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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 291  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3160 journals]
  • Editors for Acta Materialia
    • Abstract: Publication date: 15 June 2019Source: Acta Materialia, Volume 172Author(s):
       
  • Direct observation of growth and stability of Al-Cu-Fe quasicrystal thin
           films
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): Hadi Parsamehr, Chun-Liang Yang, Wei-Ting Liu, Shi-Wei Chen, Shou-Yi Chang, Lih-Juann Chen, An Pang Tsai, Chih-Huang Lai Al-Cu-Fe based quasicrystal thin films exhibit unique surface and mechanical properties. To better understand the formation of the quasicrystal thin films, we observe direct growth of quasicrystals, prepared in a multilayer Al-Cu-Fe thin films with subsequent heat treatment, by in-situ synchrotron x-ray diffraction and in-situ transmission electron microscopy during heating and cooling. Using these two methods, we show that the ternary phase is more thermodynamically stable compared to the binary phases at temperature higher than 470 °C during the heating process, and quasicrystal formation occurs during the cooling process, specifically at 660 °C, after the sample has reached a liquid state. To distinguish quasicrystal from approximant crystals in the obtained thin film samples, we use high resolution x-ray diffraction to analyze the sample at room temperature. We reveal that the peak broadening increases monotonically along the twofold, threefold, and fivefold high-symmetry directions with the physical scattering vector but does not have systematic dependence on the phason momentum, which suggests that the thin film sample is indeed a quasicrystal instead of approximant crystals and it is almost free of phason strain. Our study provides a complete understanding of the growth mechanism for thin film Al-Cu-Fe quasicrystals, which is of particular importance for developing versatile applications of quasicrystal thin films.Graphical abstractImage 1
       
  • Highly efficient charge separation in model Z-scheme TiO/TiSi/Si
           photoanode by micropatterned titanium silicide interlayer
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): M. Hannula, H. Ali-Löytty, K. Lahtonen, J. Saari, A. Tukiainen, M. Valden Atomic layer deposited (ALD) TiO is an attractive material for improving the photoactivity and chemical stability of semiconductor electrodes in artificial photosynthesis. Using photoelectrochemical (PEC) measurements, we show that an interfacial, topographically microstructured TiSi layer inside the TiO/Si heterojunction improves the charge carrier separation and shifts the water dissociation onset potential to more negative values. These observations are correlated with the X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) measurements, which reveal an increased band bending due to the TiSi interlayer. Combined with the UV-Vis absorption results, the photoelectron spectroscopy measurements allow the reconstruction of the complete energy band diagram for the TiO/TiSi/Si heterojunction and the calculation of the valence and conduction band offsets. The energy band alignment and improvements in PEC results reveal that the charge transfer across the heterojunction follows a Z-scheme model, where the metal-like TiSi islands act as recombination centers at the interface.Graphical abstractImage 1
       
  • Relating microstructure to defect behavior in AA6061 using a combined
           computational and multiscale electron microscopy approach
    • Abstract: Publication date: Available online 20 May 2019Source: Acta MaterialiaAuthor(s): Yung Suk Jeremy Yoo, Hojun Lim, John Emery, Josh Kacher In this study, a multiscale electron microscopy-based approach is applied to understanding how different aspects of the microstructure in a notched AA6061-T6, including grain boundaries, triple junctions, and intermetallic particles, promote localized dislocation accumulation as a function of applied tensile strain and depth from the sample surface. Experimental measurements and crystal plasticity simulations of dislocation distributions as a function of distance from specified microstructural features both showed preferential dislocation accumulation near intermetallic particles relative to grain boundaries and triple junctions. High resolution electron backscatter diffraction and site-specific transmission electron microscopy characterization showed that high levels of dislocation accumulation near intermetallic particles led to the development of an ultrafine sub-grain microstructure, indicative of a much higher level of local plasticity than predicted from the coarser measurements and simulations. In addition, high resolution measurements in front of a crack tip suggested a compounding influence of intermetallic particles and grain boundaries in dictating crack propagation pathways.Graphical abstractImage 1
       
  • Resolving the FCC/HCP interfaces of the (Ag2Al) precipitate phase in
           aluminium
    • Abstract: Publication date: Available online 18 May 2019Source: Acta MaterialiaAuthor(s): Zezhong Zhang, Julian M. Rosalie, Nikhil V. Medhekar, Laure Bourgeois The γ' (Ag2Al) phase in the Al-Ag alloy system has served as a textbook example for understanding phase transformations, precipitating hexagonal close packed (HCP) crystals in the face-centred cubic (FCC) aluminium matrix. The γ' precipitates display fully coherent interfaces at their broad facets and semicoherent interfaces at their edges. Shockley partial dislocations are expected to decorate the semicoherent interface due to the FCC-HCP structural transformation. Determining the exact locations and core structures of interfacial dislocations, however, remains challenging. In this study, we used aberration-corrected scanning transmission electron microscopy and atomistic simulations to re-visit this classical system. We characterised and explained the Ag segregation at coherent interfaces in the early stage of precipitation. For semicoherent interfaces, interfacial dislocations and reconstructions were revealed by bridging advanced microstructure characterisation and atomistic simulations. In particular, we discovered a new FCC/HCP interfacial structure that displays a unique combination of Shockley partial, Lomer-Cottrell and Hirth dislocations that evolve from the known interfacial structure purely composed by Shockley partial dislocations. Our findings show that the FCC-HCP transformation is more complex than hitherto considered, due to the interplay between structure and composition confined at interfaces.Graphical abstractImage 1
       
  • A multi-scale study of the interaction of Sn solutes with dislocations
           during static recovery in α-Fe
    • Abstract: Publication date: Available online 18 May 2019Source: Acta MaterialiaAuthor(s): N. Mavrikakis, C. Detlefs, P.K. Cook, M. Kutsal, A.P.C. Campos, M. Gauvin, P.R. Calvillo, W. Saikaly, R. Hubert, H.F. Poulsen, A. Vaugeois, H. Zapolsky, D. Mangelinck, M. Dumont, C. Yildirim The properties of engineering materials can be improved by optimising the microstructural developments during annealing processes. Here, we investigate the effect of Sn on the recovery annealing of cold rolled Fe-3%Si alloys. We use a multiscale approach combining micro hardness, electron back scattering diffraction (EBSD), and dark field X-ray microscopy (DFXM): a recent, non-destructive synchrotron-based technique that allows 3D mapping of orientation and lattice strain within individual grains embedded in bulk samples. Micro hardness results show that the Sn solute has a strong effect on the recovery kinetics. These results are compared to a physical kinetic model suggesting that Sn limits the softening. This observation is further discussed by a complementary atomistic modelling that demonstrates solute-dislocation interaction around edge dislocations. In situ DFXM experiments reveal the 3D microstructural evolution upon annealing at the grain level with high angular resolution. The DFXM observations show that Sn slows the recovery kinetics within individual grains, in agreement with the other microscopic investigations. Furthermore, the DFXM results provide a direct observation of strain fields around dislocation loops in an embedded single grain, which is argued to remain static due to solute effect during recovery.Graphical abstractImage 1
       
  • Assessment of the impact of hydrogen on the stress developed ahead of a
           fatigue crack
    • Abstract: Publication date: Available online 18 May 2019Source: Acta MaterialiaAuthor(s): Shuai Wang, Akihide Nagao, Petros Sofronis, Ian M. Robertson The microstructure generated in a low carbon steel under cyclic loading in air and a 40 MPa gaseous hydrogen environment has been compared as a function of distance from the crack tip. The presence of hydrogen resulted in the formation of a smaller and more equiaxed dislocation cell structure that extended further from the crack tip than the one generated in air. This enhancement and extension of the dislocation structure by hydrogen is consistent with it modifying the generation rate and mobility of dislocations as well as dislocation interactions. Qualitative assessment of the dislocation structure ahead of the crack tip found the stress ahead of the crack tip to vary linearly as ln(1/x), where x is the distance from the crack tip irrespective of the test environment. Hydrogen caused a shift to higher stresses, implying the critical damage level for crack propagation will be achieved more rapidly with a concomitant increase in the crack propagation rate.Graphical abstractImage 1
       
  • Neutron diffraction study of temperature-dependent elasticity of B19′
           NiTi--Elinvar effect and elastic softening
    • Abstract: Publication date: Available online 18 May 2019Source: Acta MaterialiaAuthor(s): A. Ahadi, R. Khaledialidusti, T. Kawasaki, S. Harjo, A. Barnoush, K. Tsuchiya The temperature-dependent elasticity of the B19′ NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19′ crystal, we present results of in-situ neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young’s modulus (TDYM) of the B19′ crystal is strongly anisotropic. For different crystallographic orientations, the change in Young’s modulus over the temperature range of 300-50 K (ΔE(hkl)=E(hkl)50K−E(hkl)300K), ranges from ΔE(102¯) = 2.8 ± 3.5 GPa (extremely weak dependence) to ΔE(103)= 59.6 ± 9.1 GPa (strong dependence). Moreover, it is found that the orientation-specific TDYM and thermal expansion (TE) of the B19′ crystal are correlated. The crystallographic orientations with weak and negative TE responses exhibit a weaker TDYM than the orientations with positive TE. The DFT-MD and QHA results capture qualitatively the above experimental observations and further show that there are orientations in a B19′ crystal exhibiting elastic softening (ΔE(hkl)
       
