Acta Materialia, Volume 181Author(s):
  • Solidification-driven Orientation Gradients in Additively Manufactured
           Stainless Steel
    • Abstract: Publication date: Available online 8 November 2019Source: Acta MaterialiaAuthor(s): Andrew T. Polonsky, William C. Lenthe, McLean P. Echlin, Veronica Livescu, George T. Gray, Tresa M. Pollock A sample of 304L stainless steel manufactured by Laser Engineered Net Shaping (LENS) was characterized in 3D using TriBeam tomography. The crystallographic, structural, and chemical properties of the as-deposited microstructure have been studied in detail. 3D characterization reveals complex grain morphologies and large orientation gradients, in excess of 10∘, that are not easily interpreted from 2D cross-sections alone. Misorientations were calculated via a methodology that locates the initial location and orientation of grains that grow during the build process. For larger grains, misorientation increased along the direction of solidification. For grains with complex morphologies, K-means clustering in orientation space is demonstrated as a useful approach for determining the initial growth orientation. The gradients in misorientation directly tracked with gradients in chemistry predicted by a Scheil analysis. The accumulation of misorientation is linked to the solutal and thermal solidification path, offering potential design pathways for novel alloys more suited for additive manufacturing.Graphical abstractGraphical abstract for this article
  • The stability of irradiation-induced defects in Zr3AlC2, Nb4AlC3 and
           (Zr0.5,Ti0.5)3AlC2 MAX phase-based ceramics
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): D. Bowden, J. Ward, S. Middleburgh, S. de Moraes Shubeita, E. Zapata-Solvas, T. Lapauw, J. Vleugels, K. Lambrinou, W.E. Lee, M. Preuss, P. Frankel This work is a first assessment of the radiation tolerance of the nanolayered ternary carbides (MAX phases), Zr3AlC2, Nb4AlC3 and (Zr0.5,Ti0.5)3AlC2, using proton irradiation followed by post-irradiation examination based primarily on x-ray diffraction analysis. These specific MAX phase compounds are being evaluated as candidate coating materials for fuel cladding applications in advanced nuclear reactor systems. The aim of using a MAX phase coating is to protect the substrate fuel cladding material from corrosion damage during its exposure to the primary coolant. Proton irradiation was used in this study as a surrogate for neutron irradiation in order to introduce radiation damage into these ceramics at reactor-relevant temperatures. The post-irradiation examination of these materials revealed that the Zr-based 312-MAX phases, Zr3AlC2 and (Zr0.5,Ti0.5)3AlC2 have a superior ability for defect-recovery above 400°C, whilst the Nb4AlC3 does not demonstrate any appreciable defect recovery below 600°C. Density functional theory calculations have demonstrated that the structural differences between the 312 and 413-MAX phase structures govern the variation of the irradiation tolerance of these materials.Graphical abstractImage, graphical abstract
  • 1 x Sr x Al2Si2O8:1%Eu 2 + , 1%Pr 3 + +Anorthite&rft.title=Acta+Materialia&rft.issn=1359-6454&">Relating Structural Phase Transitions to Mechanoluminescence: The Case of
           the Ca 1 − x Sr x Al2Si2O8:1%Eu 2 + , 1%Pr 3 +
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Ang Feng, Simon Michels, Alfredo Lamberti, Wim Van Paepegem, Philippe F. Smet The phenomenon of mechanoluminescence (ML), where phosphors emit light when pressure is applied, is considered to be closely related to the crystallographic structure of those phosphors. In this work we unravel this connection for the anorthite solid solution Ca1−xSrxAl2Si2O8, which displays two important phase transitions as a function of strontium content x (denoted as xSr), i.e., the nearly second-order P1¯-I1¯ transition and the ferroelastic I1¯-I2c transition at ambient temperature and pressure. The spontaneous strains reveal that the ferroelastic transition takes place when xSr ∈ (0.70, 0.75), while other optical methods suggest that the second-order P1¯−I1¯ transition takes place when xSr is around 0.4. The ML intensity reaches its maximum when the second order transition takes place and drops to zero when the phosphors undergo the ferroelastic transition. The first transition already brings significant changes to electron occupations at traps in this solid solution. The structural phase transitions in the anorthite solid solutions are reflected in specific ML properties, such as the ML intensity and the load threshold. Further analysis suggests this is due to the structural change of the hosts and the trap properties (trap density and electron population function). Analysis of the ML dynamics may therefore serve as a useful tool to investigate phase transitions in ML phosphors.Graphical abstractGraphical abstract for this article
  • Hafnia-doped Silicon Bond Coats manufactured by PVD for SiC/SiC CMCs
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Ronja Anton, Vito Leisner, Philipp Watermeyer, Michael Engstler, Uwe Schulz SiC/SiC ceramic matrix composites (CMCs) demand an environmental barrier coating (EBC) system when implemented in the hot section of a turbine engine. The connection between EBC and CMC is provided by a bond coat (BC). Numerous reasons make silicon the state-of-the-art BC material but it has some disadvantages regarding long time mechanical behaviour and oxidation resistance. To overcome this, a Si-BC doped with the refractory metal oxide HfO2 is introduced. Two different compositions have been deposited on monolithic SiC by magnetron sputtering. After deposition the coatings are X-ray amorphous, homogenous, columnar structured and virtually free of cracks and pores. Furnace cycle tests up to 1000 cycles were performed at 1523 K. The evolution of microstructure and phases of the coatings were examined employing Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) serial sectioning, Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). During high temperature exposure, the coatings crystallized and the silicon phase started to form a mixed thermally grown oxide (mTGO) layer. The BCs showed evenly distributed hafnia precipitates within the silicon. During testing Ostwald ripening of the precipitates took place. Hafnia slowly reacted with silicon oxide to hafnon (HfSiO4). Compared to a pure silicon reference BC, the doped coatings show a better resistance towards crack initiation and spallation up to 1000 h testing time. The results demonstrate that sputtered hafnia-doped Si-BCs are more advantageous for SiC/SiC CMCs with respect to longevity, TGO adherence, and protection of the underlying SiC in comparison to pure Si bond coats.Graphical Image, graphical abstract
  • Phase transformation induces plasticity with negligible damage in
           ceria-stabilized zirconia-based ceramics
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Aléthéa Liens, Helen Reveron, Thierry Douillard, Nicholas Blanchard, Vanni Lughi, Valter Sergo, René Laquai, Bernd R. Müller, Giovanni Bruno, Sven Schomer, Tobias Fürderer, Erik Adolfsson, Nicolas Courtois, Michael Swain, Jérôme Chevalier Ceramics and their composites are in general brittle materials because they are predominantly made up of ionic and covalent bonds that avoid dislocation motion at room temperature. However, a remarkable ductile behavior has been observed on newly developed 11 mol.% ceria-stabilized zirconia (11Ce-TZP) composite containing fine alumina (8vol.% Al2O3) and elongated strontium hexa-aluminate (8vol.% SrAl12O19) grains. The as-synthesized composite also has shown full resistance to Low Temperature Degradation (LTD), relatively high strength and exceptionally high Weibull modulus, allowing its use in a broader range of biomedical applications. In this study, to deepen the understanding of plastic deformation in Ce-TZP based composites that could soon be used for manufacturing dental implants, different mechanical tests were applied on the material, followed by complete microstructural characterization. Distinct from pure Ce-TZP material or other zirconia-based ceramics developed in the past, the material here studied can be permanently strained without affecting the Young modulus, indicating that the ductile response of tested samples cannot be associated to damage occurrence. This ductility is related to the stress-induced tetragonal to monoclinic (t-m) zirconia phase transformation, analogue to Transformation-Induced Plasticity (TRIP) steels, where retained austenite is transformed to martensite. The aim of this study is to corroborate if the observed plasticity can be associated exclusively to the zirconia t-m phase transformation, or also to microcraking induced by the transformation. The t-m transformed-zones produced after bending and biaxial tests were examined by X-ray refraction and SEM/TEM coupled with Raman. The results revealed that the observed elastic-plastic behavior occurs without extensive microcracking, confirming a purely elastic-plastic behavior driven by the phase transformation (absence of damage).Graphical abstractImage, graphical abstract
  • Controlling the domain structure of ferroelectric nanoparticles using
           tunable shells
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Anna N. Morozovska, Eugene A. Eliseev, Yevhen M. Fomichov, Yulian M. Vysochanskii, Victor Yu. Reshetnyak, Dean R. Evans The possibility of controlling the domain structure in spherical nanoparticles of uniaxial and multiaxial ferroelectrics using a shell with tunable dielectric properties is studied in the framework of Landau-Ginzburg-Devonshire theory. Finite element modeling and analytical calculations are performed for Sn2P2S6 and BaTiO3 nanoparticles covered with polymer, temperature dependent isotropic paraelectric strontium titanate, or anisotropic liquid crystal shells with a strongly temperature dependent dielectric permittivity tensor. It appeared that the “tunable” paraelectric shell with a temperature dependent high dielectric permittivity (∼300 – 3000) provides much more efficient screening of the nanoparticle polarization than the polymer shell with a much smaller (∼10) temperature-independent permittivity. The tunable dielectric anisotropy of the liquid crystal shell (∼ 1 – 100) adds a new level of functionality for the control of ferroelectric domains morphology (including a single-domain state, domain stripes and cylinders, meandering and labyrinthine domains, and polarization flux-closure domains and vortexes) in comparison with isotropic paraelectric and polymer shells. The obtained results indicate the opportunities to control the domain structure morphology of ferroelectric nanoparticles covered with tunable shells, which can lead to the generation of new ferroelectric memory and advanced cryptographic materials.Graphical abstractImage, graphical abstract
  • Microstructure-dependent deformation behaviour of a low γ′ volume
           fraction Ni-base superalloy studied by in-situ neutron diffraction
    • Abstract: Publication date: Available online 7 November 2019Source: Acta MaterialiaAuthor(s): Nitesh Raj Jaladurgam, Hongjia Li, Joe Kelleher, Christer Persson, Axel Steuwer, Magnus Hörnqvist Colliander Ni-base superalloys are critical materials for numerous demanding applications in the energy and aerospace sectors. Their complex chemistry and microstructure require detailed understanding of the operating deformation mechanisms and interaction between the matrix and the hardening phase during plastic deformation. Here we use in-situ neutron diffraction to show that the dependence of the deformation mechanisms and load redistribution on γ′ particle size in a Ni-base superalloy with a γ′ volume fraction of around 20% can exhibit distinct differences compared to their high volume fraction counterparts. In particular, the load redistribution in the coarse microstructure occurs immediately upon yielding in the present case, whereas high γ′ volume fractions have been observed to initially lead to shear mediated co-deformation before work hardening allows looping to dominate and cause load partitioning at higher stresses. The fine microstructure, on the other hand, behaved similar to high volume fraction alloys, exhibiting co-deformation of the phases due to particle shearing. A recently developed elasto-plastic self-consistent (EPSC) crystal plasticity model, specifically developed for the case of coherent multi-phase materials, could reproduce experimental data with good accuracy. Furthermore, the finite strain formulation of the EPSC model allowed deformation induced texture predictions. The correct trends were predicted by the simulations, but the rate of lattice rotation was slower than experimentally observed. The insights point towards necessary model developments and improvements in order to accurately predict e.g. texture evolution during processing and effect of texture and microstructure on component properties.Graphical abstractGraphical abstract for this article
  • Characterization of polyhedral nano-oxides and helium bubbles in an
           annealed nanostructured ferritic alloy
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Tiberiu Stan, Yuan Wu, Jim Ciston, Takuya Yamamoto, G. Robert Odette Nanostructured ferritic alloys (NFAs) contain an ultra-high density of 2-4 nm fcc pyrochlore Y2Ti2O7 nano-oxides (NOs) embedded in a bcc Fe-14Cr ferritic matrix. Characterization of helium interactions with NOs and associated Fe-Y2Ti2O7 interfaces is important to the development of structural materials for nuclear fusion and fission applications. A benchmark 14YWT NFA was first annealed to coarsen the NOs, then insoluble helium was implanted at 700°C to produce a high number density of bubbles. High-resolution scanning transmission electron microscopy characterization shows two dominant Fe-Y2Ti2O7 crystallographic orientation relationships (cube-on-edge and cube-on-cube). The smallest NOs (≈ 2 nm) are associated with the smaller bubbles (≈ 1.5 nm), while some of the largest NOs (> 6 nm) have larger, and sometimes multiple, bubbles. NO corner {111} facets are the preferred sites for He bubble nucleation. A refined sequence of events for He trapping and bubble formation is presented. These observations offer new insight on He management in NFAs, and provide a foundation for detailed modeling studies.Graphical abstractImage, graphical abstract
  • Segmentation crack formation dynamics during air plasma spraying of
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Shalaka V. Shinde, Edward J. Gildersleeve V, Curtis A. Johnson, Sanjay Sampath Air Plasma Sprayed (APS) Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coatings (TBCs) is a well-established technology in the gas turbine industry. A conventional APS TBC is a layered structure with heterogenous distribution of defects (microcracking, pores, etc.) which allow it to simultaneously possess low thermal conductivity and elastic modulus compared to its bulk counterpart. However, interfacial defects can be a source of delamination failure during thermal cycling. In addition, conventional porous coatings can experience sintering during sustained exposure, which augments failure through stiffening-induced delamination. Electron Beam Physical Vapor Deposition (EB-PVD) TBC coatings, due to their dense columnar structures, are less susceptible to both sintering and delamination. This has led to the consideration of more economically-applied APS TBCs that are dense with periodic vertical segmentation cracks. Such dense vertically cracked coatings (DVCs) have been successfully developed and implemented in gas turbine engines. These microstructures are produced in-situ through control of the process conditions with high deposition temperatures. However, the mechanism of such segmentation cracks is unclear. In this study, formation dynamics of segmentation cracks were observed through in-situ beam curvature monitoring during deposition in combination with microstructural evaluations. It was observed the initial layers of the coating are dense without segmentation cracking. As subsequent layers are deposited, periodic macrocracking initiates and typically propagates through the remaining coating thickness. The in-situ in-plane coating stress is significantly reduced after segmentation cracking begins. These results are reconciled through interpretation of thin-film fracture literature, and an initial framework to interpret the experimental observations is provided.Graphical abstractImage, graphical abstract
  • Electrical conduction mechanism of rare-earth calcium oxyborate high
           temperature piezoelectric crystals
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Shiwei Tian, Lili Li, Xinyu Lu, Fapeng Yu, Yanlu Li, Chao Jiang, Xiulan Duan, Zhengping Wang, Shujun Zhang, Xian Zhao The electrical conduction behavior of piezoelectric crystals is critical in the design of piezoelectric sensors for use at elevated temperatures. The electrical resistivity and dielectric properties of rare-earth calcium oxyborate crystals ReCa4O(BO3)3 (ReCOB, Re= Y, Gd and Pr) are investigated in this report. The relationships between the electronic structures and electrical properties are then determined using X-ray photoelectron spectra and first principle calculations. Among the ReCOB type crystals, YCOB is found to possess the highest electrical resistivity and lowest dielectric loss. Nonlinearity of the electrical resistivity as a function of temperature for ReCOB crystals is then confirmed, corresponding to different conduction mechanisms. It is also revealed that the electrical conductivity of ReCOB type crystals is heavily influenced by oxygen vacancy defects at relatively lower temperatures (below ∼600 °C), while both vacancy defects and band gap contribute to conductivity at elevated temperatures (above ∼600 °C).Graphical The electrical conduction mechanism of ReCa4O(BO3)3 (Re: rare-earth elements and Y) type high-temperature piezoelectric crystals.Image, graphical abstract