  • Deformation Kinetics and Constitutive Relation Analyses of Bifurcation in
           Work-Hardening of Face-Centred Cubic Metals at Cryogenic Temperatures
    • Abstract: Publication date: Available online 17 May 2019Source: Acta MaterialiaAuthor(s): S. Saimoto Crystal plasticity phenomena are described as anomalous at cryogenic temperatures if back-extrapolated trends observed at ambient temperatures are not found based on kinetic models of flow stress. The theoretical predictions are based on the Orowan relation which relates the dislocation velocity to the applied strain rate. The usual kinetic relation for plastic flow correlate the strain rate to the probability of dislocation overcoming obstacles with stress-assisted thermal activation. However at cryogenic temperatures, the obstacles can become athermal such that dislocation velocity can be approximated by a power-law relation. This change in kinetic reaction is attributed to be responsible for the observed bifurcation of work-hardening in face-centred cubic metals. The operative temperature range of these kinetic relations can be illustrated using the activation work versus temperature plot. The temperature-independent range of activation work give rise to one master curve wherein the stress-strain curve collapse into a common locus and a different master curve for the range dependent on thermally activation. The examinations of the theory and experiments which led to these deductions are elucidated.Graphical abstractImage 1
       
  • Quantitative 3D mesoscopic modeling of grain interactions during equiaxed
           dendritic solidification in a thin sample
    • Abstract: Publication date: Available online 16 May 2019Source: Acta MaterialiaAuthor(s): Antonio Olmedilla, Miha Založnik, Hervé Combeau A 3D mesoscopic envelope model is used to numerically simulate the experimental X-ray observations of isothermal equiaxed dendritic solidification of a thin sample of Al-20 wt%Cu alloy. We show the evolution of the system composed of multiple grains growing under influence of strong solutal interactions. We emphasize the three-dimensional effects in the thin sample thickness on the growth kinetics, focusing on three aspects: (i) the impact of the third dimension on the solute diffusion, (ii) the influence of the orientation of the preferential grain growth directions on the interactions with the confining sample walls, and (iii) the influence of the grain position along the sample thickness. We demonstrate the importance of considering the three-dimensional structure of the thin samples despite the small thickness. We further show that the mesoscopic envelope model can accurately describe the shape and the time-evolution of the equiaxed grains growing under influence of strong solutal interactions.Graphical abstractImage 1
       
  • Experimental assessment of damage-thermal diffusivity relationship in
           unidirectional fibre-reinforced composite under axial tensile test
    • Abstract: Publication date: Available online 16 May 2019Source: Acta MaterialiaAuthor(s): Jalal El Yagoubi, Jacques Lamon, Jean-Christophe Batsale, Marion Le Flem The influence of damage on thermal properties is an important issue for the design of reliable ceramic matrix composites for high temperature applications. This work is devoted to the experimental study of this influence on SiC/SiC minicomposite. This single tow reinforced composite is appropriate to investigating the damage mechanisms and thermal behavior. Relationships between the properties of constituents (fibre/matrix/interface) and behavior of the assembly can be established. Nevertheless, the literature review showed that no experimental study has focused on the thermal behavior during mechanical tests. Lock-in thermography was used to measure the thermal diffusivity during tensile tests on minicomposites while acoustic emission technique allowed monitoring damage. In this work, it is demonstrated that the evolution of the thermal diffusivity competes favorably with acoustic emission and Young’s modulus as a reliable indicator of matrix damage.Graphical abstractImage 1
       
  • Fundamentals of isothermal austenite reversion in a Ti-stabilized 12Cr –
           6 Ni – 2 Mo super martensitic stainless steel: thermodynamics versus
           experimental assessments
    • Abstract: Publication date: Available online 15 May 2019Source: Acta MaterialiaAuthor(s): J.D. Escobar, G.A. Faria, E.L. Maia, J.P. Oliveira, T. Boll, S. Seils, P.R. Mei, A.J. Ramirez This work addresses the fundamentals of inter-critical austenite reversion in a Ti-stabilized 12Cr-6Ni-2Mo (at.%) supermartensitic stainless steel, combining thermodynamic and experimental assessments. The calculation of the temperature and composition at which ferrite and austenite phases have the same free energy, i.e. T0 and C0(T), respectively, is discussed as a methodology to understand the austenite reversion and stabilization mechanisms. An ultra-fast heating rate of 500 °C.s-1 provided isothermal austenite nucleation and growth from a fully solubilized martensite, allowing direct comparison with the compositional tie-lines and the transformation paths described by the free energy calculations. Isothermal transformation temperatures below and above T0 (625 °C) were used. Below T0, massive reversion was suppressed since it would imply a free energy increase. The opposite occurred above T0, since the critical Ni concentration for austenite reversion was lower than for the solubilized case. Transmission electron microscopy and atom probe tomography evidenced that, in all cases, lath growth occurred by local equilibrium partitioning of Ni, along with co-segregation of ferrite-stabilizing elements (Cr and Mo) at the advancing interface. The complex interaction between Cr, Ni and Mo on the energy gain upon nucleation of austenite revealed that Cr segregation can be beneficial while the adverse effect of Mo can be quickly outbalanced by Ni. The most stable reverted laths were obtained for transformation temperatures at least 15 °C below T0 with average austenite/martensite Ni partitioning factors higher than 2.0.Graphical abstractImage 1
       
  • Large scale 3-dimensional atomistic simulations of screw dislocations
           interacting with coherent twin boundaries in Al, Cu and Ni under uniaxial
           and multiaxial loading conditions
    • Abstract: Publication date: Available online 15 May 2019Source: Acta MaterialiaAuthor(s): Maxime Dupraz, Satish I. Rao, Helena Van Swygenhoven Large scale 3D atomistic simulations are performed to study the interaction between a curved dislocation with a dominant screw character and a Coherent Twin Boundary (CTB). Three FCC metals (Al, Cu and Ni) are addressed using 6 embedded-atom method (EAM) potentials. The reaction mechanisms are studied first under uniaxial stress showing that transmission mechanism and critical transmission stress depend on the material considered and differ from results reported in quasi- 2D simulations. Then, the influence of multiaxial stresses including shear components in the CTB is investigated. It is shown that the influence of the loading conditions, which can be represented in terms of the Escaig stress, is material dependent. In Al and Cu, the critical transmission stress is largely dependent on the Escaig stress while only mildly for Ni. The presence of a shear component in the CTB tends to increase the critical transmission stress for all three materials. The absorption and desorption mechanisms of the screw dislocation are correlated with a potential energy barrier.Graphical abstractImage 1
       
  • Fine alpha in current and newly developed Ti alloys
    • Abstract: Publication date: Available online 15 May 2019Source: Acta MaterialiaAuthor(s): M. Wang, Y. Lu, B. Pang, Z.T. Kloenne, H.L. Fraser, Y.L. Chiu, M.H. Loretto Analytical transmission electron microscopy has been used to determine the chemical compositions of beta grains in samples of HIPped powder Ti6Al4V, cooled in the HIP at approximately 5°C/min in order to understand their different microstructures. It has been found that beta grains, which contain a high density of fine secondary alpha phase, have compositions (wt%) of about 81%Ti; 3%Al; 14%V and 2%Fe. This analysis includes both the fine secondary alpha and the beta in the grain and thus corresponds to the composition of the parent beta grain. Grains of retained beta, which do not contain any fine secondary alpha, are stabilised by a much higher V-content and contain about 75%Ti; 3%Al, 20%V and 2%Fe. The diffraction patterns from the grains that contain secondary alpha show maxima from several habits of alpha as well as beta maxima, whereas the beta grains, which contain no secondary alpha, show only beta maxima together with diffuse scattering. These observations provide the data needed to understand the factors that lead to the different microstructures observed in beta grains in slowly cooled HIPped powder Ti64 and suggest that an alloy, of a composition close to that of the beta grains in Ti64, which contain fine secondary alpha, should also contain a large fraction of fine alpha, when slowly cooled. This has been confirmed and the microhardness of slowly cooled samples has been shown to be higher than that of air-cooled Ti64.Graphical abstractMicrostructure of new alloy (a) Slowly cooled sample (b) Held for 2h at 600°C after slow cooling.Image 1
       
  • Geometry of kink microstructure analysed by rank-1 connection
    • Abstract: Publication date: Available online 15 May 2019Source: Acta MaterialiaAuthor(s): Tomonari Inamura The kinematical relationships among the geometric quantities that characterise kink bands, ridge kinks, and ortho kinks and the existence of disclinations in connecting kink bands were revealed based on the rank-1 connection, which is a condition of the continuity of deformation. Owing to the simple geometry of the kinks, the kink plane and the crystallographic rotation of the kink band were obtained in analytic forms as functions of the magnitude of the shears inside kinks. The geometry of the kink band predicted by the rank-1 connection agreed well with literature experimental data. Ridge kinks and ortho kinks were treated as the rank-1 connection of two kink bands. Positive and negative partial wedge disclinations were inevitably formed in any kink band connections when the junction plane of the kink bands did not reach the surface of the body. The Frank vector of the disclinations was also obtained as a function of the shear magnitudes in the kink bands. However, there are numerous kinds of microstructures in which the disclinations are cancelled. Modes of complete relaxation and annihilation of disclinations by combinations of kinks or slip deformations were revealed. The nature of the kink microstructure and kink strengthening was discussed.Graphical abstractImage 1
       
  • Macroscopic Energy Barrier and Rate-Independent Hysteresis in Martensitic
           Transformations
    • Abstract: Publication date: Available online 14 May 2019Source: Acta MaterialiaAuthor(s): Yongmei M. Jin, Yu U. Wang, Armen G. Khachaturyan A thermodynamic theory for the lower-bound hysteresis in martensitic transformations is developed. It is shown that the elastic energy generated by the transformation-induced crystal lattice misfit makes the system free energy a nonlinear function of the volume fractions of the phases, lifts the additivity principle and ergodicity hypothesis of the conventional Gibbsian thermodynamics, and produces a macroscopic energy barrier between the parent and product phases. Since the energy barrier is proportional to the macroscopic volume of the system and thus cannot be surmounted by the thermally assisted nucleation of the new phase, a rate-independent hysteresis of thermodynamic nature is produced, which sets the lower bound for the total transformation hysteresis. This lower-bound hysteresis is characterized by two critical temperatures of the martensitic and reverse transformations, one during cooling and another during heating, with their difference defining the corresponding transformation hysteresis. The necessary undercooling and overheating are intrinsic material properties determined by the transformation elastic energy. The theory is tested against experimental observations, explains the ubiquitous temperature hysteresis in the martensitic transformations, and correlates the hysteresis with the volumetric misfit strain of the transformations. The proposed theory is generic and is applicable to any displacive (diffusionless and martensitic) phase transformations.Graphical abstractImage 1
       