  • Unique high-temperature deformation dominated by grain boundary sliding in
           heterogeneous necklace structure formed by dynamic recrystallization in
           HfNbTaTiZr BCC refractory high entropy alloy
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Rajeshwar R. Eleti, Atul H. Chokshi, Akinobu Shibata, Nobuhiro Tsuji Microstructural evolution of dynamically recrystallized (DRX) grains and grain boundary sliding (GBS) in the heterogeneous necklace structure of HfNbTaTiZr refractory high entropy alloy (RHEA) was studied systematically during high temperature deformation. Uniaxial compression testing was carried out to different strains at 1000°C and a strain rate 10−3 s−1. Significant bulging of grain boundaries led initially to the formation of DRX grains. The fraction of DRX grains increased with strain, and typical necklace structures of fine (d ≤ 1.5 µm) DRX grains formed at strain ε ≥ 0.3. The DRX grains showed very limited grain growth, and heterogeneous microstructures composed of coarse unrecrystallized regions surrounded by the characteristic DRX necklace structure were formed at larger strains. Interrupted testing with marker grids revealed that DRX grains deformed by a GBS mechanism. The DRX necklace regions connected mesoscopically and also displayed diamond network morphologies, with unique “Y-shaped”, “T-shaped” and “X-shaped” junctions. The formation of different types of junctions were rationalized on the basis of GBS accommodated by local dislocation slip in unrecrystallized regions. The unrecrystallized regions showed preferred / micro-texture, consistent with conventional dislocation slip in the BCC crystals. On the other hand, the newly formed DRX grains initially had similar orientations to those of the parent grains, but they displayed a random texture with increasing strain, as expected from GBS. The randomized texture of DRX grains and the stability of DRX grains size represented GBS in the DRX necklace regions.Graphical abstractImage, graphical abstract
  • Structural and vibrational properties of α- and π-SnS polymorphs for
           photovoltaic applications
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Maxim Guc, Jacob Andrade-Arvizu, Ibbi Y. Ahmet, Florian Oliva, Marcel Placidi, Xavier Alcobé, Edgardo Saucedo, Alejandro Pérez-Rodríguez, Andrew L. Johnson, Victor Izquierdo-Roca Tin sulphide (SnS) has attracted the attention of the photovoltaic (PV) community due to the combination of desirable optical properties, and its binary and earth abundant elemental composition, which should lead to relatively simple synthesis. However, currently the best SnS based PV device efficiency remains at 4.36 %. Limited performance of this material is attributed to band gap alignment issues, deviations in doping concentration and poor film morphology. In this context Raman spectroscopy (RS) analysis can be useful as it facilitates the accurate evaluation of material properties. In this study we present a RS study, supported by X-ray diffraction and wavelength dispersive X-ray measurements, of α- and π-SnS thin films. In particular a complete description of SnS vibrational properties is made using six excitation wavelengths, including excitation energies coupled with certain optical band to band transitions, which leads to close to resonance measurement conditions. This study describes an in-depth analysis of the Raman spectra of both SnS structural polymorphs, including the differences in the number of observed peaks, with their relative intensities and Raman shift. Additionally, we evaluate the impact of low temperature heat treatment on SnS. These results explicitly present how the variation of the [S]/[Sn] ratio in samples deposited by different methods can lead to significant and correlated shifts in the relative positions of Raman peaks, which is only observed in the α-SnS phase. Furthermore, we discuss the suitability of using Raman spectroscopy based methodologies to extract fine stoichiometric variations in different α-SnS samples.Graphical Image, graphical abstract
  • Deformation in nanocrystalline ceramics: A microstructural study of
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Barak Ratzker, Avital Wagner, Maxim Sokol, Louisa Meshi, Sergey Kalabukhov, Nachum Frage Contrary to the characteristic strengthening of polycrystalline ceramics with a decrease in grain size, extremely fine nanocrystalline ceramics exhibit softening, increased plasticity and an inverse Hall-Petch relation. Despite experimental evidence, questions remain regarding the underlying deformation mechanisms governing this abnormal mechanical behavior. In the present study, an in-depth microstructural examination was performed on nanostructured transparent magnesium aluminate spinel (MgAl2O4) subjected to microhardness tests. Microstructural observations revealed regions strained to various degrees below the point of indentation, containing varying amounts of dislocations and nano-cavities. Furthermore, the residual strain in different areas was estimated by local electron diffraction. These observations and analysis provided evidence for grain boundary (GB) mediated mechanisms (e.g., GB sliding and rotation). Moreover, shear bands formed and were found to be associated with micro-cracking. By combining the microstructural analysis with suitable models, it was concluded that these mechanisms govern plastic deformation. By elucidating how strain is accommodated within nanocrystalline ceramics, a deeper understanding of their unique mechanical behavior is gained.Graphical Image, graphical abstract
  • Magnetic-field-induced strain-glass-to-martensite transition in a Fe-Mn-Ga
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Xiaoming Sun, Daoyong Cong, Yang Ren, Klaus-Dieter Liss, Dennis E. Brown, Zhiyuan Ma, Shijie Hao, Weixing Xia, Zhen Chen, Lin Ma, Xinguo Zhao, Zhanbing He, Jian Liu, Runguang Li, Yandong Wang Strain glass is a frozen disordered strain state with local strain order manifested by nano-sized strain domains, which is formed as a result of doping sufficient point defects into the normal martensitic system. Exploration of the transition between strain glass and long-range strain-ordered martensite is of both great fundamental importance and practical interest. However, it remains a mystery whether magnetic field can induce a transition from strain glass to martensite. Here, we report for the first time the magnetic-field-induced strain-glass-to-martensite transition, in a model system Fe-Mn-Ga. It was found that the martensitic transformation temperature of the Fe43-xMn28Ga29+x alloys decreases rapidly with increasing x and the martensitic transformation disappears when x reaches the critical value xc = 2.0. Strain glass transition occurs in the alloy with x = 2.0 (Fe41Mn28Ga31), which is confirmed by the invariance of the average structure during cooling, the frequency dispersion of the ac storage modulus and internal friction following the Vogel-Fulcher relation, and the formation of nanodomains. The magnetic-field-induced transition from strain glass to non-modulated tetragonal martensite in Fe41Mn28Ga31 was indicated by the abrupt magnetization jump on the M(H) curve and directly evidenced by the crystal structure evolution with magnetic field change revealed by in-situ neutron diffraction experiments. The microscopic mechanism for this magnetic-field-induced strain-glass-to-martensite transition is discussed. The present study may not only help establish the unified theory for strain-glass-to-martensite transition under external fields but also open a new avenue for designing advanced materials with novel functional properties.Graphical abstractImage, graphical abstract
  • Solute segregation induced sandwich structure in Al-Cu(-Au) alloys
    • Abstract: Publication date: Available online 6 November 2019Source: Acta MaterialiaAuthor(s): Yunhe Zheng, Yixian Liu, Nick Wilson, Shiqi Liu, Xiaojun Zhao, Houwen Chen, Jinfeng Li, Ziqiao Zheng, Laure Bourgeois, Jian-Feng Nie In this work atomic-resolution techniques of high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDXS)-STEM and first-principles calculations have been combined to study a hitherto unreported sandwich structure formed in aged samples of Al-Cu and Al-Cu-Au alloys. This sandwich structure comprises a stack of regularly spaced plates of metastable precipitate phases of GP zones, θ'' and θ'. Within the sandwich structure, the separation between the broad surface of θ' and its adjacent GP zone, as well as that between two neighbouring GP zones, is always three {002}α planes. This sandwich structure is observed for θ' precipitates of various thicknesses. Based on experimental results and calculations, it is inferred that the formation of the sandwich structure is induced by the θ'/Al interfacial segregation of Cu atoms, rather than the misfit associated with θ' formation. In the sandwich structure formed in the ternary alloy, Au atoms distribute mainly in the central part of θ' plate, but not at the θ'/Al interface or in the GP zone. Calculations confirm the experimental observations and further indicate that, energetically, Au atoms prefer to substitute for Cu, rather than Al, atoms within θ'.Graphical Image, graphical abstract
  • A multiscale study on the morphology and evolution of slip bands in a
           nickel-based superalloy during low cycle fatigue
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): F.D. León-Cázares, R. Schlütter, T. Jackson, E.I. Galindo-Nava, C.M.F. Rae Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ′ precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700 ∘C for 1, 40 and 500 cycles. The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ′-etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.Graphical abstractGraphical abstract for this article
  • Irradiation induced creep in nanocrystalline high entropy alloys
    • Abstract: Publication date: 1 January 2020Source: Acta Materialia, Volume 182Author(s): Gowtham Sriram Jawaharram, Christopher M. Barr, Anthony M. Monterrosa, Khalid Hattar, Robert S. Averback, Shen J. Dillon Irradiation induced creep (IIC) compliance in NiCoFeCrMn high entropy alloys is measured as a function of grain size (30 < x < 80 nm) and temperature (23–500 °C). For 2.6 MeV Ag3+ irradiation at a dose rate of 1.5×10–3 dpa−1s−1 the transition from the recombination to sink limited regimes occurs at ∼ 100 °C. In the sink-limited regime, the IIC compliance scales inversely with grain size, consistent with a recently proposed model for grain boundary IIC. The thermal creep rate is also measured; it does not become comparable to the IIC rate, however, until ∼ 650 °C. The results are discussed in context of defect kinetics in irradiated HEA systems.Graphical abstractImage, graphical abstract
  • Optimizing composition in MnBi permanent magnet alloys
    • Abstract: Publication date: December 2019Source: Acta Materialia, Volume 181Author(s): Brandt A. Jensen, Wei Tang, Xubo Liu, Alexandra I. Nolte, Gaoyuan Ouyang, Kevin W. Dennis, Jun Cui MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 × 106 J m−3), and good magnetization (81 emu g−1) at room temperature. Although the theoretical maximum energy product (BH)max of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 °C, which makes it a potential candidate for high temperature applications such as permanent magnet motors. However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn55Bi45 resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. Here we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (MS) of 79 emu g−1 at 300 K in the annealed Mn52Bi48 ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5–5 µm particles with MS of 75.5 emu g−1, coercivity Hci of 10.8 kOe, and (BH)max of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased Hci by up to 21%, and also resulted in an increase in MS up to 78.8 emu g−1.Graphical Image, graphical abstract
  • Thermodynamics of solute capture during the oxidation of multicomponent
    • Abstract: Publication date: December 2019Source: Acta Materialia, Volume 181Author(s): Q.C. Sherman, P.W. Voorhees, L.D. Marks In the classical theories of oxidation of metals it is assumed that the interface between the oxide and metal is in thermodynamic equilibrium. However, in many cases this is not true, the oxide grows too fast or the fluxes through the interface are too large for local interfacial equilibrium to exist, leading to nonequilibrium solute capture. We present a thermodynamic analysis using both an available database as well as density functional theory calculations of the thermodynamic conditions for this during the oxidation of Ni–Cr alloys. The analysis indicates that nickel atoms can be captured in the rocksalt or corundum crystallographies for a very wide range of compositions, consistent with recent experimental observations. The density functional theory analysis also provides information about the electronic structure of these oxides which is important to understand their properties, and also indicates that interpretation of spectroscopic data is not simple as mixed valence states as well as Cr4+ can occur under oxidizing conditions. We point out that across at least the first transition row of elements the thermodynamic conditions for nonequilibrium solute capture can easily be met.Graphical abstractillustration of a moving oxidation front and how the velocity of the interface connects to equilibrium or nonequilibrium formation of the oxide.Image, graphical abstract
  • Identifying heating rate dependent oxidation reactions on a nickel-based
           superalloy using synchrotron diffraction
    • Abstract: Publication date: December 2019Source: Acta Materialia, Volume 181Author(s): T.D. Reynolds, D.M. Collins, N.K. Soor, S.R. Street, N. Warnken, P.M. Mignanelli, M.C. Hardy, H.E. Evans, M.P. Taylor Synchrotron grazing incidence X-ray diffraction has been used to newly reveal the heating rate dependent oxidation reactions that develop on a polycrystalline nickel-based superalloy. A continuous layer of precursor oxide was shown to form during the heating stage. Their approximate growth rates, their effect on local surface compositions of the alloy substrate, and their degree of interface planarity are considered critical in determining subsequent oxidation reactions when held for extended thermal exposures. The precursor oxides were predominantly nickel or cobalt based (NiO/CoO and Co3O4/NiCo2O4). Following the fastest heating rates (40 °C min−1 and above), the stable Cr2O3 phase formed, inhibiting Ni or Co diffusion to the surface. At slower heating rates (10–20 °C min−1), no evidence of the stable Cr2O3 was found, even after 200 h at elevated thermal exposure, instead continued growth of the precursor oxides was observed. Heating at 5 °C min−1 gave rise to an intriguing zone where sufficient precursor and favourable kinetics enabled the formation of a spinel, NiCr2O4, surface layer. Cross sections observed with electron microscopy confirmed this to be planar and continuous. Heating at the slowest tested 2 °C min−1 contrarily gives a non-protective surface layer comprising an outwardly growing NiO/CoO precursor oxide on top of an inwardly growing mixed oxide. The quantities, interfacial morphologies of oxides of the precursor oxide grown and the possible thermodynamic reactions that lead to their formation are discussed.Graphical abstractImage, graphical abstract
  • The effect of grain boundary structure on the intergranular degradation
           behavior of solution annealed Alloy 690 in high temperature, hydrogenated
    • Abstract: Publication date: Available online 30 October 2019Source: Acta MaterialiaAuthor(s): Wenjun Kuang, Gary S. Was The environmental degradation of four different types of grain boundaries were investigated on alloy 690 following slow strain rate tensile tests in 360 °C hydrogenated water. Random high angle boundaries (RHABs) support fast Cr diffusion that promotes the formation of a continuous surface oxide film and grain boundary migration. Surprisingly, coherent twin boundaries (CTBs) are susceptible to intergranular oxidation and do not exhibit Cr diffusion or grain boundary migration. When CTBs are changed to transformed twin boundaries (TTBs) by cold work, they behave like RHABs. Finally, incoherent twin boundaries (ITBs) undergo intergranular oxidation with limited Cr depletion but no boundary migration beyond the oxide. The Cr diffusivity along grain boundary in this alloy is directly related to the density of coincident site in the grain boundary plane and determines the morphology of oxide formed near the grain boundary. CTBs are still highly resistant to stress corrosion cracking (SCC) due to the semi-coherent interface between the intergranular chromia and grain matrix. In contrast, the intergranular oxides formed along RHABs inherit highly-disordered boundary structure from the original grain boundaries and show much higher SCC susceptibility. The grain boundary structure dependence of SCC resistance should be understood from its effects on solute diffusivity, structure of intergranular oxide and the local stress-strain state.Graphical abstractImage, graphical abstract
  • Tailoring ultra-strong nanocrystalline tungsten nanofoams by reverse phase
    • Abstract: Publication date: Available online 30 October 2019Source: Acta MaterialiaAuthor(s): Mingyue Zhao, Inas Issa, Manuel J. Pfeifenberger, Michael Wurmshuber, Daniel Kiener Bulk nanoporous tungsten as an extremely strong and low density nanocrystalline material was for the first time created to satisfy the need for advanced high performance materials that can endure harsh environments. Synthesis of nanoporous tungsten was achieved by a unique procedure involving severe plastic deformation of a coarse-grained tungsten-copper composite followed by selective dissolution of the nobler copper phase. The used ammonium persulfate etching solution, in which the less noble tungsten is chemically stable, is proved to be effective in removing the nobler copper phase. A nanoporous tungsten microstructure characterized by a network of interconnected nanocrystalline tungsten ligaments and interconnected nanopores was obtained. Based on a high-resolution interface analysis, the underlined mechanisms for the formation of nanoporous tungsten structure were elucidated. Moreover, using nanoindentation we demonstrate that, due to the nanoscale microstructure, the created nanoporous tungsten possesses outstanding strength, making it an attractive material for applications in radiation shielding fields.Graphical abstractImage, graphical abstract
  • Cyclic deformation behavior and related micro-mechanisms of a special CVD
           Ni processed with bimodal grain structures: ultrafine (UF) grains and
           large grains with UF/nano twins
    • Abstract: Publication date: Available online 18 October 2019Source: Acta MaterialiaAuthor(s): Shaohua Fu, Tzu-Yin Jean Hsu, Zhirui Wang Stress-controlled cyclic tests were conducted on bulk sheet Ni-carbonyl Chemical Vapor Deposited material (CVD Ni) with bimodal grain structures: ultrafine (UF) grains and large grains with UF/nano twins. The tests were run with the cyclic stress ratio R=0.05 and peak stress level of 0.9 to 1.5 times of the material's yield strength. Results show that within the applied peak stress ratio of 0.9-1.1, the material demonstrated cyclic hardening behavior first, followed by stress-strain saturation till fracture; upon increasing the ratio to 1.4-1.5, an additional softening stage was activated and continued till fracture. By transmission electron microscope (TEM) examination, it was found that such cyclic deformation responses were associated with the stability of the ultrafine- and nano-twin structures. Initial hardening was found mainly due to the increase in dislocation density and the activities of dislocations especially with their strong interactions with the dense twin boundaries (TBs). The saturation was contributed by the simultaneous operations of the softening effect due to massive detwinning and the hardening behavior due to dislocation interactions with existing TBs. Newly formed dislocation walls and cell structures were further found in samples with stress ratio of 1.4 at fracture, corresponding to the softening stage. Furthermore, such microstructural evolution, which were observed also through annealing and monotonic deformation of the same material, is identified as a consistent energy reduction path for the material. Thus, an energy criterion is further established to predict the massive detwinning events that cause the major softening phenomena under cyclic deformation.Graphical abstractImage, graphical abstract
  • Kinetic Pathways of Ordering and Phase Separation Using Classical Solid
           State Models within the Steepest-Entropy-Ascent Quantum Thermodynamic
    • Abstract: Publication date: Available online 14 October 2019Source: Acta MaterialiaAuthor(s): Ryo Yamada, Michael R. von Spakovsky, William T. Reynolds The kinetics of ordering and concurrent ordering and phase separation are analyzed with an equation of motion initially developed to account for dissipative processes in quantum systems. A simplified energy eigenstructure, or pseudo-eigenstructure, is constructed from a static concentration wave method to describe the configuration-dependent energy in a binary alloy. This pseudo-eigenstructure is used in conjunction with an equation of motion that follows steepest entropy ascent to calculate the kinetic path that leads to ordering and phase separation in a series of hypothetical alloys. By adjusting the thermodynamic solution parameters, it is demonstrated that the model can predict: (a) the stable equilibrium state, (b) the unique thermodynamic path and kinetics of continuous or discontinuous ordering, and (c) the kinetics of concurrent processes involving simultaneous ordering and phase separation.Graphical abstractGraphical abstract for this article
  • 2020 Acta Award Recipients
    • Abstract: Publication date: Available online 10 October 2019Source: Acta MaterialiaAuthor(s):
  • Anomalous work hardening behavior of Fe40Mn40Cr10Co10 high entropy alloy
           single crystals deformed by twinning and slip
    • Abstract: Publication date: December 2019Source: Acta Materialia, Volume 181Author(s): S. Picak, J. Liu, C. Hayrettin, W. Nasim, D. Canadinc, K. Xie, Y.I. Chumlyakov, I.V. Kireeva, Ibrahim Karaman The orientation dependence of tensile deformation in Fe40Mn40Co10Cr10 high entropy alloy (HEA) was investigated in [111], [001] and [123] oriented single crystals. Transmission electron microscopy investigations revealed three major mechanisms controlling the deformation stages, depending on the orientation: (i) deformation twinning, (ii) planar slip and (iii) dislocation wall/network formation. While twinning and planar slip were strongly orientation dependent, dislocation walls were observed in all orientations. Twinning was the dominant deformation mode in [111] crystals, while only multi-slip was observed in [001]. Both twins and planar slip were activated in [123] crystals. [111] crystals exhibited the highest strain hardening coefficients and ultimate tensile strength due to the strong twin-twin and twin-slip interactions where twin boundaries reduce the mean free path of dislocations, leading to dynamic Hall–Petch hardening. The decent ductility levels (∼45%) were attained in [111] due to nanoscale internal twins and tertiary twin system forming at the later stages of deformation and suppressing necking. In contrast, no twins or stacking faults were observed in [001] crystals, which is consistent with the Copley–Kear effect. [123] crystals had outstanding tensile ductility (∼65%), due to the activation of planar slip and twinning. Overall, in this off-stoichiometric HEA, we have determined the stacking faculty energy and critical resolved shear stresses for both twinning and slip, and demonstrated the formation of high dislocation density walls and wavy slip in [001], while the hardening stages of [123] and [111] are primarily governed by planar slip and twinning, which can be rationalized by the Copley–Kear effect.Graphical abstractImage, graphical abstract
  • Flash sintering activated by bulk phase and grain boundary complexion
    • Abstract: Publication date: December 2019Source: Acta Materialia, Volume 181Author(s): Yuanyao Zhang, Jiuyuan Nie, Jian Luo A naturally-occurring coupled thermal and electric runaway, resulted from an Arrhenius temperature-dependent specimen conductivity, can trigger flash sintering in many ceramics. This study reveals another possibility to activate flash sintering: a bulk phase transformation or a grain boundary (phase-like) complexion transition can cause an abrupt rise in the specimen conductivity to jump start flash sintering (prior to the occurrence of a natural thermal runaway). In undoped and Al2O3-doped ZnO, the flash sintering is activated by natural thermal runways that can be quantitively predicted from an Arrhenius extrapolation of low-temperature specimen conductivity. In contrast, a bulk eutectic reaction and the associated formation of premelting-like intergranular films (IGFs) in Bi2O3-doped ZnO can lead to a nonlinear rise in the specimen conductivity (above the Arrhenius extrapolation) to trigger flash sintering prior to the occurrence of the predicted natural thermal runaway. Yet, a natural thermal runaway can still take place in Bi2O3-doped ZnO before the occurrence of the interfacial and bulk transformation if the initial electric field is increased to a sufficiently high level. All five cases can be fully explained in a consistent framework so that this set of experiments systematically validate our theory of flash initiation. This work uncovers the roles of the bulk phase and interfacial (phase-like) complexion transformations in initiating flash sintering, thereby suggesting a new direction to understand and tailor the flash sintering process. An observation of ultra-fast field-induced migration of aliovalent cations during the flash sintering of Al2O3-doped ZnO is also reported.Graphical abstractImage, graphical abstract
  • Deformation induced grain boundary segregation in nanolaminated Al-Cu
    • Abstract: Publication date: Available online 29 October 2019Source: Acta MaterialiaAuthor(s): W. Xu, X.C. Liu, X.Y. Li, K. Lu A gradient nanostructured surface layer was formed in an Al-4 wt.% Cu alloy processed by means of surface mechanical grinding treatment at liquid nitrogen temperature. Within the deformed surface layer, laminated structures with a wide range of thickness were formed. With a decreasing depth from the treated surface, lamellae thickness decreases accompanied by an increased fraction of high angle grain boundaries (HAGBs) from 10% to 70%. In the topmost surface layer, nanolaminated (NL) structures were found with an average thickness as small as 28 nm and a HAGB fraction of 70%. Composition analysis indicated that Cu atoms segregate at NL boundaries in the as-prepared sample, Cu concentration is about 3-4 times higher than that in the lattice. The obvious grain boundary (GB) segregation of Cu induced by cryogenic plastic deformation is attributed dynamic interaction between solute atoms with gliding dislocations. GB segregation of Cu is responsible for the stabilization of the NL structures with a much finer structural size than that in pure Al, resulting in higher hardness. The deformation-induced GB segregation provides an alternative strategy to achieving stable high strength nanostructures in Al alloys.Graphical abstractImage, graphical abstract
  • Contribution of intragranular misorientations to the cold rolling textures
           of ferritic stainless steels
    • Abstract: Publication date: Available online 29 October 2019Source: Acta MaterialiaAuthor(s): A. Després, M. Zecevic, R.A. Lebensohn, J.D. Mithieux, F. Chassagne, C.W. Sinclair A combined experimental and simulation study of intragranular misorientation and texture development in ferritic stainless steels is presented. Cold rolling was performed on materials having different grain shapes to reveal variations of misorientations and texture with variations in microstructure. The experimental results were compared with predictions of the Visco-Plastic Self-Consistent (VPSC) model and the recently developed Grain-Fragmentation Visco-Plastic Self-Consistent (GF-VPSC) model. It is shown that the GF-VPSC model, incorporating the development of intragranular misorientations, provides a much better prediction of the texture strength compared to the standard VPSC model. The predictions of intragranular misorientation are also in good agreement with experimental measurements. Both experiments and simulations point to the importance of anisotropy of intragranular misorientation distributions in determining texture development and, importantly, texture strength.Graphical abstractGraphical abstract for this article
  • Dynamic Martensitic Phase Transformation in Single-crystal Silver
    • Abstract: Publication date: Available online 26 October 2019Source: Acta MaterialiaAuthor(s): Ramathasan Thevamaran, Claire Griesbach, Sadegh Yazdi, Mauricio Ponga, Hossein Alimadadi, Olawale Lawal, Seog-Jin Jeon, Edwin L. Thomas The ability to transform the crystal structure of metals in the solid state enables tailoring their physical, mechanical, electrical, thermal, and optical properties in unprecedented ways. We demonstrate a martensitic phase transformation from a face-centered-cubic (fcc) structure to a hexagonal-close-packed (hcp) structure that occurs in nanosecond timescale in initially near-defect-free single-crystal silver (Ag) microcubes impacted at supersonic velocities. Impact-induced high pressure and high strain rates in Ag microcubes cause impact orientation dependent extreme micro- and nano-structural transformations. When a microcube is impacted along the [100] crystal symmetry direction, the initial fcc structure transforms into an hcp crystal structure, while impact along the [110] direction does not produce phase transformations, suggesting the predominant role played by the stacking faults generated in the [100] impact. Molecular dynamics simulations at comparable high strain rates reveal the emergence of such stacking faults that coalesce, forming large hcp domains. The formation of hcp phase through the martensitic transformation of fcc Ag shows new potential to dramatically improve material properties of low-stacking-fault energy materials.Graphical abstractImage, graphical abstract
  • On solute depletion zones along grain boundaries during segregation
    • Abstract: Publication date: Available online 25 October 2019Source: Acta MaterialiaAuthor(s): D. Scheiber, T. Jechtl, J. Svoboda, F.D. Fischer, L. Romaner We propose a model for predicting the depletion zone arising next to grain boundaries during non-equilibrium segregation. The model directly links to distribution of segregation energies as provided by atomistic simulations. We expose the theoretical framework based on the thermodynamic extremal principle and propose an efficient algorithm to solve the underlying equations. The example of the W-25at%Re is discussed to illustrate the main features of the model. Multi-component segregation kinetics is discussed for segregation of B, C, and N in Mo to illustrate site-competition scenarios. Comparison with earlier results obtained without depletion illustrates the importance of this effects. Finally the depletion zones along GBs are investigated for many different compositions to asses for which material composition and heat treatments they may be observed experimentally. We find that extended depletion zones arise for very small solute concentrations.Graphical abstractImage, graphical abstract
  • Mesoscale Modeling of Jet Initiation Behavior and Microstructural
           Evolution during Cold Spray Single Particle Impact
    • Abstract: Publication date: Available online 25 October 2019Source: Acta MaterialiaAuthor(s): Sumit Suresh, Seok-Woo Lee, Mark Aindow, Harold D. Brody, Victor K. Champagne, Avinash M. Dongare Quasi-coarse-grained dynamics (QCGD) simulations are carried out to investigate the mesoscale deformation behavior during the impact of a 20 µm pure aluminum particle onto a substrate of pure aluminum at time and length scales relevant to cold spray deposition. A rigorous analysis of the evolution of pressure, temperature, strain, flow stress and microstructure is carried out to investigate the jetting mechanisms over a range of process parameters (impact velocity and particle temperature). The QCGD simulations identify a critical role of the pressure wave propagation in the initiation of a jet, i.e. outward flow of material at the particle/substrate interface periphery (edge). Jetting is observed to initiate when the shock wave interacts with the edge and results in localized softening of the metal in this region. This localized softening enables outward flow of the material and is accompanied by a release of the pressures in the particle and the substrate at the interface. Observations of final splat microstructures of systems that showed jetting revealed several new “small” grains in the range of 2-4 µm. These grains are mainly found at the interface, suggesting that recrystallization is favored in cold sprayed impacts of aluminum.Graphical abstractThe temporal evolution of pressure in a thin vertical section through the center of the particle is plotted in Figure 2 for an impact velocity of 1300 m/s, where “jetting” is observed. The contour levels are chosen to provide a clear visual inspection of the propagation of a compressive shock wave generated as shown in (a), interaction with the particle edge as shown in (b), if any, and its role in initiating a jet. In this case the shock wave arrives the particle edge at t = 3.6 ns as shown in Figure 2 (c), that results in outward flow of the materials and a particle jet is initiated.Image, graphical abstract
  • Multi-layered domain morphology in relaxor single crystals with
           nano-patterned composite electrode
    • Abstract: Publication date: Available online 24 October 2019Source: Acta MaterialiaAuthor(s): Chengtao Luo, Wei-Yi Chang, Min Gao, Chih-Hao Chang, Jiefang Li, Dwight Viehland, Jian Tian, Xiaoning Jiang (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystals, especially with compositions near its morphotropic phase boundary (MPB), have been employed for a broad range of applications such as ultrasound transducers, sensors, and actuators. To further enhance the properties of PMN-PT, electrode patterning, as a method of domain engineering, was proved to be an effective approach. In our previous report, a 200 nm grating pattern electrode (Ti/Au-MnOx) (nano-electrode) was prepared on one surface of PMN-PT crystal, exhibiting 30% d33 enhancement. In this work, the multi-layered domain morphology and the domain engineering from nano-electrode were characterized using piezoresponse force microscopy (PFM). A hypothetical domain engineering model for nano-electrodes is established to explain the experimental results as well as the property enhancement from the nano-electrode. The electrode patterning proves that the nano-scale modification can tune the macro-scale piezoelectric properties of the bulk material.Graphical abstractNano-patterned composite electrode (Nano-electrode), as a method of domain engineering, exhibits 30% d33 enhancement on (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystal. In this work, the mechanism of the property enhancement is studied through the multi-layered domain morphology by piezoresponse force microscopy (PFM). A hypothetical domain engineering model for nano-electrodes is established in the end. Image, graphical abstract
  • Microstructural Evolution in Amorphous-Nanocrystalline ZrCu Alloy under
           Neutron Irradiation