  • Interaction between nano-voids and migrating grain boundary by molecular
           dynamics simulation
    • Abstract: Publication date: Available online 14 May 2019Source: Acta MaterialiaAuthor(s): Liang Zhang, Yasushi Shibuta, Cheng Lu, Xiaoxu Huang Understanding the interaction between void and grain boundary (GB) is important to the design of radiation resistant materials by GB engineering and to achieve high quality metallurgical diffusion joining. In this study, the interaction between nano-voids and GBs has been systematically investigated by molecular dynamics simulations. The bicrystal Cu sample was used throughout the work, and the dynamic GB-void interaction was achieved by GB migration under shear deformation. Both high-angle GBs (Σ5(310) GB, Σ5(210) GB) and low-angle GBs (Σ37(750) GB, Σ61(650) GB) were investigated, and the effect of void size and temperature on the simulation result was examined. The transition of the deformation mechanism from GB migration to dislocation propagation was observed during the interaction between voids and high-angle GBs at low temperature (T=10 K). At higher temperature (T=300 and 600 K), the migrating GB can be pinned to voids, freely traversed voids, or dissolved voids in the process of their interaction. The void-drag effect on GB motion was analyzed based on the Zener-like equation, which indicates that the retarding pressure applied to the migrating GB by a void is closely related to the surface area of the void, the degree of contact between GB and void, and GB energy. By investigating the thermal stability of a void at the stationary GB, it was found that the dissolution of voids at a moving GB cannot be attributed solely to the thermal diffusion mechanism. The dynamic migration of high-angle GBs can significantly accelerate the dissolution time of the void. Atomistic analysis indicated that the migrating GB rearranged the atoms on the void surface by the collective motion of structural units, and the GB structural phase transformation provided an efficient diffusion channel for transporting the vacancies. The low-angle GBs show a reduced ability to dissolve the voids than the high-angle GBs, which can be ascribed to their low GB energy and diffusion coefficient, the fast GB migration velocity, and the discrete GB structure.Graphical abstractImage 1Dynamic interaction of shear-coupled grain boundary motion and nano-voids.
       
  • Acta Journals Editors Transitioning to Joint Appointments
    • Abstract: Publication date: Available online 13 May 2019Source: Acta MaterialiaAuthor(s): Christopher A. Schuh
       
  • Influence of bulk energy and triple junction mobility on interface
           kinetics - a tool for interpretation of experiments
    • Abstract: Publication date: Available online 12 May 2019Source: Acta MaterialiaAuthor(s): K. Hackl, A.U. Khan, F.D. Fischer, J. Svoboda A material system consisting of a lamellar grain structure adjacent to a large single grain is investigated. The system evolution is driven by changing of interface energy of the lamellar structure as well as by difference in bulk energies stored in the single grain and the lamellar grains. The triple junctions and the grain boundaries are assumed to have finite mobilities representing kinetic material parameters of the system. A complete analysis of the kinetics of this system is provided, which involves several possible scenarios depending on the values of the geometrical and material parameters of the system. The scenarios are fully classified. Moreover, the analysis offers a way, how the values of the material parameters (interface energy densities, difference in bulk energies and mobilities) can be extracted from the measured system kinetics and geometry.Graphical abstractImage 1
       
  • Quantified Uncertainty in Thermodynamic Modeling for Materials Design
    • Abstract: Publication date: Available online 11 May 2019Source: Acta MaterialiaAuthor(s): Noah H. Paulson, Brandon J. Bocklund, Richard A. Otis, Zi-Kui Liu, Marius Stan Phase fractions, compositions and energies of the stable phases as a function of macroscopic composition, temperature, and pressure (X-T-P) are the principle correlations needed for the design of new materials and improvement of existing materials. They are the outcomes of thermodynamic modeling based on the CALculation of PHAse Diagrams (CALPHAD) approach. The accuracy of CALPHAD predictions vary widely in X-T-P space due to experimental error, model inadequacy and unequal data coverage. In response, researchers have developed frameworks to quantify the uncertainty of thermodynamic property model parameters and propagate it to phase diagram predictions. In most previous studies, uncertainty was represented as intervals on phase boundaries (with respect to composition or temperature) or invariant reactions (with respect to temperature) and was unable to represent the uncertainty in eutectoid invariant reactions or in the stability of phase regions. In this work, we propose a suite of tools that leverages samples from the multivariate model parameter distribution to represent uncertainty in forms that surpass previous limitations and are well suited to materials design. These representations include the distribution of phase diagrams and their features, as well as the dependence of phase stability and the distributions of phase fraction, composition, activity and Gibbs energy on X-T-P location - irrespective of the total number of components. Most critically, the new methodology allows the material designer to interrogate a certain composition and temperature domain and get in return the probability of different phases to be stable, which can positively impact materials design.Graphical abstractImage 1
       
  • Influence of morphological instability on grain boundary trajectory during
           directional solidification
    • Abstract: Publication date: Available online 10 May 2019Source: Acta MaterialiaAuthor(s): Supriyo Ghosh, Alain Karma, Mathis Plapp, Silvère Akamatsu, Sabine Bottin-Rousseau, Gabriel Faivre The interplay between the diffusion-controlled dynamics of a solidification front and the trajectory of a grain boundary groove at the solid-liquid interface is studied by means of thin-sample directional solidification experiments of a transparent alloy, and by numerical simulations with the phase-field method in two dimensions. We find that low-angle grain boundaries (subboundaries) with an anisotropic interfacial free energy grow tilted at an angle θtwith respect to the temperature gradient axis. θtremains essentially equal to its value imposed at equilibrium as long as the solidification velocity V remains low. When V increases and approaches the cellular instability threshold, θtdecreases, and eventually vanishes when a steady-state cellular morphology forms. The absence of mobility of the subboundary in the solid is key to this transition. These findings are in good agreement with a recent linear-stability analysis of the problem.
       
  • EBSD analysis of grain-refinement mechanisms operating during
           equal-channel angular pressing of commercial-purity titanium
    • Abstract: Publication date: Available online 10 May 2019Source: Acta MaterialiaAuthor(s): G.S. Dyakonov, S. Mironov, I.P. Semenova, R.Z. Valiev, S.L. Semiatin Electron backscatter diffraction was applied to examine the fundamental mechanisms governing the development of ultrafine-grain microstructures during equal-channel angular pressing of commercial-purity titanium. Texture analysis and examination of misorientation distributions revealed the enhanced activity of non-prismatic slip systems at prior grain boundaries, which promoted the preferential development of deformation-induced boundaries in these areas. By contrast, deformation in the interior of grains was dominated by prism slip which resulted in sluggish microstructural evolution. This difference in slip activity gave rise to the preferential nucleation of ultrafine grains at prior grain boundaries and thus the development of a bimodal grain structure. The grain refinement was concluded to result primarily from the incompatibility of deformation in neighboring grains.Graphical abstractImage 1
       
  • Effect of Oxygen Defects Blocking Barriers on Gadolinium Doped Ceria (GDC)
           Electro-Chemo-Mechanical Properties
    • Abstract: Publication date: Available online 10 May 2019Source: Acta MaterialiaAuthor(s): Ahsanul Kabir, Simone Santucci, Ngo Van Nong, Maxim Varenik, Igor Lubomirsky, Robin Nigon, Paul Muralt, Vincenzo Esposito Some oxygen defective metal oxides, such as cerium and bismuth oxides, have recently shown exceptional electrostrictive properties that are even superior to the best performing lead-based electrostrictors, e.g. lead-magnesium-niobates (PMN). Compared to piezoelectric ceramics, electromechanical mechanisms of such materials do not depend on crystalline symmetry, but on the concentration of oxygen vacancy (VO⋅⋅) in the lattice. In this work, we investigate for the first time the role of oxygen defect configuration on the electro-chemo-mechanical properties. This is achieved by tuning the oxygen defects blocking barrier density in polycrystalline gadolinium doped ceria with known oxygen vacancy concentration, Ce0.9Gd0.1O2-δ, δ = 0.05. Nanometric starting powders of ca. ∼12 nm are sintered in different conditions, including field assisted spark plasma sintering (SPS), fast firing and conventional method at high temperatures. These approaches allow controlling grain size and Gd-dopant diffusion, i.e. via thermally driven solute drag mechanism. By correlating the electro-chemo-mechanical properties, we show that oxygen vacancy distribution in the materials play a key role in ceria electrostriction, overcoming the expected contributions from grain size and dopant concentration.Graphical abstractImage 1
       
  • Ionizing vs collisional radiation damage in materials: separated,
           competing, and synergistic effects in Ti3SiC2
    • Abstract: Publication date: Available online 10 May 2019Source: Acta MaterialiaAuthor(s): William A. Hanson, Maulik K. Patel, Miguel L. Crespillo, Fuxiang Zhang, Steven J. Zinkle, Yanwen Zhang, William J. Weber Comparison between intense radiation environments present in nuclear reactors and charged particle beams is necessary for evaluating next generation fission and fusion reactor materials. However, these two irradiation environments encompass different proportions of ionizing and collisional phenomena, so exploring the different energy loss pathways is needed for appropriate analysis. Using the candidate Mn+1AXn phase, Ti3SiC2, as a test case, this work separates the effects of electronic and nuclear energy loss during ion irradiation, through a combination of 4 MeV Au, 17 MeV Pt, and 14 MeV Cl ion irradiations to examine the effects independently and systematically recombine them. Nuclear energy loss (elastic collisions) is found to be primarily responsible for the formation of a face-centered-cubic phase with anti-site defects, while intense electronic energy loss (ionization) exacerbates the effect and increases lattice strain. Further, these dissipation pathways are found to be competing or synergistic depending on their ionization and collisional ratio.Graphical abstractImage 1
       