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Fan Xiong, Ming-Fei Li, Babafemi Malomo, Liang Yang An extensive investigation on the microstructural evolution of an amorphous-nanocrystalline alloy (ANA) under neutron irradiation has been conducted by molecular dynamics simulation. The phenomenon of rapid and full annihilation of irradiation-induced vacancies was found in the nanocrystal zone and after structural relaxation, free volumes in the amorphous matrix were systematically self-recovered. An effective self-healing behavior of the nanocrystal zone subsequently sufficed, regardless of the thermal degradation effect caused by the intensity of collision cascades during quenching. As knocked-on atoms were arrested at the phase boundary, it is self-evident that, the mechanism of atomic diffusion was non-existent at the interface between the nanocrystal grain and the neighbouring amorphous zone. Consequently, from the foregoing, ANA materials have been found to demonstrate excellent resistance to neutron irradiation and prospectively, the results of this study will potentially facilitate the development of advanced materials with high irradiation resistance.Graphical Image, graphical abstract
  • High electrostrictive strain in lead-free relaxors near the morphotropic
           phase boundary
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Tangyuan Li, Chang Liu, Xiaoqin Ke, Xiao Liu, Liqiang He, Peng Shi, Xiaobing Ren, Yunzhi Wang, Xiaojie Lou Thanks to its small hysteresis, large electrostrictive strain in relaxor ferroelectrics is superior than piezoelectric strain for applications in precision microactuators. Although relaxor ferroelectrics exhibit the largest electrostrictive strain in ceramics, the magnitude of the strain is limited to ∼0.20% at room temperature due to the large amount of non-ferroelectric defects existing in relaxors. In this work, we develop a relaxor with a morphotropic phase boundary (MPB) by doping a rhombohedral (R3m) ferroelectric BaZr0.2Ti0.8O3 into a tetragonal (P4mm) ferroelectric 0.89Bi0.5Na0.5TiO3-0.11BaTiO3. A high electrostrictive strain of 0.27% is achieved at room temperature in the relaxor sample. Experimental results illustrate that the composition is near the MPB and exhibits the existence of nanodomains, favoring the achievement of high electrostrictive strain. Moreover, phase field simulations show that the high electrostrictive strain obtained at this composition originates from the low defect fields needed to induce relaxor as a result of small polarization anisotropy at the phase boundary as compared to conventional relaxors away from phase boundaries. Our work provides a new design strategy for the next generation of high-performance ferroelectric relaxors.Graphical Image, graphical abstract
  • Modeling sintering anisotropy in ceramic stereolithography of silica
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Charles Manière, Gabriel Kerbart, Christelle Harnois, Sylvain Marinel In the domain of ceramic additive manufacturing, sintering is a key step for controlling the final shape and mechanical strength of a 3D object. The thermal treatment of the printed green objects has a high influence on the specimen density, debinding, and sintered microstructure. This work focuses on the shrinkage anisotropy phenomenon that occurs during sintering. We demonstrate by dilatometry and interrupted sintering microstructure analysis that this phenomenon originates from non-ideal particle packing between the printed layers, which generates an anisotropic porosity distribution at the mesoscale. Based on this, a sintering model is developed and specially adapted for the numerical prediction of the sintering anisotropy. This model is formulated in analytic equations that can easily identify all the model parameters and reproduce the experimental dimensional changes. This numerical tool can be of great assistance in the prediction of additive manufacturing object dimensional changes during sintering.Graphical Image, graphical abstract
  • Nanocalorimetry and Ab Initio Study of Ternary Elements in
           CuZr-based Shape Memory Alloy
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Yucong Miao, Ruben Villarreal, Anjana Talapatra, Raymundo Arróyave, Joost J. Vlassak We present a computational-experimental study on the ternary alloying effect of CuZr-based shape memory alloy. The transformation behavior, including crystallization, martensite-austenite transformation temperature and hysteresis of Cu-Zr-X (X = Ni, Co, Hf) thin-film samples were investigated by nanocalorimetry. We used ab initio simulations to determine the B2-Cm transformation pathway, evaluate the lattice parameters, the relative phase stability, and the twin boundary energy as a function of composition. Experimental results show that alloying with Ni or Co reduces the hysteresis of the martensitic transformation, while Hf increases it. These observations are in agreement with the trend of the middle eigenvalue of the martensitic transformation matrix. The energy difference between the pure phases obtained from simulations suggests that both Co and Ni stabilize martensite against austenite. However, experiments show that Co decreases transformation temperature, while Ni increases it. We attribute this observation to the larger twin boundary energy and strain energy in the Co-containing alloy. Our results indicate that ab initio simulations are a helpful tool in the development of new shape memory alloys, provided the energy terms associated with the fine twin structure of the martensite are taken into account.Graphical Image, graphical abstract
  • Amorphization and dislocation evolution mechanisms of single crystalline
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Zhonghuai Wu, Weidong Liu, Liangchi Zhang, Sean Lim The amorphization and dislocation evolution mechanisms of a single crystal 6H-SiC were systematically investigated by using nano-indentation, high-resolution transmitted electron microscope (HRTEM), molecular dynamics (MD) simulations and the generalized stacking fault (GSF) energy surface analysis. Two major plastic deformation mechanisms of 6H-SiC under nano-indentation were revealed by HRTEM, i.e., (1) an amorphization region near the residual indentation mark, and (2) dislocations below the amorphization region in both the basal and prismatic planes. MD results showed that the amorphization process corresponds to the first “pop-in” event of the indentation load-displacement curve, while the dislocation nucleation and propagation are related to the consequent “pop-in” events. The amorphization is confirmed to achieve via an initial transformation from wurtzite structure to an intermediate structure, and then a further amorphization process.Graphical Image, graphical abstract
  • Dislocation density distribution at slip band-grain boundary intersections
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Yi Guo, David M. Collins, Edmund Tarleton, Felix Hofmann, Angus J. Wilkinson, T. Ben Britton We study the mechanisms of slip transfer at a grain boundary, in titanium, using Differential Aperture X-ray Laue Micro-diffraction (DAXM). This 3D characterisation tool enables measurement of the full (9-component) Nye lattice curvature tensor and calculation of the density of geometrically necessary dislocations (GNDs). We observe dislocation pile-ups at a grain boundary, as the neighbour grain prohibits easy passage for dislocation transmission. This incompatibility results in local micro-plasticity within the slipping grain, near to where the slip planes intersect the grain boundary, and we observe bands of GNDs lying near the grain boundary. We observe that the distribution of GNDs can be significantly influenced by the formation of grain boundary ledges that serve as secondary dislocation sources. This observation highlights the non-continuum nature of polycrystal deformation and helps us understand the higher order complexity of grain boundary characteristics.Graphical abstractImage, graphical abstract
  • Effect of AC field on uniaxial viscosity and sintering stress of ceria
    • Abstract: Publication date: Available online 23 October 2019Source: Acta MaterialiaAuthor(s): Chen Cao, Robert Mücke, Olivier Guillon The production of traditional and advanced ceramics is an energy-intensive activity, which requires high temperatures and long dwelling times to activate diffusional processes necessary for densification. Electric field assisted processing has received considerable attention recently, due to its potential to significantly reduce the costs of required heat treatments. However, the effect of electric fields on the densification and coarsening of oxide ceramics still not completely understood, and the mechanisms behind, in particular for fields, are still under debate. The potential influence of electric field on the sintering parameters (uniaxial viscosity and uniaxial sintering stress) and microstructure of polycrystalline yttria doped ceria were studied. Sintering parameters were measured without and with AC electric fields (14 V/cm and 28 V/cm, 50 Hz) which were below the “flash regime”. During all sintering measurements, the sample temperature was adjusted by lowering the furnace temperature according to the temperature measurements using densified samples. Major findings are: (i) The densification behavior is clearly modified by these moderate electric fields, although temperature increase due to macroscopic Joule heating is excluded. (ii) The densification rate remains proportional to the applied stress under electrical fields. (iii) Sintering parameters are significantly affected by the applied electric fields.Graphical abstractImage, graphical abstract
  • Electric field compensation effect driven strain temperature stability
           enhancement in potassium sodium niobate ceramics
    • Abstract: Publication date: Available online 22 October 2019Source: Acta MaterialiaAuthor(s): Ting Zheng, Jiagang Wu In view of unsolved key scientific problems of (K,Na)NbO3 (KNN)-based ceramics, this work focuses on intrinsic structure elucidation and physical mechanism of temperature stability in both piezoelectricity d33 and strain property d33* through domain strategy and crystal structure engineering. The introduction of BiFeO3 into KNN-BNH can stabilize multiphase coexistence structure and then results in high performance (d33=400∼450 pC/N, TC=280∼320 °C, and Suni=0.16∼0.18%) with composition in-sensitivity (x=0∼0.6%). Different from the viewpoint of electric field induced diffuse phase transition, we first verified that electric field compensation effect can greatly offset the negative effects induced by polycrystalline phase transition and thermal depolarization, leading to much better temperature stability of d33* than that of d33. We believe that this work has clarified the origin of the difference in temperature stability between d33 and d33*, which can provide some useful clues for further improving the stability of KNN-based ceramics, especially d33 temperature stability for sensor and transducer applications.Graphical abstractImage, graphical abstract
  • Integration of the Noncollinear Antiferromagnetic Metal Mn3Sn onto
           Ferroelectric Oxides for Electric-Field Control
    • Abstract: Publication date: Available online 19 October 2019Source: Acta MaterialiaAuthor(s): Xiaoning Wang, Zexin Feng, Peixin Qin, Han Yan, Xiaorong Zhou, Huixin Guo, Zhaoguogang Leng, Weiqi Chen, Qiannan Jia, Zexiang Hu, Haojiang Wu, Xin Zhang, Chengbao Jiang, Zhiqi Liu Non-collinear antiferromagnetic materials have received dramatically increasing attention in the field of spintronics as their exotic topological features such as the Berry-curvature-induced anomalous Hall effect and possible magnetic Weyl states could be utilized in future topological antiferromagnetic spintronic devices. In this work, we report the successful integration of the antiferromagnetic metal Mn3Sn thin films onto ferroelectric oxide PMN-PT. By optimizing growth, we realized the large anomalous Hall effect with small switching magnetic fields of several tens mT fully comparable to those of bulk Mn3Sn single crystals, anisotropic magnetoresistance and negative parallel magnetoresistance in Mn3Sn thin films with antiferromagnetic order, which are similar to the signatures of the Weyl state in bulk Mn3Sn single crystals. More importantly, we found that the anomalous Hall effect in antiferromagnetic Mn3Sn thin films can be manipulated by electric fields applied onto the ferroelectric materials, thus demonstrating the feasibility of Mn3Sn-based topological spintronic devices operated in an ultralow power manner.Graphical abstractImage, graphical abstract
  • Unsupervised Learning of Dislocation Motion
    • Abstract: Publication date: Available online 14 October 2019Source: Acta MaterialiaAuthor(s): Darren C. Pagan, Thien Q. Phan, Jordan S. Weaver, Austin R. Benson, Armand J. Beaudoin The unsupervised learning technique, locally linear embedding (LLE), is applied to the analysis of X-ray diffraction data measured in-situ during the uniaxial plastic deformation of an additively manufactured nickel-based superalloy. With the aid of a physics-based material model, we find that the lower-dimensional coordinates determined using LLE appear to be physically significant and reflect the evolution of the defect densities that dictate strength and plastic flow behavior in the alloy. The implications of the findings for future constitutive model development are discussed, with a focus on wider applicability to microstructure evolution and phase transformation studies during in-situ materials processing.Graphical abstractGraphical abstract for this article
  • Hierarchically-structured large superelastic deformation in
    • Abstract: Publication date: Available online 14 October 2019Source: Acta MaterialiaAuthor(s): Yu Deng, Christoph Gammer, Jim Ciston, Peter Ercius, Colin Ophus, Karen Bustillo, Chengyu Song, Ruopeng Zhang, Di Wu, Youwei Du, Zhiqiang Chen, Hongliang Dong, Armen G. Khachaturyan, Andrew M. Minor Large superelastic deformation in ferroelastic-ferroelectrics (FMs) is a complex phenomenon involving multiple mechanisms operating simultaneously. Understanding how these mechanisms contribute corporately is critical to apply this useful property to the intrinsically brittle FMs, which can therefore display both excellent functional and mechanical performance. Here, we have directly observed and quantitatively analyzed in situ in a transmission electron microscope the three main mechanisms of twinning domain, phase transformation and mobile point defect contributing to extremely large superelastic deformation in single-crystal BaTiO3 (5.0% strain) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (10.1% strain). Our results reveal the hierarchical origin of large recoverable strain in “brittle” FMs.Graphical abstractImage, graphical abstract
  • Effects of 3d Electron Configurations on Helium Bubble Formation and Void
           Swelling in Concentrated Solid-Solution Alloys
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Yanwen Zhang, Xing Wang, Yuri N. Osetsky, Yang Tong, Robert Harrison, Stephen E. Donnelly, Di Chen, Yongqiang Wang, Hongbin Bei, Brian C. Sales, Karren L. More, Pengyuan Xiu, Lumin Wang, William J. Weber Elemental specific chemical complexity is known to play a critical role in microstructure development in single-phase concentrated solid-solution alloys (SP-CSAs), including both He bubble formation and irradiation-induced void swelling. While cavity formation and evolution under ion irradiation at elevated temperature are complex nonequilibrium processes, chemical effects are revealed at the level of electrons and atoms herein in a simplified picture, using Ni and a special set of Ni-based SP-CSAs composed of 3d transition metals as model alloys. Based on Ni and the model alloys with minimized variables (e.g., atomic mass, size, and lattice structure), we discuss the effects of chemically-biased energy dissipation, defect energetics, sluggish diffusion, and atomic transport on cavity formation and evolution under both self-ion Ni irradiation and He implantation. The observed difference in microstructure evolution is attributed to the effects of d electron interactions in their integrated ability to dissipate radiation energy. The demonstrated impact of alloying 3d transition metals with larger differences in the outermost electron counts suggests a simple design strategy for tuning defect properties to improve radiation tolerance in structural alloys.Graphical abstractImage, graphical abstract
  • Phase transformation assisted twinning in face-centered-cubic
           FeCrNiCoAl 0.36
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Peijun Yu, Rui Feng, Junping Du, Shuhei Shinzato, Jyh-Pin Chou, Bilin Chen, Yu-Chieh Lo, Peter K. Liaw, Shigenobu Ogata, Alice Hu The FeNiCoCr-based high entropy alloys (HEAs) exhibit excellent mechanical properties, such as twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP) that can reach a remarkable combination of strength and ductility. In this work, the face-centered-cubic (FCC) single-crystal FeNiCoCrAl0.36 HEAs were studied, using the density functional theory (DFT) combined with the phonon calculation to estimate the stacking fault energies, temperature-dependent phase stabilities of different structures. And the kinetic Monte Carlo (kMC) is used to predict the substructures evolution based on the transition state energies obtained from DFT calculations. We proposed two different formation paths of nano-twins in this Al-composited HEA and found that short-range hexagonal-close-packed (HCP)-stacking could occur in this HEA. The DFT calculations suggest that this HEA has negative stacking fault energy, HCP formation energy, and twin-formation energy at 0 K. Phonon calculations represent that at the finite temperature, the competing FCC/HCP phase stability and propensity for twinning make it possible to form HCP-like twin boundaries. The kMC simulations suggest that under deformation, TWINs could form within the HCP laths which differs from the study of others. With the great agreement of results from kMC simulations and experiments, this twin-hcp laminated substructure formation path offers a new concept of designing TWIP HEAs containing tunable twin structures with HCP and TWIN lamellae structures, which could result in better mechanical properties of HEAs.Graphical abstractImage, graphical abstract