  • Giant enhancement of the magnetocaloric response in Ni-Co-Mn-Ti by rapid
           solidification
    • Abstract: Publication date: Available online 10 May 2019Source: Acta MaterialiaAuthor(s): Henrique Neves Bez, Arjun K. Pathak, Anis Biswas, Nikolai Zarkevich, Viktor Balema, Yaroslav Mudryk, Duane D. Johnson, Vitalij. K. Pecharsky Magnetocaloric refrigeration is a solid-state cooling approach that promises high energy efficiency and low environmental impact. It remains uncompetitive with conventional vapor-compression technologies due to lack of high-performing materials that exhibit large magnetocaloric effects in low magnetic fields. Here we report a game-changing enhancement of the magnetocaloric response in a transition-metal-based Ni-Co-Mn-Ti. Mechanically and chemically stable rapidly solidified ribbons exhibit magnetic entropy changes as high as ∼27 J⋅kg-1K-1 for a moderate field change of 2 T, comparable to or larger than the best known materials for near-room temperature applications. The ribbons can be easily manufactured in large quantities and the transition temperature can be adjusted by varying Co concentration.Graphical abstractImage 1
       
  • Quantitative prediction of texture effect on Hall–Petch slope for
           magnesium alloys
    • Abstract: Publication date: Available online 10 May 2019Source: Acta MaterialiaAuthor(s): Bo Guan, Yunchang Xin, Xiaoxu Huang, Peidong Wu, Qing Liu The high texture dependence of a Hall–Petch slope (k) for Mg alloys has been frequently reported. Several important equations used to calculate k have been previously developed, and although they seem to work well for fcc and bcc materials, they often fail to predict the highly texture-dependent k in Mg alloys. A new equation based on the dislocation pile-up model was developed in this study. The validity of this new equation was tested through a comparison of the predicted k values with the experimental values as well as the calculations from older equations. The results indicate that the new equation can achieve an accurate prediction for several previously reported texture effects on k, whereas the k values predicted by the older equations often exhibit a clear deviation. The reasons for this were analyzed and discussed. The strong deformation anisotropy for Mg alloys leads to a complex texture effect on k, including the effects from both external and internal stresses. Both effects are well expressed in the new equation. In contrast, the old equations consider the external stress effect, but do not express well the internal stress effect. In addition, the old equations consider only the predominant deformation mode. However, our results indicate that the activation of a portion of another deformation mode other than the predominant one plays an important role in the k value. In the new equation, all possible deformation modes and their fractions are considered in the calculation. Using the important parameters of the new equation, the mechanisms for several texture effects on k as previously reported were discussed and new understandings were obtained.Graphical abstractImage 1
       
  • Emergence of shallow energy levels in B-doped Q-carbon: A high-temperature
           superconductor
    • Abstract: Publication date: Available online 9 May 2019Source: Acta MaterialiaAuthor(s): Ritesh Sachan, Jordan A. Hachtel, Anagh Bhaumik, Adele Moatti, John Prater, Juan Carlos Idrobo, Jagdish Narayan We report the spectroscopic demonstration of the shallow-level energy states in the recently discovered B-doped Q-carbon Bardeen-Cooper-Schrieffer (BCS) high-temperature superconductor. The Q-carbon is synthesized by ultrafast melting and quenching, allowing for high B-doping concentrations which increase the superconducting transition temperature (Tc) to 36 K (compared to 4 K for B-doped diamond). The increase in Tc is attributed to the increased density of energy states near the Fermi level in B-doped Q-carbon, which give rise to superconducting states via strong electron-phonon coupling below Tc. These shallow-level energy states, however, are challenging to map due to limited spatial and energy resolution. Here, we use ultrahigh energy resolution monochromated electron energy-loss spectroscopy (EELS), to detect and visualize the newly formed shallow-level energy states (vibrational modes) near the Fermi level (ranging 30-100 meV) of the B-doped Q-carbon. With this study, we establish the significance of high-resolution EELS in understanding the superconducting behavior of BCS superconducting C-based materials, which demonstrate a phenomenal enhancement in the presence of shallow-level energy states.Graphical abstractImage 1
       
  • Missing information and data fidelity in digital microstructure
           acquisition
    • Abstract: Publication date: Available online 9 May 2019Source: Acta MaterialiaAuthor(s): Chongbing Bao, Chuanyi Ji, Henning Friis Poulsen, Mo Li Measuring and modeling microstructure is crucial for establishing the structure-property relations. The digitized format referred to as the digital microstructure plays an increasingly vital role in modern material design and advanced manufacturing of functional materials. However, one basic issue facing digital acquisition of microstructures has attracted little attention, namely the loss of information in the first steps of data gathering and processing. The missing information leads to serious issues in a range of topics from reconstruction of microstructures to prediction of material properties. To bring forth and quantify these issues, we developed an analytical method and a new numerical simulation scheme using Laguerre-Voronoi tessellation and Xu-Li microstructure characterization method. With these approaches, we define quantitatively the missing information and its impact.Graphical abstractComparison of the digitized microstructure in the form of voxels (or squares in the figure) with the real microstructure formed by the polygonal domains (or the grain boundary network). The missing grains in the digital microstructure are shown in red circles. Like traditional metallography that has played important roles in materials science for centuries, digitization of microstructure is the modern advance that promises to revolutionize synthesis, processing, and application of a vast number of materials. Compared to metallography, digital microstructure involves data with unprecedented magnitude of size and scale. However, one basic issue facing digital acquisition of microstructures has attracted little attention, namely, the loss of information in the first steps of data gathering and processing. The missing information leads to serious issues in data fidelity in a range of topics from reconstruction of microstructures to prediction of material properties. To address these issues, we developed an analytical method and a numerical simulation scheme using Laguerre-Voronoi tessellation and Xu-Li microstructure characterization method. With these new approaches, we define and establish this problem for the first time by showing quantitatively the missing information and its impact.Image 1
       
  • Near-ideal Compressive Strength of Nanoporous Silver Composed of Nanowires
    • Abstract: Publication date: Available online 8 May 2019Source: Acta MaterialiaAuthor(s): Peng Peng, Hao Sun, Adrian P. Gerlich, Wei Guo, Ying Zhu, Lei Liu, Guisheng Zou, Chandra Veer Singh, Norman Zhou Nanoporous materials exhibit promising applications in energy storage, catalysis, and sensing. They are typically synthesized by dealloying, a costly and environmentally detrimental technology, valid only within a narrow compositional range of alloys. Surmounting these disadvantages, we assembled nanoporous silver materials via bottom-up nanoscale joining of nanowires, a technique also suitable for other metals. Furthermore, the resulting nanoporous materials exhibit an unprecedented, near-ideal compressive yield strength (∼2.6 GPa). Such an ultra-high strength, however, does not belong to the nanoporous materials composed of nanowires with the minimum length in our samples, challenging the smaller-is-stronger tenet. According to molecular dynamics simulations, such a strength degradation as nanowires shorten is attributed to the internal compressive stress arising from the five-fold twins within nanowires. Such internal stress maximizes at the center and diminishes near free surfaces, making the center part harder to compress than that adjacent to free surfaces. The volume fraction of the latter increases as the nanowire shortens, diminishing the overall Young’s modulus. For nanowires having aspect ratios smaller than six, a reduced Young’s modulus also lowers the yield strength since yield strain is independent of aspect ratios. However, if the aspect ratio exceeds six, compression induces bending, leading to a decline of both yield strength and yield strain. Overall, this work not only provides new physical insights on the structure-mechanical property relationship for nanoporous silver, but also paves a new way for bottom-up synthesizing nanoporous metals efficiently, economically, and environmental-friendly, optimizing the strength of the nanoporous materials.Graphical abstractImage 1
       
  • Enhancement of superelastic property in Ti–Zr–Ni–Cu alloy by using
           glass alloy precursor with high glass forming ability
    • Abstract: Publication date: Available online 8 May 2019Source: Acta MaterialiaAuthor(s): Woo-Chul Kim, Yong-Joo Kim, Yeong-Seong Kim, Jae-Ik Hyun, Sung-Hwan Hong, Won-Tae Kim, Do-Hyang Kim The glass forming ability (GFA), crystallization behavior and subsequent superelastic property after crystallization in Ti50-xZrxNi35Cu15 (x = 5, 10, 15, 20 at%) alloys have been investigated with an aim to develop glassy alloy precursor for shape memory alloy having both high glass forming ability and superior superelastic property. The addition of Zr is effective in improving the GFA exhibiting a critical thickness larger than 100 μm by increasing both liquid stability and resistance to crystallization. The glassy Ti50-xZrxNi35Cu15 alloys are polymorphically crystallized into B2 structure. With increasing Zr content, the grain size after crystallization decreases significantly by accelerated nucleation process. Thus, the stability of austenite increases, resulting in the decrease of Ms. The critical stress for slip deformation gradually increases with alloying Zr up to 15 at%, improving the superelastic recovery. The Ti35Zr15Ni35Cu15 alloy exhibits the highest remnant depth ratio value of 7.2%. By contrast, the superelasticity is significantly deteriorated in the Ti30Zr20Ni35Cu15 nanocrystalline alloy due to the difficulty in stress-induced martensitic transformation which is caused by extremely small grain size and irregular morphology of grain boundary. Considering both glass forming ability and sueprelastic property, the Ti35Zr15Ni35Cu15 alloy is confirmed to be the most suitable alloy satisfying wide supercooled liquid region (∼40 K), large maximum thickness for amorphous alloy formation (∼100 μm) and excellent superelasticity after crystallization.Graphical abstractImage 1
       