  • Twinning and sequential kinking in lamellar Ti-6Al-4V alloy
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Xiaodong Zheng, Shijian Zheng, Jian Wang, Yingjie Ma, Hao Wang, Yangtao Zhou, Xiaohong Shao, Bo Zhang, Jiafeng Lei, Rui Yang, Xiuliang Ma Fully lamellar Ti-6Al-4V alloys comprise body-centered cubic (BCC) β lamellae in large-sized, hexagonal close-packed (HCP) α colonies and exhibit outstanding toughness. Although α/β interfaces are considered to play a key role in plastic deformation connected to the toughness, the interface effects have not been revealed so far. In this work, we studied underlying deformation mechanisms of interface-related deformation modes at an atomic scale. After the cyclic loading, {11¯02} deformation twins were observed in the vicinity of fatigue crack surfaces. Moreover, the α/β interface structures before and after cyclic loading deformation were characterized via transmission electron microscopy (TEM). The initial α/β interfaces can be described by the terrace ledge kink model, consisting of (011¯0)α (1¯21)β terrace plane and (1¯100)α (1¯01)β ledge plane. TEM investigations reveal that deformation twins nucleate at the α/β interface and the corresponding nucleation is ascribed to the dissociation of basal type dislocations. More importantly, these twins can continuously propagate through multiple β phase lamella. The continuous propagation of twinning is accomplished through double kinking mechanism. In this manner, twinning in α phases and sequential kinking in β phases can effectively release the stress intensification at the crack tip and dissipate plastic work/energy, correspondingly enhancing fracture toughness of fully lamellar Ti-6Al-4V.Graphical Image, graphical abstract
  • Ternary diagrams of the phase, optical bandgap energy and
           photoluminescence of mixed-halide perovskites
    • Abstract: Publication date: Available online 11 October 2019Source: Acta MaterialiaAuthor(s): Se-Yun Kim, Ho-Chang Lee, Yujin Nam, Yeonghun Yun, Si-Hong Lee, Dong Hoe Kim, Jun Hong Noh, Joon-Hyung Lee, Dae-Hwan Kim, Sangwook Lee, Young-Woo Heo Halide perovskites attract enormous attention as promising light absorption and emission materials for photovoltaics and optoelectronic applications. Here we report ternary diagrams of the phase, optical bandgap energy (Eg) and photoluminescence intensity of methylammonium lead halide (MAPbX3, where X = I, Br and Cl) perovskites, with three vertices of MAPbI3, MAPbBr3 and MAPbCl3. All the compositions were synthesized via a facile mechanochemical reaction at room temperature, to ensure the desired stoichiometries of the final products. Through structural study on MAPbX3, the phase diagram comprising a single phase region and two multi-phase regions was obtained. In the single phase region, the a-axis lattice constant increases almost linearly with increasing the average size of the X site ions. Interestingly, Eg decreases almost linearly with increasing the average size of the X site ions, giving negligible deviation from Vegard's law. As the result, a certain bandgap value, in the range of 1.55 - 2.9 eV, can be easily designed with infinite numbers of compositions. For the last, the ternary diagram of the photoluminescence intensity reveals the effective compositions for red, green and blue light emission. The comprehensive structural and optical information reported in this study is useful for designing halide perovskites for various applications. In addition, our approach for compositional mapping various characteristics using a solid-state reaction is an efficient and robust way to studying halide perovskites.Graphical abstractImage, graphical abstract
  • Insights into the fivefold symmetry of the amorphous Sb-based change
           materials in the rapid phase change from first principles
    • Abstract: Publication date: Available online 10 October 2019Source: Acta MaterialiaAuthor(s): Kewu Bai In materials science, it is widely believed that the fivefold symmetry in the amorphous phase will suppress crystallization because of its structural incompatibility with the crystal phase. The Sb-based phase change materials dominated by the fivefold rings, however, display the ultrafast growth-dominated phase change from amorphous to crystalline phase and exhibit significant property contrast consequently when they are heated by a laser or electrical pulse. To resolve the paradox, the long-time ab initio molecular dynamics calculation is carried out to simulate the crystallization cycle of the amorphous Ge15Sb85 under non-isothermal condition. The calculation results are in a reasonable agreement with experimental data. In particular, it is found that a transient phase state exists just before the crystallization onset temperature of the amorphous Ge15Sb85, in which the predominant fivefold rings in half-chair conformation undergo rapid conversion into the puckered sixfold rings. It is further demonstrated that such rings conversion via a route of the bond exchange model involves small atomic displacements and results in the structural motif similarity between the transient and the crystalline states. This thus reduces the crystal–amorphous interfacial energy promoting crystal nucleation consequently. The subsequent electronic calculations indicated that such conversion of the dominant rings in the amorphous Ge15Sb85 may be triggered by the increased pp orbital hybridizations and modulated by the diminished sp electron mixing. It is anticipated that the findings presented will provide a stepping-stone for the rational design of the Sb-based phase change materials.Graphical abstractImage, graphical abstract
  • Quantitative identification of constituent phases in a Nd-Fe-B-Cu sintered
           magnet and temperature dependent change of electron density of Nd2Fe14B
           studied by synchrotron X-ray diffraction
    • Abstract: Publication date: Available online 8 October 2019Source: Acta MaterialiaAuthor(s): Hiroyuki Okazaki, David Billington, Naruki Tsuji, Wakana Ueno, Yoshinori Kotani, Shogo Kawaguchi, Kunihisa Sugimoto, Kentaro Toyoki, Tomoki Fukagawa, Takeshi Nishiuchi, Kazuhiro Hono, Satoshi Hirosawa, Tetsuya Nakamura We have measured the temperature dependent XRD profiles of an isotropic Nd-Fe-B-Cu sintered magnet during the annealing process. Through Rietveld refinement, we demonstrate the changes in the volume fractions of a Nd2Fe14B main-phase and the other secondary phases as a function of increasing temperature up to 1047°C. The secondary phases mainly include dhcp-Nd, fcc-NdOx, and hcp-Nd2O3 at room temperature and fcc-Nd at elevated temperatures. The main phase starts melting above 900°C but remains relatively stable up to 600°C, while the dhcp-Nd phase completely disappears at around 600°C. Taking an advantage of the excellent quality of the XRD profiles in the powdered single crystal sample, we have also investigated the electron density distribution of main-phase by MEM/Rietveld analysis in order to elucidate the origin of the large magnetic anisotropy. The change in asymmetric part of electron density is derived by the subtraction of the electron density distributions between those recorded at -123 and -173°C, where the spin reorientation transition at -138°C changes the magnetic anisotropy. This change is attributed to be a difference of electron density distribution between Nd f and g sites of the Nd2Fe14B, relating to the magnetic anisotropy.Graphical abstractImage, graphical abstract
  • Collaborative ductile rupture mechanisms of high-purity copper identified
           by in situ X-ray computed tomography
    • Abstract: Publication date: Available online 5 October 2019Source: Acta MaterialiaAuthor(s): Brendan P. Croom, Helena Jin, Philip J. Noell, Brad L. Boyce, Xiaodong Li The competition between ductile rupture mechanisms in high-purity Cu and other metals is sensitive to the material composition and loading conditions, and subtle changes in the metal purity can lead to failure either by void coalescence or Orowan Alternating Slip (OAS). In situ X-ray computed tomography tensile tests on 99.999% purity Cu wires have revealed that the rupture process involves a sequence of damage events including shear localization; growth of micron-sized voids; and coalescence of microvoids into a central cavity prior to the catastrophic enlargement of the coalesced void via OAS. This analysis has shown that failure occurs in a collaborative rather than strictly competitive manner. In particular, strain localization along the shear band enhanced void nucleation and drove the primary coalescence event, and the size of the resulting cavity and consumption of voids ensured a transition to the OAS mechanism rather than continued void coalescence. Additionally, the tomograms identified examples of void coalescence and OAS growth of individual voids at all stages of the failure process, suggesting that the transition between the different mechanisms was sensitive to local damage features, and could be swayed by collaboration with other damage mechanisms. The competition between the different damage mechanisms is discussed in context of the material composition, the local damage history, and collaboration between the mechanisms.Graphical Image, graphical abstract
  • Misorientation Dependence Grain Boundary Complexions in <111>
           Symmetric Tilt Al Grain Boundaries
    • Abstract: Publication date: Available online 4 October 2019Source: Acta MaterialiaAuthor(s): Prakash Parajuli, David Romeu, Viwanou Hounkpati, Rubén Mendoza-Cruz, Jun Chen, Miguel José Yacamán, Jacob Flowers, Arturo Ponce Since polycrystalline alloys consist of a complex network of various types of grain boundaries (GBs), detailed atomic-scale analysis of how some impurities are distributed at every type of GBs is necessary to fully understand the implications of GB segregation on material’s performance. In this study, we present the atomic-scale structural combined with a chemical analysis of segregation induced GB complexions across the various types of Al alloy 7075 GBs using aberration-corrected microscopy and crystal orientation mapping assisted with precession electron diffraction. The result shows multilayer Cu GB segregation containing non-uniformly segregated mixed atomic columns across the interfaces. Two distinct types of Cu GB segregation behavior were observed, point and parallel array, analyzed by means of a displacement field obtained from the dichromatic pattern. Atomistic simulations were performed to test the energetic feasibility of the observed segregation behavior. As per the knowledge of the authors, this is the first report on experimental analysis of segregation induced periodic ordered structured GB complexions on Al alloy system. Furthermore, every GBs of the films were segregated uniquely forming ordered structures along the interface. The distance between two consecutive high segregated units was periodic for the point segregated GBs and followed a trend of a theoretical model of dislocation spacing. Based on the distance between two high segregated units, it is inferred that highly misorientated GBs are more segregated than low misoriented GBs. This study demonstrates that the misorientation between the neighboring grains significantly influences the segregation behavior across the interface and consequently, the structure of segregation-induced GB complexions.Graphical abstractImage 1
  • Mesoscale Characterization of Continuous Fiber Reinforced Composites
           Through Machine Learning: Fiber Chirality
    • Abstract: Publication date: Available online 4 October 2019Source: Acta MaterialiaAuthor(s): Samuel Sherman, Jeff Simmons, Craig Przybyla A method of quantifying fiber chirality, the twist of continuous fibers through a volume, is defined and applied to both phantom data and real data. Specifically, a field quantity termed the fiber chirality based on the anti-symmetric part of the gradient of the fiber orientation is introduced. For this method of estimation, the input is the set of fiber positions gathered from the stack of images which represent the sample volumes. The phantom sample is generated and several different real continuous fiber reinforced matrix composites are experimentally characterized. Each phantom dataset contains a bundle of fibers that rotate about a center axis with a user-defined angle at each step in the z-direction. The chirality of the fibers is calculated based on the pre-characterized positions using a machine learning algorithm. To validate the method of quantification, our chirality estimation method results in a colormap with an angle of rotation that becomes increasingly more similar to the user-defined angle with decreasing inter-slice distance. Fiber positions from real data are then input into the estimation method and the results are compared.Graphical abstractGraphical abstract for this article
  • Synergetic effects of solute and strain in biocompatible Zn-based and
           Mg-based alloys
    • Abstract: Publication date: Available online 4 October 2019Source: Acta MaterialiaAuthor(s): Y.Q. Guo, S.H. Zhang, I.J. Beyerlein, D. Legut, S.L. Shang, Z.K. Liu, R.F. Zhang Zn-based and Mg-based alloys have been considered highly promising biodegradable materials for cardiovascular stent applications due to their excellent biocompatibility and moderate in vitro degradation rates. However, their strength is too poor for use in cardiovascular stents. The strength of these metals can be related to the sizes of the dislocation cores and the threshold stresses needed to activate slip, i.e., the Peierls stress. Using density functional theory (DFT) and an ab initio-informed semi-discrete Peierls-Nabarro model, we investigate the coupled effect of the solute element and mechanical straining on the stacking fault energy, basal dislocation core structures and Peierls stresses in both Zn-based and Mg-based alloys. We consider several biocompatible solute elements, Li, Al, Mn, Fe, Cu, Mg and Zn, in the same atomic concentrations. The combined analysis here suggests some elements, like Fe, can potentially enhance strength in both Zn-based and Mg-based alloys, while other elements, like Li, can lead to opposing effects in Zn and Mg. We show that the effect of solute strengthening and longitudinal straining on SFEs is much stronger for the Zn-based alloys than for the Mg-based alloys. DFT investigations on electronic structure and bond lengths reveal a coupled chemical-mechanical effect of solute and strain on electronic polarization, charge transfer, and bonding strength, which can explain the weak mechanical effect on Zn-based alloys and the variable strengthening effect among these solutes. These findings can provide critical information needed in solute selection in Zn-based and Mg-based alloy design for biomedical applications.Graphical abstractImage, graphical abstract
  • Non-synchronized rotation of layered spin configurations in
           La0.825Sr0.175MnO3 /SrTiO3 film
    • Abstract: Publication date: Available online 3 October 2019Source: Acta MaterialiaAuthor(s): Xin Li, Jingzhi Han, Xiongzuo Zhang, Rui Wu, Yinfeng Zhang, Haidong Tian, Mingzhu Xue, Xin Wen, Zhichao Li, Shunquan Liu, Wenyun Yang, Changsheng Wang, Honglin Du, Xiaodong Zhang, Yingchang Yang, Jinbo Yang Magnetic properties of single perovskite-structure epitaxial film are reported to have layered distribution recently. However, the different responses of spin configurations in individual layers to the variation of temperature and strain state, especially the subtle response of layered film due to structural phase transformation of SrTiO3 (STO) substrate around 105 K, have not been revealed completely. Drastic drop and concomitant abnormal increase of remnant magnetic moments (REM) and coercivity of low-doped La1-xSrxMnO3 (x=0.175) /SrTiO3 film were observed around 105 K only in the in-plane direction, and layered magnetic structures were further inferred based on investigation of microstructure, strain distribution and chemical inhomogeneity. Abnormal change of magnetic properties around 105 K was discussed by a three-layer model, in which softer ferromagnetic layer was supposed to form in the middle layer of strain-induced layered structure, and the spin configuration of middle layer underwent non-synchronous transformation relative to other parts of the film, which may be attributed to the change of in-plane strain inside the film around 105 K. Our work reveals the variation of individual magnetic layers with the increase of temperature through utilizing the remnant magnetic field in measuring system, and the abrupt reversal of spin configurations at the intermediate layer can also serve the design of novel spintronics devices in the future.Graphical abstractImage, graphical abstract
  • Thermodynamic and structural evolution of mechanically milled and swift
           heavy ion irradiated Er2Ti2O7 pyrochlore