  • Simple geometrical aspects of grain growth in the framework of Herring’s
           analysis and a disclination model
    • Abstract: Publication date: Available online 8 May 2019Source: Acta MaterialiaAuthor(s): Reiner Kirchheim The time change of the area of a single grain is calculated in 2D by applying Herring’s analysis. The grain is a regular n-sided polygon with its corners representing triple junctions (TJs). The resulting change of grain area is compared to the Von Neumann–Mullins analysis, where grain boundaries (GB) are curved and angles at TJs are equilibrium angles. The rates of area change are similar with the largest deviation for triangles. For both cases of different angles at TJs polygons with n6 grow. Pieces of evidence are provided for describing the GB-movement by disclination generation and motion due to thermal fluctuations. Thus besides energy considerations a stochastic element comes into play.Graphical abstractImage 1
       
  • Corrigendum to “Nanocomposite microstructures dominating anisotropic
           elastic modulus in carbon fibers” [Acta Mater. 166 (2019) 75–84]
    • Abstract: Publication date: Available online 8 May 2019Source: Acta MaterialiaAuthor(s): Masakazu Tane, Haruki Okuda, Fumihiko Tanaka
       
  • Thermal Conductivity of Architected Cellular Metamaterials
    • Abstract: Publication date: Available online 7 May 2019Source: Acta MaterialiaAuthor(s): A. Mirabolghasemi, A.H. Akbarzadeh, D. Rodrigue, D. Therriault Periodic architected cellular metamaterials, as a novel class of low-density materials, possess unprecedented multifunctional properties mainly due to their underlying microarchitecture. In this paper, we study the thermal conductivity of cellular metamaterials and evaluate their performance for thermal management applications. To understand the relations between the microarchitecture and the thermal response, we analyze the thermal conductivity of a wide range of cellular metamaterials with strategically developed microarchitectures from two-dimensional (2D) cells with Supershape pores to three-dimensional (3D) thin-walled open lattices and shellular materials. We implement standard mechanics homogenization on the periodic representative volume elements (RVEs) of these cellular metamaterials to examine the effect of pore architecture (relative density, pore shape, pore orientation, and pore elongation) on their effective thermal conductivity. The numerical results show how the thermal conductivity of an isotropic material can be modified by pore introduction and how the pore architecture could lead to an anisotropic effective thermal conductivity tensor. To examine the impact of having 2D Supershape cuts on 3D RVEs, thin-walled open lattices are designed as an assembly of thickened 2D RVEs with Supershape pores. A mathematical model is derived based on the effective thermal properties of the constituent 2D RVEs to predict the effective thermal properties of these lightweight cellular materials. Effective thermal conductivity of shellular materials based on triply periodic minimal surfaces is also compared with those of the previously introduced architectures. Unlike the shellular materials, which only cover a narrow region of thermal conductivity versus relative density chart, cellular materials with a wide range of anisotropic effective thermal conductivities can be engineered by using 2D Supershape pores on 2D or 3D thin-walled cells. Finally, we show how the concept of architected functionally graded cellular materials can be used to tune the heat flow within cellular media to guide it in a specific direction to control the temperature inside advanced 3D printed materials. As a case study, the optimum spatial distribution of pore rotation angle is found to maximize or minimize the heat flow passing through different sides of a square-shaped porous slab. This paper opens an avenue for developing thermal metamaterials with programmable anisotropic thermal properties.Graphical abstractImage 1
       
  • Synthesis of novel hybrid carbon nanomaterials inside silicon carbide
           nanotubes by ion irradiation
    • Abstract: Publication date: Available online 7 May 2019Source: Acta MaterialiaAuthor(s): Tomitsugu Taguchi, Shunya Yamamoto, Hironori Ohba A novel hybrid carbon nanomaterial was synthesized by ion irradiation of a C–SiC coaxial nanotube. The hybrid consisted of one-dimensionally stacked graphene nanodisks with diameters less than 50 nm and cylindrical multiwalled carbon nanotubes inside an amorphous SiC tubular layer. A sudden emergence of new continuous graphitic layers in the microstructure were observed by in situ transmission electron microscopy following ion irradiation, where these layers were perpendicular to the nanotube’s length direction. The SiC crystals in the C–SiC coaxial nanotube became amorphous, also due to the ion irradiation, although the critical amorphization dose was higher than that for bulk SiC. Most remarkably, the carbon layer remained crystalline, even after an irradiation dose higher than 20 dpa (displacement per atom). Such results show that these carbon layers possess better resistance against amorphization when subjected to ion irradiation than the SiC layers in the C–SiC coaxial nanotube. The lattice plane spacing of the carbon layer increases up to the point when irradiation damage lead to the complete amorphization of SiC crystals, after which it starts to decrease. This demonstrates that the carbon layer experiences high compression stress during ion irradiation. Thus, the ion irradiation of C–SiC coaxial nanotubes gives rise to a novel development of the microstructure and can be considered as one of the new synthetic processes for making novel carbon nanomaterials.Graphical abstractImage 1
       
  • Freeze Casting of Iron Oxide Subject to a Tri-axial Nested Helmholtz-coils
           Driven Uniform Magnetic Field for Tailored Porous Scaffolds
    • Abstract: Publication date: Available online 7 May 2019Source: Acta MaterialiaAuthor(s): Isaac Nelson, Levi Gardner, Krista Carlson, Steven E. Naleway In this research, Fe3O4 particles were magnetically manipulated to create porous scaffolds using a tri-axial nested Helmholtz coil-based freeze-casting setup. This novel setup allowed for a uniform magnetic field to be applied in any direction and for it to effectively change directions at any time. Applying a uniform low magnetic field of 7.8 mT in various directions was investigated to fabricate a variety of tailored microstructures and mechanical properties in the resultant scaffolds. It was observed that the using the magnetic field aligned up to 81% of the lamellar walls and also altered the area and shape of the pores of the resultant scaffolds. This lamellar wall alignment occurred at every applied magnetic field direction due to the Fe3O4 particles aligning during the freeze-casting process. As a result of this alignment, increases in the mechanical properties of up to 4.1x were observed. The results provide a novel experimental technique for the fabrication of user-defined microstructures in Fe3O4-based freeze-cast materials that provides significant advantages over previous experimental setups for magnetic freeze casting.Graphical abstractImage 1
       
  • Frenkel defect recombination in Ni and Ni‒containing concentrated
           solid‒solution alloys
    • Abstract: Publication date: Available online 7 May 2019Source: Acta MaterialiaAuthor(s): Shijun Zhao, Yuri Osetsky, Alexander V. Barashev, Yanwen Zhang Recombination of Frenkel defects is an important process that contributes to the performance of materials under irradiation. In this work, the recombination mechanisms of Frenkel defects in face-centered cubic Ni and Ni‒containing solid‒solution alloys are investigated based on interatomic potentials and ab initio calculations. It is found that, in pure Ni, the spontaneous recombination volume for a [100] dumbbell interstitial is 18Ω and 34Ω (Ω is the atomic volume) with the empirical potential and ab initio method respectively. Addition of Fe atoms increases the spontaneous recombination volume of Frenkel defects significantly. For those stable Frenkel defects that cannot recombine, a stronger attractive force between the interstitial and the vacancy is found in concentrated Ni‒Fe alloys compared to pure Ni, which provides the driving force for enhanced recombination. The distribution of life-time for Frenkel defects at finite temperature suggests that recombination in Ni‒Fe alloys is delayed due to the sluggish diffusion of interstitials. Finally, recombination in Ni‒Fe‒Cr alloys is studied by substituting a portion of Fe in Ni‒Fe with Cr. A remarkable increase in recombination probability is observed in this case because of the presence of less stable Cr‒containing than Fe‒containing dumbbell interstitials and lower migration barriers of vacancies. This work reveals that higher defect recombination probability in concentrated alloys is responsible for the experimentally observed enhanced irradiation resistance.Graphical abstractThe number of recombiantion sites increases signifiantly when alloying Fe into Ni, which can promote defect recombiantion and enhance irradiation tolerance.Image 1
       
  • Selective laser melting enabled additive manufacturing of Ti–22Al–25Nb
           intermetallic: Excellent combination of strength and ductility, and unique
           microstructural features associated
    • Abstract: Publication date: Available online 7 May 2019Source: Acta MaterialiaAuthor(s): Y.H. Zhou, W.P. Li, D.W. Wang, L. Zhang, K. Ohara, J. Shen, T. Ebel, M. Yan To realize near net-shaping of hard-to-process intermetallics is an often challenging but critical issue to their wider industrial applications. In this work, we report that an intermetallic Ti–22Al–25Nb has been successfully fabricated by selective laser melting (SLM). The as-printed samples show a high room-temperature ultimate tensile strength ∼1090 MPa and excellent ductility ∼22.7%; both values are higher than most conventionally fabricated Ti–22Al–25Nb intermetallic. We clarify the mechanical performance achieved by detailed microstructure analysis, including dislocation and phase constitution. It is proposed that high-density dislocation networks significantly contribute to the strength and ductility, which are further enhanced by the favorable phase constitution, including the nano-scale O phase precipitates within the disordered β phase and disappearance of the brittle α2 phase in the microstructure. Phase evolution during solidification, particularly regarding the O phase’s formation, has also been clarified using in-situ laser heating, high-temperature synchrotron X-ray diffraction and Scheil simulation. It is demonstrated that the O phase formation involves both displacive transition (B2 → B19) and chemical ordering (B19 → O). The metastable B19 phase (as an intermediate stage) may be formed by shearing cubic B2 phase along (110)[1−11]direction into an orthorhombic structure under high residual stress. Furthermore, a demonstrative part of turbine blade has been fabricated to highlight the importance of SLM in fabricating critical structural part like the hard-to-process intermetallics.Graphical abstractImage 1
       