    • Abstract: Publication date: Available online 20 September 2019Source: Acta MaterialiaAuthor(s): Cheng-Kai Chung, Eric C. O'Quinn, Joerg C. Neuefeind, Antonio F. Fuentes, Hongwu Xu, Maik Lang, Alexandra Navrotsky Design and synthesis of thermodynamically metastable yet kinetically achievable materials possessing various desired functional and physical properties have recently drawn tremendous scientific-attention. In addition to conventional heat treatments and wet chemistry approaches, energy deposition into materials can induce unique nonequilibrium phases with distinct structures, chemistry, energetics, and properties. Mechanochemical synthesis and ion beam irradiation are two processing techniques that provide access to phases and states far from equilibrium. By a combination of high temperature oxide melt solution calorimetry, differential scanning calorimetry (DSC), neutron pair distribution function (PDF) analysis, and supplementary powder X-ray diffraction (XRD), the energetics and multiscale structural evolution on annealing of ball milled and swift heavy ion irradiated Er2Ti2O7 pyrochlore were investigated. Despite very similar structural modifications of local atomic arrangements and only minor differences in the long range structure, both types of damage yield significant difference in the energetics of the produced material. The energy of destabilization in the milled sample (70.2 ± 8.2 kJ/mol) is much less endothermic than that in the irradiated sample (457.3 ± 8.0 kJ/mol). The DSC profiles, supported by neutron scattering, X-ray diffraction, and solution calorimetry, reveal decoupled annealing events in different temperature ranges, separating crystallization of long range pyrochlore structure from annealing of short range weberite-like domains.Graphical Image, graphical abstract
  • First-principles and Machine Learning Predictions of Elasticity in
           Severely Lattice-distorted High-Entropy Alloys with Experimental
    • Abstract: Publication date: Available online 20 September 2019Source: Acta MaterialiaAuthor(s): George Kim, Haoyan Diao, Chanho Lee, A.T. Samaei, Tu Phan, Maarten de Jong, Ke An, Dong Ma, Peter K. Liaw, Wei Chen Stiffness usually increases with the lattice-distortion-induced strain, as observed in many nanostructures. Partly due to the size differences in the component elements, severe lattice distortion naturally exists in high entropy alloys (HEAs). The single-phase face-centered-cubic (FCC) Al0.3CoCrFeNi HEA, which has large size differences among its constituent elements, is an ideal system to study the relationship between the elastic properties and lattice distortion using a combined experimental and computational approach based on in-situ neutron-diffraction (ND) characterizations, and first-principles calculations. Analysis of the interatomic distance distributions from calculations of optimized special quasi random structure (SQS) found that the HEA has a high degree of lattice distortion. When the lattice distortion is explicitly considered, elastic properties calculated using SQS are in excellent agreement with experimental measurements for the HEA. The calculated elastic constant values are within 5% of the ND measurements. A comparison of calculations from the optimized SQS and the SQS with ideal lattice sites indicate that the lattice distortion results in the reduced stiffness. The optimized SQS has a bulk modulus of 177 GPa compared to the ideal lattice SQS with a bulk modulus of 194 GPa. Machine learning (ML) modeling is also implemented to explore the use of fast, and computationally efficient models for predicting the elastic moduli of HEAs. ML models trained on a large dataset of inorganic structures are shown to make accurate predictions of elastic properties for the HEA. The ML models also demonstrate the dependence of bulk and shear moduli on several material features which can act as guides for tuning elastic properties in HEAs.Graphical abstractImage, graphical abstract
  • The effect of large plastic deformation on elevated temperature mechanical
           behavior of dynamic strain aging Al-Mg alloys
    • Abstract: Publication date: Available online 20 September 2019Source: Acta MaterialiaAuthor(s): C. Meng, W. Hu, S. Sandlöbes, S. Korte-Kerzel, G. Gottstein The tensile behavior at elevated temperature of heavily plastically deformed Al-Mg alloys with an Mg content of 1 to 5% was investigated. Large plastic deformation was imposed by confined channel die pressing at room temperature up to 18 passes. During heating to test temperature the specimens were observed to undergo recovery and partly recrystallization. The strain rate sensitivity was found to increase with increasing test temperature, and the previously reported asymmetry of the stress differential upon instantaneous up and down strain rate changes disappeared at elevated temperatures. The transition temperature from the low temperature to the high temperature behavior depended on the amount of pre-deformation. The observed phenomena were attributed to a change in deformation mechanism which occurred the earlier, i.e. at lower temperature, the higher the dislocation density introduced by pre-deformation.Graphical abstractImage, graphical abstract
  • The Effects of Ultra-Fine-Grained Structure and Cryogenic Temperature on
           Adiabatic Shear Localization in Titanium
    • Abstract: Publication date: Available online 11 September 2019Source: Acta MaterialiaAuthor(s): Zezhou Li, Shiteng Zhao, Bingfeng Wang, Shuang Cui, Renkun Chen, Ruslan Z. Valiev, Marc A. Meyers The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic shear localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano-recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40 to 200 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.Graphical abstractImage 1
  • Helium-plasma–Induced Straight Nanofiber Growth on HCP Metals
    • Abstract: Publication date: Available online 3 October 2019Source: Acta MaterialiaAuthor(s): Shin Kajita, Tomohiro Nojima, Tatsuki Okuyama, Yuta Yamamoto, Naoaki Yoshida, Noriyasu Ohno Low-energy helium (He) plasma irradiations were conducted on ruthenium (Ru) and rhenium (Re), which have hexagonal close packed (HCP) crystal structures. Growth of linear shaped fiberform nanostructures were identified on the surfaces of the both metals after the He plasma irradiation. We also conducted He plasma irradiation while Re particles were deposited on tungsten substrate; 3-mm-thick large scale fiberform nanostructures were grown on the surface. The crystal orientation was analyzed using diffraction patterns of Re and Ru nanofibers together with detailed transmission electron microscope observations. It was found that the growth of linear nanofibers has a preferential crystal orientation in the growth direction and it is always in the c-direction of the HCP crystals. Potential growth processes and mechanisms are proposed based on the experimental observations.Graphical abstractGraphical abstract for this article
  • Ultra-broad Temperature Insensitive Ceramics with Large Piezoelectricity
           by Morphotropic Phase Boundary Design
    • Abstract: Publication date: Available online 2 October 2019Source: Acta MaterialiaAuthor(s): Haiyan Zhao, Yudong Hou, Xiaole Yu, Mupeng Zheng, Mankang ZhuABSTRACTImproving the operating characteristics of piezoelectric devices in high temperature environments urgently requires the development of piezoceramics with both high piezoelectric coefficient and excellent temperature stability. However, it is difficult for existing piezoceramics to take care of both at the same time. Generally, high piezoelectricity can be obtained at morphotropic phase boundary (MPB) in the binary systems, such as PbZrO3-PbTiO3 and Pb(Mg1/3Nb2/3)O3-PbTiO3, but the temperature stability is unsatisfactory, which seriously restricts the practical application. Here, an optimum composition having excellent comprehensive properties is constructed by designing multiple MPBs in the novel xPb(In1/2Nb1/2)O3-yBiScO3-zPbTiO3 ternary system. When x = 0.04, y = 0.345 and z = 0.615, the specimen has a high piezoelectric coefficient d33 of 478 pC/N at 200°C, meanwhile, the fluctuation of d33 is less than ± 10% over an ultra-broad temperature range of 50-350°C. Combined with a variety of in situ analysis techniques, it can be determined that the temperature-insensitive high piezoelectric coefficient is related to the multiple MPBs design, which is beneficial to the optimization of the hierarchical domain configuration. The developed phase boundary design strategy paves a new way to building next generation high performance high temperature piezoceramics.Graphical abstractImage, graphical abstract
  • Low temperature deformation of MoSi2 and the effect of Ta, Nb
           and Al as alloying elements
    • Abstract: Publication date: Available online 2 October 2019Source: Acta MaterialiaAuthor(s): Carolin Zenk, James S.K.-L. Gibson, Verena Maier-Kiener, Steffen Neumeier, Mathias Göken, Sandra Korte-Kerzel Molybdenum disilicide (MoSi2) is a very promising material for high temperature structural applications due to its high melting point (2030∘C), low density, high thermal conductivity and good oxidation resistance. However, MoSi2 has limited ductility below 900∘C due to its anisotropic plastic deformation behaviour and high critical resolved shear stresses on particular slip systems.Nanoindentation of MoSi2 microalloyed with aluminium, niobium or tantalum showed that all alloying elements cause a decrease in hardness. Analysis of surface slip lines indicated the activation of the additional {1 1 0} slip system in microalloyed MoSi2, which is not active below 300∘C in pure MoSi2. This was confirmed by TEM dislocation analysis of the indentation plastic zone. Further micropillar compression experiments comparing pure MoSi2 and the Ta-alloyed sample enabled the determination of the critical resolved shear stresses of individual slip systems even in the most brittle [0 0 1] crystal direction.Graphical abstractImage 1
  • On the high creep strength of the W containing IRIS-TiAl alloy at
    • Abstract: Publication date: Available online 1 October 2019Source: Acta MaterialiaAuthor(s): Alain Couret, Jean-Philippe Monchoux, Daniel Caillard This paper presents a study of the creep at 850°C under 150 MPa of the IRIS alloy (Ti-Al48-W2-B0.1) densified by spark plasma sintering. The dislocation microstructure in a sample strained up to 1.5% was studied by post-mortem transmission electron microscopy. The deformation is mainly due to ordinary dislocations. Several populations of dislocations are evidenced. Their Burgers vectors, the plane in which they are moving and the corresponding deformation mechanisms are determined. In the discussion section, the deformation mechanisms, the factors controlling their activation and the role of tungsten as hardening element are examined.Graphical abstractImage, graphical abstract
  • Understanding solid solution strengthening at elevated temperatures in a
           creep-resistant Mg-Gd-Ca alloy
    • Abstract: Publication date: Available online 1 October 2019Source: Acta MaterialiaAuthor(s): Ning Mo, Ingrid McCarroll, Qiyang Tan, Anna Ceguerra, Ying Liu, Julie Cairney, Hajo Dieringa, Yuanding Huang, Bin Jiang, Fusheng Pan, Michael Bermingham, Ming-Xing Zhang The present work studies the strengthening mechanisms of a creep-resistant Mg-0.5Gd-1.2Ca (at.%) alloy at both room and elevated temperatures. Although peak-ageing (T6) at 180 ∘C for 32 h led to a significant increase in room temperature strength due to the precipitation strengthening by three types of precipitates (Mg2Ca, Mg5Gd on prismatic planes and a new type of Mg-Gd-Ca intermetallic compound on the basal plane), the as-solid solution treated (T4) alloy exhibited better resistance to temperature softening during compression and to stress relaxation at 180 °C and better creep resistance at 210 °C/100 MPa. The Gd-Ca co-clusters with short-range order in the Mg solid solution, which was verified, at the first time, by atom probe tomography (APT) analysis and atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), were responsible for the solid solution hardening, offering a more effective strengthening effect through local order-strengthening. Such solid solution strengthening increased the thermal stability of the alloy structure at elevated temperatures, at least at early stage of the creep. Subsequently, dynamic precipitation started contributing to the creep resistance due to the formation of higher density of precipitates. However, in the T6 alloy, creep testing at elevated temperatures, particularly at 210 °C that was higher than the ageing temperature, led to coarsening of the precipitates, which acted as over ageing. As a result of such over ageing, the resistance of the T6 alloy to heat-induced softening was weakened, leading to lower creep resistance than the T4 alloy.Graphical abstractImage, graphical abstract
  • Effects of Zn and Cr Additions on Precipitation and Creep Behavior of a
           Dilute Al-Zr-Er-Si Alloy
    • Abstract: Publication date: Available online 1 October 2019Source: Acta MaterialiaAuthor(s): Richard A. Michi, Jacques Perrin Toinin, Amir R. Farkoosh, David N. Seidman, David C. Dunand The effects of adding 0.08 at.% Cr or 1 at.% Zn to a dilute Al-0.11Zr-0.005Er-0.02Si at.% alloy are studied in terms of the precipitation behavior of Al3Zr (L12-structure) nanoprecipitates and the resulting alloy's creep resistance. Although Cr and Zn additions do not affect measurably the precipitation kinetics or coarsening resistance, the modified alloys exhibit changes in dislocation creep resistance at 300°C: the creep threshold stress is decreased by 2 MPa in the Zn-modified alloy and increased by 3 MPa in the Cr-modified alloy. This is attributed to a modification of the lattice parameter of Al3Zr(L12), which affects the ease with which matrix dislocations climb over the nanoprecipitates. The Zn-modified alloy exhibits Al3Zr(L12) nanoprecipitates containing 6–7 at.% Zn, as determined by atom-probe tomography, which reduces the lattice parameter by 0.17% as a result of Zn substituting for Al on its sublattice, as calculated utilizing density functional theory. The Al3Zr(L12) nanoprecipitates in the Cr-modified alloy contain 0.10–0.20 at.% Cr (1.2 –2.5 times more than the matrix) and 0.28–0.51 at.% Er (a 60- to 100-fold enrichment); the Er increases the lattice parameter misfit of Al3Zr(L12), with the Al matrix. Erbium is confirmed to be particularly potent in increasing the creep resistance of aluminum alloys containing L12 nanoprecipitates. An alloy design methodology for creep resistance is also validated, whereby precipitate compositions measured by APT are input to DFT calculations to determine their effects on lattice parameter misfit.Graphical Image, graphical abstract
  • On distances between grain interfaces in macroscopic parameter space
    • Abstract: Publication date: Available online 30 September 2019Source: Acta MaterialiaAuthor(s): A. Morawiec There is a considerable activity in acquisition of large data sets of macroscopic grain boundary parameters. Analysis of these sets requires grouping boundaries with similar parameters, and thus one needs a measure of similarity or a distance between interfaces. A number of such functions have been previously defined. Interface distances are constructed in two steps: first, an underlying metric is devised, and then it is modified to account for equivalences between points in the space of macroscopic parameters. The paper characterizes and compares the known underlying distance functions. Features important for a function to be applicable as the interface distance are considered. The main aspects are the invariance with respect to symmetries in the parameter space and the property of metric intrinsicality which is essential for analysis of interface distributions. Also ways of devising alternative metrics are described, and suggestions for choosing a standard metric which may serve in conventional computations are given. Results of the metric characterization are of importance for laying out foundations of the statistical analysis of macroscopic grain boundary data.Graphical abstractGraphical abstract for this article
  • A eutectic dual-phase design towards superior mechanical properties of
           heusler-type ferromagnetic shape memory alloys
    • Abstract: Publication date: Available online 30 September 2019Source: Acta MaterialiaAuthor(s): Zhigang Wu, Zhiwen Liang, Yajiu Zhang, Zhuhong Liu, Junsong Zhang, Fakhrodin Motazedian, Sam Bakhtiari, Bashir Samsam Shariat, Yinong Liu, Yang Ren, Hong Yang Heusler-type ferromagnetic shape memory alloys possess attractive multifunctional properties, including magnetic field induced shape memory effect, magnetoresistance and magnetocaloric effect, owing to the unique concurrent magnetic and martensitic transformations. However, these intermetallics generally exhibit intrinsic high brittleness and low strength, which severely impede their workability for processing and applicability in real use. In this study, we demonstrate a new grain refining strategy by means of eutectic solidification to improve the mechanical properties in Ni-Mn-Sn-Fe alloys. In a fully eutectic microstructure, the average γ lamellae thickness was refined to ∼170 nm and the composite showed a compressive strength of 1950 MPa, ductility of 19.5%, Young's Modulus of 38 GPa, pseudoelasticity of 3.2% and high mechanical cyclic stability. The high mechanical performance is attributed to the effect of departmentalization of the brittle Heusler alloy by the densely distributed γ phase fine lamellae in resisting crack propagation. The eutectic Heusler composite exhibited a metamagnetic phase transformation, with a magnetic entropy change of 10.2 J/kg•K and a refrigeration capacity of 168 J/kg in a field change of 5 T.Graphical abstractImage, graphical abstract
  • Experimental observations of amorphization in stoichiometric and
           boron-rich boron carbide
    • Abstract: Publication date: Available online 30 September 2019Source: Acta MaterialiaAuthor(s): Ankur Chauhan, Mark C. Schaefer, Richard A. Haber, Kevin J. Hemker Boron carbide is extremely hard but has been shown to undergo stress-induced amorphization when subjected to large nonhydrostatic stresses. This localized amorphization has been associated with the sudden loss of shear strength and poor ballistic performance. Recent quantum mechanics predictions suggest that boron-enrichment may be used to mitigate amorphization in boron carbide. As a means to test this hypothesis, stoichiometric boron carbide (nominally B4C) and a novel composition of B-rich boron carbide (nominally B6.3C) were investigated. Nanoindentation followed by Raman spectroscopy revealed an obvious reduction in the Raman peaks associated with amorphization in the B-rich material. Transmission electron microscopy observations of the region below the nanoindents facilitated direct observation of amorphization, confirmed the Raman finding that amorphization is reduced in the B-rich specimens, and provided additional insight into deformation mechanisms. It is surmised that boron-rich alloys offer a path to reducing local amorphization in boron carbide.Graphical abstractImage, graphical abstract
  • Development of strong and ductile metastable face-centered cubic
           single-phase high-entropy alloys
    • Abstract: Publication date: Available online 29 September 2019Source: Acta MaterialiaAuthor(s): Daixiu Wei, Xiaoqing Li, Stephan Schönecker, Jing Jiang, Won-Mi Choi, Byeong-Joo Lee, Hyoung Seop Kim, Akihiko Chiba, Hidemi Kato Face-centered cubic (fcc)-phase high-entropy alloys (HEAs) have attracted much academic interest, with the stacking fault energy (SFE) playing an important role in regulating their mechanical behaviors. Here, we revealed the principles for regulating both the elastic and plastic behaviors by composition modification and Mo addition in an fcc-phase quaternary CoCrFeNi system with the assistance of ab initio and thermodynamics calculations. An increase in Co content and a decrease in Fe and Ni contents reduced the fcc phase stability and SFE, but enhanced the elastic modulus, anisotropy, and lattice friction stress. A minor substitution of Co by Mo increased the lattice constant, but decreased the SFE and elastic modulus. Based on these findings, we developed a series of strong and ductile metastable fcc-phase CoxCr25(FeNi)70-xMo5 (x= 30, 40, 50) HEAs with mechanical properties superior to those of the CoCrFeNi HEAs. The careful investigation revealed that the enhanced mechanical properties are due to the Mo-addition-induced strengthening accompanied with a low-SFE-induced restriction of planar behavior of dislocations, mechanical twinning, and strain-induced martensitic transformation. The findings shed light on the development of high-performance HEAs.Graphical abstractImage, graphical abstract
  • Oxygen effects on ω and α phase transformations in a metastable
           β Ti-Nb alloy
    • Abstract: Publication date: Available online 29 September 2019Source: Acta MaterialiaAuthor(s): Kathleen Chou, Emmanuelle A. Marquis Oxygen is known to have substantial influence on metastable β titanium alloys through martensite suppression and phase stability changes that significantly affect mechanical behavior. Here, we have investigated the influence of oxygen in solid solution on ω and α precipitation during ageing in a metastable β-type Ti-20Nb atomic (at.) % alloy with up to about 5 at. % O obtained through an oxidation exposure. Ageing results show that elevated oxygen induced a shape change for ω precipitates from an ellipsoid shape to an elongated rod shape and resulted in a higher ω number density. Additionally, the growth rate of ω precipitates was slowed with oxygen. Oxygen partitioned to the ω phase during ageing and was shown to expand the region of ω phase stability to higher temperatures, suggesting that oxygen increases ω phase stability. Prolonged ageing revealed that α eventually nucleated at all oxygen levels. However, the rate of α precipitation depended on oxygen content, and the slowest rate was observed with intermediate levels of oxygen (∼2-3 at. %) compared to elevated and minimal levels. A mechanism for this non-linear effect on α precipitation is discussed based on oxygen acting as both an ω-stabilizer and α-stabilizer in β titanium alloys.Graphical Image, graphical abstract
  • A phase-field model for hydride formation in polycrystalline metals:
           Application to δ-hydride in zirconium alloys
    • Abstract: Publication date: Available online 28 September 2019Source: Acta MaterialiaAuthor(s): Tae Wook Heo, Kimberly B. Colas, Arthur T. Motta, Long-Qing Chen We report a phase-field model for simulating metal hydride formation involving large volume expansion in single- and polycrystals. As an example, we consider δ-hydride formation in α-zirconium (Zr), which involves both displacive crystallographic structural change and hydrogen diffusion process. Thermodynamic Gibbs energy functions are extracted from the available thermodynamic database based on the sublattice model for the interstitial solid solutions. Solute-grain boundary interactions and inhomogeneous elasticity of polycrystals are taken into consideration within the context of diffuse-interface description. The stress-free transformation strains of multiple variants for hcp-Zr (α) to fcc-hydride (δ) transformation are derived based on the well-established orientation relationship between the α and δ phases as well as the corresponding temperature-dependent lattice parameters. In particular, to account for the large volume expansion, we introduced the mixed interfacial coherency concept between those phases—basal planes are coherent and prismatic planes are semi-coherent in computing the strain energy contribution to the thermodynamics. We analyzed the morphological characteristics of hydrides involving multiple structural variants and their interactions with grain boundaries. Moreover, our simulation study allows for the exploration of the possible hydride re-orientation mechanisms when precipitating under applied tensile load, taking into account the variation in the interfacial coherency between hydrides and matrix, their elastic interactions with the applied stress, as well as their morphology-dependent interactions with grain boundaries. The phase-field model presented here is generally applicable to hydride formation in any binary metal-hydrogen systems.Graphical abstractImage, graphical abstract
  • The mechanical response of a α 2(Ti3Al) + γ(TiAl)-submicron grained
           Al2O3 cermet under dynamic compression: modeling and experiment
    • Abstract: Publication date: Available online 28 September 2019Source: Acta MaterialiaAuthor(s): B. Amirian, HY. Li, J.D. Hogan Novel experimental data, obtained using an advanced digital image correlation technique coupled to ultra-high-speed photography, have been used to develop and validate a microstructure-dependent constitutive model for a α2(Ti3Al) + γ(TiAl)-submicron grained Al2O3 cermet. Utilizing experimental characterization for important simulation inputs (e.g., microstructural features size, constituent stiffness), the numerical model makes use of a variational form of the Gurson model, based on the nonlinear homogenization approach, to account for the experimentally observed deformation features in this composite (e.g., void deformation and growth, particle fracture). By considering the variability in microstructural features (e.g., particle shape, size, and aspect ratio), as well as densely packed ceramic particles, the proposed model is evaluated by comparing the numerical responses to experimental results for quasi-static and dynamic stress-strain behavior of the material. The results show that the proposed approach is able to accurately predict the mechanical response and deformation of the microstructure. Once validated, the model is expanded for studying the predominant damage mechanisms in this material, as well as determining important mechanical response features such as transitional strain rates, flow stress hardening, extensive flow softening, and energy absorbing efficiency of the material as a function of void and particle volume fraction under high strain rate loading. The totality of this work opens promising avenues for qualitative (damage micromechanisms) and quantitative (stress-strain curve) understanding of ceramic-metal composites under various loading conditions, and offer insights for designing and optimizing cermet microstructures.Graphical abstractGraphical abstract for this article
  • Grain Boundary Serration in Nickel Alloy Inconel 600: Quantification and
    • Abstract: Publication date: Available online 26 September 2019Source: Acta MaterialiaAuthor(s): Yuanbo T. Tang, Phani Karamched, Junliang Liu, Jack C. Haley, Roger C. Reed, Angus J. Wilkinson The serration of grain boundaries in Inconel 600 caused by heat treatment is studied systematically. A new method based on Fourier transforms is used to analyse the multiple wave-like character of the serrated grain boundaries. A new metric – the serration index – is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between serrated and non-serrated boundaries. By considering the variation of the serration index with processing parameters, a causal relationship between degree of serration and solution treatment/cooling rate is elucidated. Processing maps for the degree of serration are presented. Two distinct formation mechanisms arise which rely upon grain boundary interaction with carbides: (i) Zener-type dragging which hinders grain boundary migration and (ii) a faceted carbide growth-induced serration.Graphical abstractGraphical abstract for this article
  • Interaction of precipitation with austenite-to-ferrite phase
           transformation in vanadium micro-alloyed steels
    • Abstract: Publication date: Available online 26 September 2019Source: Acta MaterialiaAuthor(s): Chrysoula Ioannidou, Zaloa Arechabaleta, Alfonso Navarro-López, Arjan Rijkenberg, Robert M. Dalgliesh, Sebastian Kölling, Vitaliy Bliznuk, Catherine Pappas, Jilt Sietsma, Ad A. van Well, S. Erik Offerman The precipitation kinetics of vanadium carbides and its interaction with the austenite-to-ferrite phase transformation is studied in two micro-alloyed steels that differ in vanadium and carbon concentrations by a factor of two, but have the same vanadium-to-carbon atomic ratio of 1:1. Dilatometry is used for heat-treating the specimens and studying the phase transformation kinetics during annealing at isothermal holding temperatures of 900, 750 and 650°C for up to 10 h. Small-Angle Neutron Scattering (SANS) and Atom Probe Tomography (APT) measurements are performed to study the vanadium carbide precipitation kinetics. Vanadium carbide precipitation is not observed after annealing for 10 h at 900 and 750°C, which is contrary to predictions from thermodynamic equilibrium calculations. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650°C. The precipitate volume fraction and mean radius continuously increase as holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the austenite-to-ferrite phase transformation is finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth with soft impingement (overlapping diffusion fields) and coarsening take place. APT shows gradual changes in the precipitate chemical composition during annealing at 650°C, which finally reaches a 1:1 atomic ratio of vanadium-to-carbon in the core of the precipitates after 10 h.Graphical Image, graphical abstract
  • Controlling surface morphology by nanocrystalline/amorphous competitive
           self-phase separation in thin films: thickness-modulated reflectance and
           interference phenomena
    • Abstract: Publication date: Available online 26 September 2019Source: Acta MaterialiaAuthor(s): A. Borroto, S. Bruyère, S. Migot, J.F. Pierson, T. Gries, F. Mücklich, D. Horwat Controlling surface morphology is a key issue for obtaining functional materials with surface-based properties. In this paper, we explore the possibility of using the self-separation of phases as a way of controlling the surface morphology features. We demonstrate using X-ray diffraction and transmission electron microscopy that a competitive self-separation of a nanocrystalline and an amorphous phases occurs in co-sputtered Zr-Mo thin films with a Mo content of 60 at%, corresponding to a composition intermediate to those necessary to form single-phased amorphous and nanocrystalline films. The dependence of the residual stress with the thickness at the biphased composition is discussed in terms of the morphology evolution and a possible mechanism for the self-separation of phases is presented. We show that the self-separation of phases as presented here is not limited to Zr-Mo alloys and can be extended to other systems. By changing the film thickness, it is possible to change the surface morphology of the films at the biphasic composition, due to the competitive growth of the nanocrystalline phase in the amorphous phase. In this way, it was possible to control the surface roughness and, because of this, tuning the film reflectance at a determined wavelength. The occurrence of an interference pattern in the reflectance spectra was discussed and associated to the presence of two different height levels at the film surface.Graphical abstractGraphical abstract for this article
  • Transition from ductilizing to hardening in Tungsten: the dependence on
           Rhenium distribution
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Yu-Hao Li, Hong-Bo Zhou, Linyun Liang, Ning Gao, Huiqiu Deng, Fei Gao, Gang Lu, Guang-Hong Lu Mechanical responses of tungsten (W) and its alloys are strongly controlled by the properties of 1/2 screw dislocations. Rhenium (Re), as a typical alloying and transmutation element in W, can substantially modify the properties of the dislocations, thus the plasticity of the materials. In this study, we investigate the interaction of Re and Re clusters with the screw dislocations in W by first-principles calculations in combination with theoretical models. Specifically, we propose two competing and Re-distribution dependent mechanisms, i.e. “ductilizing effect” and “hardening effect”; both are crucial to the mechanical properties of W. For the ductilizing effect, dispersed Re atoms weaken the surrounding interatomic interaction and reduce the shear resistance, thus facilitating the motion of the dislocation. In contrast, for the hardening effect, Re clusters formed by aggregated Re atoms due to irradiation can increase the Peierls stress and energy, thus hindering the motion of the dislocations. The proposed mechanisms shed light on the experimental observations that there is a Re-induced transition from ductilizing to hardening due to irradiation. The current work provides a theoretical guidance to the development of W-based future fusion materials in search of ductilizing alloying elements.Graphical abstractImage, graphical abstract
  • High Temperature Stability and Mechanical Quality Factor of Donor-acceptor
           Co-doped BaTiO3 Piezoelectrics
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Ruixuan Song, Yu Zhao, Weili Li, Yang Yu, Jie Sheng, Ze Li, Yulei Zhang, Hetian Xia, Wei-Dong Fei Low temperature stability has limited the applications at elevated temperature for ABO3-type lead-free ceramics. And multi-grade resonances of piezoelectric ceramics are hardly obtained although the resonances are very important in some applications such as filter and resonator. High piezoelectric properties and large mechanical quality factors with multi-grade resonances can be obtained in (Li+-La3+) co-doped BaTiO3 ceramics, and excellent temperature stabilities are achieved in the ceramics. It has been shown that the thermal treatments both with and without electric field at 200 °C improve the piezoelectric properties and thermal stability further. Large piezoelectric constant (272 pC N−1) and huge mechanical quality factor (2010) was obtained after thermal-electrical treatment, which is caused by Li+-La3+ ionic pair aligning along the external electric filed during thermal-electrical treatment. Moreover, the present study provides an effective method to design combination properties with high temperature stability for ABO3 perovskite ferroelectric ceramics.Graphical Image, graphical abstract
  • Electron beam induced rejuvenation in a metallic glass film during in-situ
           TEM tensile straining
    • Abstract: Publication date: Available online 25 September 2019Source: Acta MaterialiaAuthor(s): Christian Ebner, Jagannathan Rajagopalan, Christina Lekka, Christian Rentenberger Rejuvenation of an amorphous TiAl thin film under external tensile stress by high energy electron irradiation is observed via in-situ transmission electron microscopy (TEM). Electron beam (e-beam) irradiation results in a characteristic change of the elastic properties over time, as measured by the atomic-level elastic strain contained in the TEM diffraction pattern. Specifically, a time dependent increase/decrease of elastic strain is observed along the tensile direction, the saturation value of which correlates linearly with the preceding stress increment/decrement but shows little dependence on the e-beam condition. The low sensitivity of the saturation value to the e-beam condition indicates that the elastic strain change is induced by the structural transitions of a population (dependent on stress increment/decrement) of unstable atomic configurations to local, elastically soft areas. Classical molecular dynamics (MD) simulations including high energy electron scattering events are performed under tensile load to obtain insights into the structural modification that leads to time dependent changes in elastic strain under irradiation. The simulations reveal a change in quantities that are characteristic of structural rejuvenation, with a reduction of the local shear modulus manifesting as time dependent increase in the atomic-level elastic strain at fixed external stress. This link to the experimental data is confirmed by tracking elliptic distortions of simulated diffraction patterns calculated from MD configurations. The presented findings are highly relevant for experimental characterization of amorphous materials using TEM and give a new perspective on local structural modifications by electron irradiation.Graphical abstractGraphical abstract for this article
  • Tracking pores during solidification of a Ni-based superalloy using 4D
           synchrotron microtomography.