  • Effect of boundary conditions on reduction during early stage flash
           sintering of YSZ
    • Abstract: Publication date: Available online 6 May 2019Source: Acta MaterialiaAuthor(s): Carolyn A. Grimley, Andre L.G. Prette, Elizabeth C. Dickey The onset of flash sintering is generally considered to be predominantly a function of the conductivity of a given ceramic, the furnace temperature, and the electric field. However, the evolution of the point defect profiles in ionic conductors such as yttria-stabilized zirconia (YSZ) can complicate the picture of homogeneous Joule heating and thermal runaway during DC flash sintering. Here, 8 mol% YSZ pellets were partially flash sintered under a DC bias using various current densities and hold times. The electrode geometry was varied to modulate the oxygen ion flux available to the cathode to compare the effects on the resulting oxygen vacancy inhomogeneity. The contribution to the cathodic reduction reaction from fundamental and experimental factors such as interface reaction kinetics and sample geometry are also discussed. Local reduction of the ceramic was inevitably observed under all current densities and the resulting microstructural inhomogeneity was explained as the result of a transient conductivity asymmetry. This asymmetry was the result of the enhanced electronic conductivity in the cathode region to a value significantly greater than the ionic conductivity of near-stoichiometric YSZ. The link between the local conductivity, voltage and Joule heating is mathematically demonstrated to result in an asymmetric heating profile.Graphical abstractImage 1
       
  • Corrigendum to “Revealing ionization-induced dynamic recovery in
           ion-irradiated SrTiO3” [Acta Mater. 149 (2018) 256–264]
    • Abstract: Publication date: Available online 3 May 2019Source: Acta MaterialiaAuthor(s): Gihan Velişa, Elke Wendler, Haizhou Xue, Yanwen Zhang, William J. Weber
       
  • Thermal and magnetic effects in quasi-binary Tb1-xDyxNi2 (x = 0.25, 0.5,
           0.75) intermetallics
    • Abstract: Publication date: Available online 3 May 2019Source: Acta MaterialiaAuthor(s): J. Ćwik, Y. Koshkid’ko, N. Kolchugina, K. Nenkov, N.A. de Oliveira Measurements of the low-temperature heat capacity performed for the polycrystalline Tb1-xDyxNi2 intermetallic compounds with x = 0.25, 0.5 and 0.75 enable us to determine their Debye temperatures and estimate the lattice, electron and magnetic contributions to the heat capacity. The measurements in magnetic fields of 1 and 2 T were performed to find the isothermal magnetic entropy and adiabatic temperature changes which characterize the magnetocaloric effect of these compounds. The experimental data obtained are paralleled by theoretical calculations performed in the frame of a microscopic model, which takes into account the exchange interaction and the crystal electric field anisotropy. For the Tb0.75Dy0.25Ni2 compound, direct measurements of the adiabatic temperature change in the temperature range of the magnetic transition were carried out in magnetic fields up to 14 T. The found regularities are discussed in terms of Landau`s theory of second-order magnetic phase transitions. In addition, for the parent binary compound, TbNi2, the change of emitted (absorbed) quantity of heat ΔQ was determined using direct measurements.Graphical abstractImage 1
       
  • Effect of neutron and ion irradiation on the metal matrix and oxide
           corrosion layer on Zr-1.0Nb cladding alloys
    • Abstract: Publication date: Available online 2 May 2019Source: Acta MaterialiaAuthor(s): Jing Hu, Alistair Garner, Philipp Frankel, Meimei Li, Mark Kirk, Sergio Lozano-Perez, Michael Preuss, Chris Grovenor A detailed study has been carried out on recrystallised Zr-1.0Nb alloys corroded and irradiated under different conditions, including ex-autoclave and ex-reactor samples. After 540 days in reactor and damage around 5 dpa, the neutron irradiated sample shows no serious evidence for radiation enhanced corrosion and is still in pre-transition stage. The good corrosion resistance of the neutron irradiated Zr-1.0Nb can be related to the higher volume fraction of tetragonal phase and fewer interconnected nano porosity/cracks in the oxide. This indicates less tetragonal to monoclinic transition, leads to more protective oxide in the neutron irradiated sample, containing little evidence for short circuit paths for the penetration of oxygen or water towards the metal-oxide interface. These observations on a sample with a slow overall oxidation rate are consistent with the hypothesis that interconnected porosity can lead to early transitions and rapid oxidation. Tetragonal oxide can be either stabilised by irradiation, or stabilised by local release of impurity species from SPPs such as dissolution of Fe from Zr-Nb-Fe precipitates or radiation introduced precipitates (RIPs) which is likely to be small β-Nb clusters. The oxide consists of well-aligned columnar-equiaxed microstructure in the autoclave sample while a more complex oxide grain structure was observed in the neutron-irradiated sample. As oxide continues to grow, there are more and loops, dissolved Fe and RIPs in the metal matrix, however, the corrosion rate is low enough for the tetragonal oxide to stabilise and suboxide + Zr(Osat) phases exist for protectiveness, so there is no enhanced corrosion after radiation. In situ ion irradiation in the TEM revealed no visible defect clusters or voids in the oxide, suggesting that radiation damage to the metal matrix rather than oxide may have a stronger effect on corrosion mechanisms after neutron irradiation, however, cascade damages are not visible in this case. Neutron irradiation also seems to have little effect on promoting fast oxidation or dissolution of β-Nb precipitates into the surrounding oxide or metal during irradiation. These results are discussed in the light of the current mechanisms for corrosion of nuclear fuel cladding alloys.Graphical abstractImage 1
       
  • X-Ray Diffraction and Stress Relaxations to Study Thermal and
           Stress-assisted Annealings in Nanocrystalline Gold Thin Films
    • Abstract: Publication date: Available online 1 May 2019Source: Acta MaterialiaAuthor(s): P. Godard, D. Faurie, T. Sadat, M. Drouet, D. Thiaudière, P.O. Renault The dependence on thermal history of the plasticity mechanisms occurring in nanocrystalline gold thin films is evidenced thanks to relaxation tests combined with in-situ synchrotron X-ray diffraction. The two techniques complement one another. The activation parameters show that the films deform mainly by dislocations and give indications about their mean free paths, whereas the Bragg peak positions, widths and areas bring invaluable information on residual stress as well as on some plasticity properties, like dislocation storage inside the grains or grain rotations. For the demonstration, two sputter-deposited nanocrystalline 50nm-thin films deposited onto stretchable substrates are studied. It is shown that an as-grown sample (at a homologous temperature of 0.22) presents a stress-assisted annealing, thus decreasing its initial defect density, whereas if a thermal annealing (three hours at 200∘C, corresponding to a homologous temperature of 0.35) has been applied to the sample before the tensile test, it deforms by conventional plasticity, and the dislocations are not stored inside the grains. These mechanisms lead to different work-hardening properties. This work shows how a moderate annealing can have a profound influence on the mechanical behaviour of these thin films.Graphical abstractImage 1
       
  • Experimental study of the γ-surface of austenitic stainless steels
    • Abstract: Publication date: Available online 1 May 2019Source: Acta MaterialiaAuthor(s): Dávid Molnár, Göran Engberg, Wei Li, Song Lu, Peter Hedström, Se Kyun Kwon, Levente Vitos We introduce a theory-guided experimental approach to study the γ-surface of austenitic stainless steels. The γ-surface includes a series of intrinsic energy barriers (IEBs), which are connected to the unstable stacking fault (USF), the intrinsic stacking fault (ISF), the unstable twinning fault (UTF) and the extrinsic stacking fault (ESF) energies. The approach uses the relationship between the Schmid factors and the effective energy barriers for twinning and slip. The deformation modes are identified as a function of grain orientation using in situ electron backscatter diffraction measurements. The observed critical grain orientation separating the twinning and slip regimes yields the USF energy, which combined with the universal scaling law provides access to all IEBs. The measured IEBs and the critical twinning stress are verified by direct first-principles calculations. The present advance opens new opportunities for modelling the plastic deformation mechanisms in multi-component alloys.Graphical abstractImage 1
       
  • 3D Printing of Biomimetic Composites with Improved Fracture Toughness
    • Abstract: Publication date: Available online 30 April 2019Source: Acta MaterialiaAuthor(s): Zian Jia, Lifeng Wang Recent researches show that material microstructural designs mimicking biomaterials offer an effective way to produce tougher materials. To reveal the underlying microstructure-toughness relationship, five bioinspired material microstructures are investigated experimentally, including the brick-and-mortar, cross-lamellar, concentric hexagonal, and rotating plywood microstructure. The feature sizes of these microstructures are controlled to be one order smaller than the specimen size, providing better pictures of how crack resistance interacts with heterogeneity. Fracture theories are further used to analyze the toughening mechanisms and find the design criteria of these microstructures. Results show that the rotating plywood structure presents a “J” shaped R-curve, while other structures show “Γ” shaped R-curves. The “J” shaped R-curve gives a larger critical energy release rate and tolerates a longer crack, thus preferable for crack arresting. By contrast, the “Γ” shaped R-curve provides a larger critical failure stress and is beneficial for preventing crack initiation. Moreover, combined experimental results and theoretical analysis suggest that heterogeneity improves toughness by 1) creating stiffness variations to slow down crack propagation and prevent crack penetration and 2) guiding cracks along weak interfaces to promote progressive damage. Our results shed new light on the structure-property relationships which will facilitate the design of tougher and better crack resistant composites.Graphical abstractImage 1
       
  • Electronic Impact of Concentrated Interstitial Carbon on Physical
           Properties of AISI-316 Austenitic Stainless Steel
    • Abstract: Publication date: Available online 30 April 2019Source: Acta MaterialiaAuthor(s): Zhe Ren, Frank Ernst Applying low-temperature carburization to foils of AISI-316 austenitic stainless steel, we obtained precipitate-free solid solutions of interstitial carbon in austenite with uniform, constant carbon levels ≈0.1, orders of magnitude above the equilibrium solubility limit. In this study, we report the impact of such concentrated interstitial carbon on the mass density, electrical conductivity, thermal conductivity, conduction electron concentration, and electron mobility. Compared to the as-received state, the carburized material only has 0.9 of the mass density, 0.8 of the electrical conductivity, and 0.7 of the thermal conductivity. Hall-effect data indicate that the reduced conductivity is not caused by reduced electron mobility, but exclusively by reduced conduction electron concentration. These observations suggest that the interstitial carbon atoms form covalent bonds with the transition metal atoms. With their unique combination of properties, free-standing uniform concentrated solid solutions of interstitial carbon in austenite, realized here in the form of fully low-temperature-carburized foils, can be regarded as a new material that warrants further studies.Graphical abstractImage 1
       
  • Tuning correlative atomic scale fluctuation and related properties in
           Ni-Nb-Zr metallic glasses
    • Abstract: Publication date: Available online 30 April 2019Source: Acta MaterialiaAuthor(s): T.G. Park, S.Y. Kim, H.S. Ahn, H.S. Oh, D.H. Kim, H.J. Chang, E.S. Park Herein, we systematically investigated how to tailor correlative fluctuation and related properties in Ni-Nb-Zr metallic glasses (MGs). The Ni60Nb40-xZrx MGs (x=0-20 at.%) with Nb-Zr atomic pair (ΔHmix=+4 kJ/mol) exhibited correlative atomic scale fluctuations in composition and topological orders upon Zr addition up to 20 at.% in XAFS analysis, which would, in turn, lead to atomic scale elastic fluctuations. Interestingly, a statistical analysis of strain burst sizes along with in situ bending test showed that more effective elastic interaction between weak spots upon loading is promoted in Ni60Nb20Zr20, which has correlative atomic scale fluctuations, due to a high concentration of loosely-packed regions, thereby resulting in multiple shear bands. The bulk specimens (d=1 mm) for 8 and 10 at.% of Zr with increased glass-forming ability (GFA) exhibited enhanced plasticity without the reduction of fracture strength, implying cooling rate effect on fluctuation-induced plasticity. We believe that the results of this study would provide an effective guideline for tuning correlative fluctuation and related properties in MGs via manipulation of enthalpy relationship and cooling rate.Graphical abstractImage 1
       
  • Incorporation of Sc into the structure of barium-hexaferrite nanoplatelets
           and its ex-traordinary finite-size effect on the magnetic properties
    • Abstract: Publication date: Available online 27 April 2019Source: Acta MaterialiaAuthor(s): Darko Makovec, Matej Komelj, Goran Dražić, Blaž Belec, Tanja Goršak, Sašo Gyergyek, Darja Lisjak In this investigation we analyze an unprecedented difference in the behavior of nanoparticles when compared to the corresponding bulk. We have found that a chemical substitution can have the opposite effect on the magnetic properties of nanoparticles compared to the bulk, as revealed for the first time in the case of Sc-substituted barium-hexaferrite nanoplatelets. Even though the Sc substitution is known to greatly decrease the saturation magnetization, MS, of the bulk barium hexaferrite, it showed the opposite effect for nanoplatelets. The MS values of the nanoplatelets (in average 50 nm wide and approximately 3 nm thick) increased to over 38 Am2/kg, compared to ∼16 Am2/kg for unsubstituted nanoplatelets of comparable average size. The Sc incorporation was investigated with a combination of atomic-resolution imaging and elemental mappings in a scanning-transmission electron microscope. As in the bulk, the Sc3+ ions showed a clear preference for incorporation into an R block of the hexaferrite SRS∗R∗ structure for the nanoplatelets (R and S represent a hexagonal (BaFe6O11)2- and a cubic (Fe6O8)2+ structural block, respectively). A clear difference between the nano and the bulk observed for the first time was in the partial substitution of the Sc3+ for the Ba2+ in the nanoplatelets; however, this cannot explain the large increase in MS. Ab-initio calculations suggest that the opposite effect of the Sc substitution in the nanoplatelets to that in the bulk can be ascribed to specific, two-dimensional magnetic ordering in the platelets.Graphical abstractImage 1
       
  • Dynamic Precipitation and Recrystallization in Mg-9wt.%Al During
           Equal-Channel Angular Extrusion: A Comparative Study to Conventional Aging
           
    • Abstract: Publication date: Available online 26 April 2019Source: Acta MaterialiaAuthor(s): Xiaolong Ma, Suhas Eswarappa Prameela, Peng Yi, Matthew Fernandez, Nicholas M. Krywopusk, Laszlo J. Kecskes, Tomoko Sano, Michael L. Falk, Timothy P. Weihs Precipitation of fine intermetallic particles during conventional thermal aging can significantly enhance the mechanical properties of Al alloys. However, this method offers only limited strengthening in Mg alloys as thermal aging usually leads to intermetallic particles that are too coarse in size and too sparse in spacing. Dynamic precipitation during low-temperature deformation processing offers a chance to rectify this limitation. In addition, mechanical processing often drives precipitation and recrystallization concurrently, therefore, a careful analysis of their interaction and interdependent thermodynamic driving forces is needed. Herein, we investigate dynamic precipitation and recrystallization in a coarse-grained, fully solutionized Mg-9wt.%Al alloy following low-temperature Equal Channel Angular Extrusion (ECAE) using electron microscopy, theoretical calculations, and mechanical property evaluations. Through comparisons with conventionally aged samples, we find that dynamic precipitation during extrusion produces continuous, nanoscale Mg17Al12 particles within grain interiors with a high number density and a low aspect ratio due to strain-induced, defect-assisted nucleation. We quantitatively analyzed the dislocation-accelerated nucleation rate, and the excess vacancy concentration in comparison to the reports in Al alloys. We also find a combined set of reactions that includes discontinuous precipitation and recrystallization along grain boundaries due to the extrusion process. The volume fraction of the combined-reaction region that contains submicron Mg grains and submicron intergranular Mg17Al12 particles grows as the number of passes increases. Using a thermodynamic analysis, we estimate the individual and combined driving forces for precipitation and recrystallization processes at grain boundaries. We identify the chemical energy of the supersaturated Mg matrix as a major driving force for the combined reactions, which indirectly promotes recrystallization and the formation of submicron Mg grains. Our results offer key insights into the evolution of microstructure during dynamic precipitation and recrystallization, and thus provide guidance for the design of improved microstructures in Mg alloys.Graphical abstractImage 1
       
  • Statistical analysis of twin/grain boundary interactions in pure rhenium
    • Abstract: Publication date: Available online 26 April 2019Source: Acta MaterialiaAuthor(s): Josh Kacher, Julian E. Sabisch, Andrew M. Minor Using electron backscatter diffraction, over 1,000 grain boundary/twin interactions were investigated in pure Re after uniaxial compression. Crystallographic factors such as grain boundary disorientation angle, displacement gradient tensor accommodation, and twin plane and shear vector alignment were taken into account as well as the macroscopic Schmid factor. It was found that the crystallographic relationship between coincident twin pairs at grain boundaries fall into two categories. In the first category, the twin pairs satisfied two criteria: the twin plane alignment was maximized across the boundary and the twin variant was kept constant across the boundary. In the second category, comprising approximately 5% of the characterized interactions, twin variant selection appeared to be driven by the macroscopically applied stress state, as resolved by the Schmid factor. In comparison to published results on twin/grain boundary interactions in Mg and Zr, it was found that twins transmit across grain boundaries in Re far more readily, regardless of the grain boundary disorientation angle. This is likely to be a function of the anomalously low twin boundary energy in Re as well as the difference in magnitude of the twinning-induced deformation that must be accommodated at the boundary, with the {112¯1} system being dominant in Re and the {101¯2} being dominant in Mg and Zr.Graphical abstractImage 1
       
  • Permeability of Porous Ceramics by X-ray CT Image Analysis
    • Abstract: Publication date: Available online 26 April 2019Source: Acta MaterialiaAuthor(s): Seth Nickerson, Yin Shu, Danhong Zhong, Carsten Könke, Adama Tandia This work presents a method to 1) statistically characterize the complex geometry of porous material microstructures, 2) parameterize these random microstructures into statistically condense, feature rich metrics, and 3) relate these metrics to material properties such as permeability. This method is applied to porous synthetic cordierite and aluminum titanate materials covering a range of 40%-70% porosity and 8-18 μm median pore size (as measured by mercury intrusion porosimetry), using X-ray computed tomography images of the porous microstructure. Direct simulations of the microstructures were made to estimate effective permeability and correlations are presented to microstructural metrics. High quality relationships are established, specifically utilizing 2-point correlation functions and lineal path function characteristic dimensions. Results are compared to legacy methods such as the widely known relationships proposed by Kozeny-Carman and Kuwabara.Graphical abstractImage 1
       
  • In-situ Study of Planar Slip in a Commercial Aluminum-Lithium Alloy using
           High Energy X-ray Diffraction Microscopy
    • Abstract: Publication date: Available online 25 April 2019Source: Acta MaterialiaAuthor(s): Wesley A. Tayon, Kelly E. Nygren, Roy E. Crooks, Darren C. Pagan Aluminum-lithium alloys offer high specific-strength and -stiffness which makes them attractive for aerospace applications. However, several material related challenges have limited the use of these alloys including a tendency for grain boundary cracking due to localized damage known as delamination. Prior studies of these alloys have found evidence of a relationship between delamination and the occurrence of intense planar slip due to the shearing of the δ' (Al3Li) precipitate phase. In this study, far-field high-energy X-ray diffraction microscopy is used to quantify the effects of slip activity and subsequent stress response of the Al-Cu-Li alloy (AA2099) in both individual grains and various grain populations. Results show that grains with high Schmid factors, interpreted as an indicator of propensity for single slip at the elastic-plastic transition, are most likely to undergo grain softening consistent with precipitate-shear-driven planar slip. Conversely, grains with low Schmid factors are less prone to this deformation mode. A corresponding rise in triaxiality is found in grains which soften. The implications of pairings of grains exhibiting dissimilar micromechanical behaviors for delamination are discussed.Graphical abstractImage 1
       