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Emeric. Plancher, Pauline Gravier, Edouard Chauvet, Jean-Jacques Blandin, Elodie Boller, Guilhem Martin, Luc Salvo, Pierre Lhuissier Time-resolved in situ microtomography is employed to track the nucleation and growth of individual pores during solidification of a commercial nickel-based superalloy. Three cooling rates (0.1, 0.5 and 1°C/s) are investigated to evaluate the effect of this key processing parameter on the formation of porosity. Phase contrast obtained with a coherent X-ray beam is used to visualize the evolution of dendritic structures in absence of a sufficient absorption contrast. Two mechanisms leading to shrinkage pores have been identified. The first mechanism (mechanism A) is associated with the coalescence of secondary dendrite arms at temperature during the early stages of solidification. The second mechanism (mechanism B) is related to insufficient liquid feeding in the interdendritic region during the last stages of solidification, at lower temperatures. A variation of cooling rate by a factor 2 does not affect the nucleation rate of pores generated through mechanism B. However, it seems to affect the nucleation rate of small pores obtained through the mechanism A. The kinetics of growth for the majority of individual pores can be described using an exponential-like function. This kinetics is faster for mechanism B compared to mechanism A.Graphical abstractImage, graphical abstract
  • Compression-compression fatigue behaviour of Gyroid-type Triply Periodic
           Minimal Surface porous structures fabricated by Selective Laser Melting
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Lei Yang, Chunze Yan, Wenchao Cao, Zhufeng Liu, Bo Song, Shifeng Wen, Cong Zhang, Yusheng Shi, Shoufeng Yang Triply Periodic Minimal Surface (TPMS) porous structures are recognized as the most promising bionic artificial structures for tissue engineering. The fatigue properties of additive manufactured porous structures are essential for long-term use in a dynamical bio-skeletal environment. The aim of this study is to study the compression-compression fatigue behaviour and the underlying fatigue mechanism of Gyroid cellular structures (GCS), a typical TPMS porous structure. The high-cycle fatigue results show that both cyclic ratcheting and fatigue damage phenomena contribute to the failure of GCS during fatigue testing. For most fatigue loading stress, the failure samples have nearly 45° fracture bands along the diagonal surface. The fatigue ratio of GCS reaches 0.35 for as-built samples and can be raised to 0.45 after sandblasting treatment. The fatigue ratio values are higher than most of the other bending-dominated lattice structures, suggesting superior fatigue resistance properties of GCSs due to the smooth surface connection between struts. Besides, a systematic investigation of the crack initiation and propagation was conducted by both deformation analysis and finite element method to support experimental phenomena. The results also indicate that the fatigue resistance properties of GCSs are significantly enhanced by sandblasting post-treatment, through removing the adhered powder particles, inducing compressive residual stress on the surface and generating a nanocrystalline zone.Graphical abstractImage, graphical abstract
  • Interfacial origins of visible-light photocatalytic activity in ZnS-GaP
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Paria Sadat Musavi Gharavi, Lin Xie, Richard Francis Webster, Collin Keon Young Park, Yun Hau Ng, Jiaqing He, Judy Nancy Hart, Nagarajan Valanoor The origins of recently reported visible-light photoelectrochemical activity in ZnS-GaP (ZG) multilayer films are investigated using aberration-corrected scanning transmission electron microscopy (STEM). It is revealed that the multilayers carry a large volume fraction of defects, specifically stacking faults and twins, at the interfaces. The defects act as excellent channels for diffusion. For each ZG interface, a ∼5 nm-interdiffused region with an effective chemical composition of a ZnS-GaP solid solution is observed. Previous theoretical calculations have found that ZnS-GaP solid solutions possess a lower band gap than either GaP or ZnS and thus are expected to have better visible-light photo-activity. These findings are thus able to explain the observed commensurate increase in the visible-light photoelectrochemical response with increasing number of ZG layers. This work suggests that interfaces with intentionally designed lattice imperfections and/or intentionally driven interdiffusion leading to local solid solution formation provide a new materials design strategy for achieving efficient visible-light photo-activity.Graphical abstractImage, graphical abstract
  • Dissecting the influence of nanoscale concentration modulation on
           martensitic transformation in multifunctional alloys
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): Jiaming Zhu, Hong-Hui Wu, Xu-Sheng Yang, He Huang, Tong-Yi Zhang, Yunzhi Wang, San-Qiang Shi Nanoscale concentration modulation (CM) is a novel and effective approach of manipulating martensitic transformations (MTs) for developing next-generation high-performance shape memory alloys (SMAs). Spinodal decomposition is one of the most economic methods to obtain bulk compositionally modulated materials for practical applications. The wavelength, amplitude, and statistical distribution of CM generated by spinodal decomposition are tunable via adjusting the ageing temperature, or the ageing time. However, how these features influence the effect of CM on MTs still remains largely unexplored. In this study, theoretical analyses and computer simulations are combined to dissect the influence of these features on the kinetic process of MTs and mechanical properties of SMAs. The findings of this study provide insights and guidance on the design of SMAs for desired mechanical properties via CM engineering. Moreover, the findings are applicable to not only SMAs but also other materials that have MTs, e.g. steels and high-entropy alloys.Graphic abstractImage, graphical abstract
  • In Situ Characterization of Work Hardening and Springback in Grade 2
           α-Titanium Under Tensile Load
    • Abstract: Publication date: Available online 24 September 2019Source: Acta MaterialiaAuthor(s): K. Sofinowski, M. Šmíd, S. Van Petegem, S. Rahimi, T. Connolley, H. Van Swygenhoven Plastic effects during sheet metal forming can lead to undesirable distortions in formed components. Here, the three-stage work hardening and plastic strain recovery ("springback") in a cold-rolled, α-phase commercially pure titanium is examined. Interrupted standard tensile tests with in situ x-ray diffraction and quasi-in situ electron backscatter diffraction show that twinning plays a minor role in both of these phenomena. The experiments give evidence that the observed work hardening plateau is the result of an abrupt activation and multiplication of 〈c+a〉 slip and a subsequent redistribution of load between grain families. The springback can be attributed to inelastic backwards motion and annihilation of dislocations, driven by backstresses from dislocation-based hardening during loading. The peak broadening behavior, observed by x-ray diffraction, suggests that the internal stress state is highest in the rolling direction, resulting in consistently higher springback magnitude along this direction.Graphical_abstractImage, graphical abstract
  • Helical Dislocations: Observation of vacancy defect bias of screw
           dislocations in neutron irradiated Fe-9Cr
    • Abstract: Publication date: Available online 23 September 2019Source: Acta MaterialiaAuthor(s): J.C. Haley, F. Liu, E. Tarleton, A.C.F. Cocks, G.R. Odette, S. Lozano-Perez, S.G. Roberts We have analysed the microstructure of a model alloy of Fe9Cr irradiated with neutrons to a dose of 1.6 dpa at 325°C. Helical dislocations comprise a major part of the damage; these formed from the interaction of pre-existing screw dislocations with irradiation-induced defects. We have investigated the process behind how these helices form, and how they cause local clustering of dislocation loops. Specifically, we have shown experimentally that the interaction of vacancy defects with pre-existing screw dislocations causes the formation of mixed screw-edge helical dislocations. Interstitials and vacancies were generated in equal numbers, which shows that the screw dislocations must have acted as vacancy-biased sinks.Helical dislocations in general were analysed from a theoretical perspective, and three Dimensional Discrete Dislocation Dynamics (3D-DDD) was used to develop a model for the formation and growth of a vacancy-fed helical dislocation.Since the helical dislocations cause the removal of vacancies from the local microstructure, this leaves a higher supersaturation of interstitials close to the dislocations. We argue that this supersaturation is responsible for enhanced interstitial loop coarsening, leading to a higher proportion of visible interstitial clusters in the vicinity of helical dislocations. These findings offer a new perspective on how dislocations affect the spatial homogeneity of radiation damage.Graphical abstractImage, graphical abstract
  • Influence of phase decomposition on mechanical behavior of an equiatomic
           CoCuFeMnNi high entropy alloy
    • Abstract: Publication date: Available online 22 September 2019Source: Acta MaterialiaAuthor(s): Benjamin E. MacDonald, Zhiqiang Fu, Xin Wang, Zhiming Li, Weiping Chen, Yizhang Zhou, Dierk Raabe, Julie Schoenung, Horst Hahn, Enrique J. Lavernia Phase decomposition is commonly observed experimentally in single-phase high entropy alloys (HEAs). Hence, it is essential for the consideration of HEAs for structural applications to study and understand the nature of phase decomposition in HEAs, particularly the influence it has on mechanical behavior. This paper describes the phase decomposition in the equiatomic CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior. Thermomechanical processing, followed by systematic post deformation annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix phase. Low temperature annealing treatments at 600°C and below led to the nucleation of Fe-Co rich ordered B2 precipitates that contributed precipitation hardening while sufficiently small in size, on the order of 140 nm in diameter. At temperatures < 800°C Cu segregation, due to its immiscibility with the other constituents, eventually forms a Cu-rich disordered FCC phase that is determined to increase the yield strength of the alloy while reducing the ductility, likely attributable to the presence of additional interfaces. The thermal stability and chemistry of these phases are compared to those predicted on the basis of calculated phase diagram (CALPHAD) analyses.Graphical Image, graphical abstract
  • Microstructural evolution of helium-implanted 6H-SiC subjected to
           different irradiation conditions and annealing temperatures: a multiple
           characterization study
    • Abstract: Publication date: Available online 22 September 2019Source: Acta MaterialiaAuthor(s): N. Daghbouj, B.S. Li, M. Callisti, H.S. Sen, M. Karlik, T. Polcar The microstructural phenomena occurring in 6H-SiC subjected to different irradiation conditions and annealing temperatures were investigated to assess the suitability of 6H-SiC as a structural material for nuclear applications. To this aim, a single crystal of 6H-SiC was subjected to He+ irradiation at 300 keV with different fluences and at temperatures ranging from 25 to 750°C. Rutherford backscattering/channeling (RBS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were combined to shed light on the microstructural changes induced by irradiation and subsequent annealing (750 to 1500°C). At room temperature, amorphization starts to occur at a fluence of 2.5 × 1016 cm−2 (0.66 dpa). On the contrary, amorphization was prevented at high irradiation temperatures and fluences. Furthermore, a thin and highly strained region located around the maximum He concentration (Rp) formed. This region results from the accumulation of interstitial atoms which are driven toward the highly damaged region under the actions of a strain gradient and high temperature. Regardless of the fluence and irradiation temperature, the material stores elastic energy, which leads to the trapping of He in dissimilar defect geometries. For irradiation temperatures below 750°C, helium was accumulated in bubbles which coarsened after annealing. On the other hand, for an implantation temperature of 750°C, helium was trapped in platelets (even for medium fluence), which evolved into a homogenous dense array of cavities during annealing. DFT calculations show that the bubbles are under high pressure and contribute to developing the overall tensile strain in the single crystal 6H-SiC.Graphical abstractImage, graphical abstract
  • A 3D analysis of the onset of slip activity in relation to the degree of
           micro-texture in Ti-6Al-4V
    • Abstract: Publication date: Available online 22 September 2019Source: Acta MaterialiaAuthor(s): S. Hémery, A. Naït-Ali, M. Guéguen, J. Wendorf, A.T. Polonsky, M.P. Echlin, J.C. Stinville, T.M. Pollock, P. Villechaise The mechanical properties of titanium alloys result from their complex multi-scale microstructural features, including micron scale precipitates and millimeter scale microtextured regions (MTRs). While previous investigations have revealed that the presence of mm-scale MTRs can degrade mechanical properties, particularly fatigue, the accompanying strain localization processes that operate at the microscale within the α grains in MTRs are not well understood. The present work is a mechanistic investigation of MTRs using crystal plasticity simulations of mm3-scale experimentally captured and synthetically generated 3D microstructure datasets. The explicit modeling of both the α grains and MTRs in Ti-6Al-4V enables assessment of the effect microtexture and local structure variations within the MTR on overall deformation behavior and the onset of plastic slip in MTRs. The presence of MTRs with a dominant [0001] orientation results in both stress and plastic strain hotspots during the early stages of straining. Crystal plasticity predictions are compared to previous digital image correlation studies on early strain localization. The influence of MTRs on the local stress and strain fields is discussed with regard to the monotonic tension, fatigue and dwell-fatigue behavior of titanium alloys.Graphical abstractImage, graphical abstract
  • Spectrum of Grain Boundary Segregation Energies in a Polycrystal
    • Abstract: Publication date: Available online 21 September 2019Source: Acta MaterialiaAuthor(s): Malik Wagih, Christopher A. Schuh Solute segregation at grain boundaries (GBs) is emerging as an alloy design tool, uses of which include the stabilization of nanocrystalline alloys. To predict the equilibrium segregation state in a given alloy, most thermodynamic models treat the full network of GBs as a single “entity”, and thus use an “effective” segregation energy to describe it. This simplification ignores the spectral nature of available GB segregation energies in a polycrystal, which we elucidate here computationally for a Mg solute in an Al polycrystal; the distribution is found to be captured accurately with a skew-normal function. A thermodynamic segregation isotherm that incorporates this spectrum is outlined and employed to study the effect of such a spectrum on predictions of the equilibrium GB segregation state. The ramifications for experimentally-extracted GB segregation energies are shown to be potentially significant, and nanocrystalline stability criteria are extended to account for this spectral nature of GB segregation.Graphical abstractImage, graphical abstract
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