  • Coercivity enhancement of selective laser sintered NdFeB magnets by grain
           boundary infiltration
    • Abstract: Publication date: Available online 25 April 2019Source: Acta MaterialiaAuthor(s): Christian Huber, Hossein Sepehri-Amin, Michael Goertler, Martin Groenefeld, Iulian Teliban, Kazuhiro Hono, Dieter Suess Laser powder bed fusion is a well-established additive manufacturing method that can be used for the production of net-shaped Nd-Fe-B magnets. However, low coercivity has been one of the drawbacks in the laser powder bed fusion processed Nd-Fe-B magnets. In this work, we have demonstrated that the grain boundary diffusion process using low-melting Nd-Cu, Nd-Al-Ni-Cu, and Nd-Tb-Cu alloys to the selective laser sintered NdFeB magnets can results in a substantial enhancement of coercivity from 0.65 T to 1.5 T. Detailed microstructure investigations clarified that the formation of Nd-rich grain boundary phase, the introduction of Tb-rich shell at the surface of Nd2Fe14B grains, and maintaining the grain size in nano-scale are responsible for the large coercivity enhancement.Graphical abstractImage 1
       
  • Quantifying competitive grain overgrowth in polycrystalline ZnO thin films
    • Abstract: Publication date: Available online 25 April 2019Source: Acta MaterialiaAuthor(s): A. Brian Aebersold, Lorenzo Fanni, Aïcha Hessler-Wyser, Sylvain Nicolay, Christophe Ballif, Cécile Hébert, Duncan T.L. Alexander The grain size evolution of polycrystalline thin films, which form by competitive grain overgrowth as commonly interpreted using the van der Drift model, is understood to follow a power-law scaling of the average grain size d with film thickness h, i.e. d ∝ hα. While simulations have identified a growth exponent α = 0.4 for three-dimensional growth, previous experimental studies did not confirm this value, instead finding α values in the range of ∼0.5–0.7. Here we study competitive grain overgrowth using a system of ZnO thin films grown by low-pressure metal–organic chemical vapor deposition. We present quantitative data on the evolution of grain size and orientation across the thickness of thin films, obtained by automated crystal orientation mapping of “double-wedge” transmission electron microscopy samples. The data from a-textured ZnO films, grown under three different conditions, are compared against van der Drift model predictions of self-similarity of the grain size distribution and the power-law scaling. The results are further interpreted by comparing to simulations of facetted polycrystalline film growth, which we adapt to the ZnO system by including idiomorphic growth shapes with a six-fold symmetry and random or biased nuclei orientations. As well as showing the predicted self-similarity of grain size distributions during growth, for the first time our experimental data confirm a power-law growth exponent of α = 0.4, as also predicted by the simulations using randomly oriented nuclei. Nevertheless, interpretation of this result is contingent on the absence of factors such as textured nucleation and renucleation during film growth. Indeed, only one film, grown at a higher ratio of H2O/DEZ precursor gases, displaying random initial nucleation, and minimal grain renucleation during growth, shows a proper conformance to the model nature and predictions.Graphical abstractImage 1
       
  • Hardening mechanisms and impact toughening of a high-strength steel
           containing low Ni and Cu additions
    • Abstract: Publication date: Available online 24 April 2019Source: Acta MaterialiaAuthor(s): H.J. Kong, C. Xu, C.C. Bu, C. Da, J.H. Luan, Z.B. Jiao, G. Chen, C.T. Liu Aging treatments at 400–550˚C are commonly used to attain a peak strengthening for the Cu-rich nanocluster-strengthened high-strength low-alloy (HSLA) steels. However, these temperatures fall within the dangerous 300–600˚C temper-embrittlement regime, leading to poor impact toughness. On the other hand, aging at temperatures above the embrittlement regime can improve the impact toughness but at a great expense of strength. In this work, the strengthening mechanisms as well as the toughening of a low cost weldable HSLA steel with a low content of carbon (C ∼0.08 wt.%), nickel (Ni =0.78 wt.%), and copper (Cu =1.3 wt.%) were carefully investigated. Our findings show that the low-C-Ni-Cu HSLA steel is insensitive to the aging temperatures and can achieve a yield strength (YS) and ultimate tensile strength (UTS) over 1000 and 1100 MPa, respectively, with tensile ductility>10% (reduction of area>60%) at a heat-treat temperature of 640˚C through multiple strengthening mechanisms. Besides, a good low-temperature (-40˚C) impact performance (∼200 J) with high YS (∼900 MPa) and UTS (∼1000 MPa) can be obtained by seeking a strength balance among the fine grain size (∼2.5 μm), medium-sized (∼14 nm) overaged Cu-rich precipitates, tempered martensite, and fresh martensite (or carbides). Moreover, a relatively lower YS (∼800 MPa) and UTS (∼900 MPa) useful for steel manufacturing can be attained by a prolonged aging at 640˚C. In addition, the dislocation-precipitate interactions were also explored based on the dislocation theories in this study.Graphical abstractImage 1
       
  • Effects of lamellar structure on tensile properties and resistance to
           hydrogen embrittlement of pearlitic steel
    • Abstract: Publication date: Available online 24 April 2019Source: Acta MaterialiaAuthor(s): Sang-Hyun Yu, Sang-Min Lee, Sukjin Lee, Jae-Hoon Nam, Jae-Seung Lee, Chul-Min Bae, Young-Kook Lee The hydrogen embrittlement (HE) and H trapping sites of pearlitic steel specimens with various lamellar spacings (λ) were evaluated through slow strain rate tensile testing and thermal desorption analysis. When λ decreases, both tensile strength and resistance to HE were unusually improved. This is because tearing, which is the initiation of H cracking, was delayed in the specimen with fine λ and short cementite (θ) platelets. Undeformed H-charged specimens showed a peak (peak 1), which is separable into two sub-peaks (peak 1-1 and peak 1-2) in their H desorption rate curves, regardless of λ. Peak 1-1 and peak 1-2 were generated by H atoms detrapped from FP/θ interfaces and from dislocations inside FP, respectively. The Ea values of H desorption for peak 1-1 and peak 1-2 were 23.2 kJ/mol, and 26.1 kJ/mol, respectively. Meanwhile, deformed H-charged specimens exhibited the second peak (peak 2) with peak temperature (TP) of ∼600 K, as well as peak 1 with TP of ∼375 K. When tensile strain increased, peak 2 increased at the expense of peak 1. Primary H trapping sites for peak 2 are strained FP/θ interfaces with interfacial dislocations.Graphical abstractImage 1
       
  • Colossal magnetoresistance in the insulating ferromagnetic double
           perovskites Tl2NiMnO6: A neutron diffraction study
    • Abstract: Publication date: Available online 24 April 2019Source: Acta MaterialiaAuthor(s): Lei Ding, Dmitry D. Khalyavin, Pascal Manuel, Joseph Blake, Fabio Orlandi, Wei Yi, Alexei A. Belik In the family of double perovskites, colossal magnetoresistance (CMR) has been so far observed only in half-metallic ferrimagnets such as the known case Sr2FeMoO6 where it has been assigned to the tunneling MR at grain boundaries due to the half-metallic nature. Here we report a new material−Tl2NiMnO6, a relatively ordered double perovskite stablized by the high pressure and high temperature synthesis−showing CMR in the vicinity of its Curie temperature. We explain the origin of such effect with neutron diffraction experiment and electronic structure calculations that reveal the material is a ferromagnetic insulator. Hence the ordered Tl2NiMnO6 (∼70% of Ni2+/Mn4+ cation ordering) represents the first realization of a ferromagnetic insulating double perovskite, showing CMR. The study of the relationship between structure and magnetic properties allows us to clarify the nature of spin glass behaviour in the disordered Tl2NiMnO6 (∼31% of cation ordering), which is related to the clustering of antisite defects and associated with the short-range spin correlations. Our results highlight the key role of the cation ordering in establishing the long range magnetic ground state and lay out new avenues to exploit advanced magnetic materials in double perovskites.Graphical abstractImage 1
       
  • Systematic design and realization of double-negative acoustic
           metamaterials by topology optimization
    • Abstract: Publication date: Available online 24 April 2019Source: Acta MaterialiaAuthor(s): Hao-Wen Dong, Sheng-Dong Zhao, Peijun Wei, Li Cheng, Yue-Sheng Wang, Chuanzeng ZhangDouble-negative acoustic metamaterials (AMMs) offer the promising ability of superlensing for applications in ultrasonography, biomedical sensing and nondestructive evaluation. However, the systematic design and realization of broadband double-negative AMMs are stilling missing, which hinder their practical implementations. In this paper, under the simultaneous increasing or non-increasing mechanisms, we develop a unified topology optimization framework involving different microstructure symmetries, minimal structural feature sizes and dispersion extents of effective parameters. The optimization framework is applied to conceive the heuristic resonance-cavity-based and space-coiling metamaterials with broadband double negativity. Meanwhile, we demonstrate the essences of double negativity derived from the novel artificial multipolar LC (inductor-capacitor circuit) and Mie resonances which can be induced by controlling mechanisms in optimization. Furthermore, abundant numerical simulations validate the corresponding double negativity, negative refraction, enhancement of evanescent waves and subwavelengh imaging. Finally, we experimentally show the desired broadband subwavelengh imaging by using the 3D-printed optimized space-coiling metamaterial. The present design methodology provides an ideal approach for constructing the constituent “atoms” of metamaterials according to any artificial physical and structural requirements. In addition, the optimized broadband AMMs and superlens lay the structural foundations of subwavelengh imaging technology.Graphical abstractImage 1
       
  • Atomic Origins of Radiation-induced Defects and the Role of Lamellar
           Interfaces in Radiation Damage of Titanium Aluminide Alloy Irradiated with
           Kr-ions at Elevated Temperature
    • Abstract: Publication date: Available online 24 April 2019Source: Acta MaterialiaAuthor(s): Hanliang Zhu, Mengjun Qin, Robert Aughterson, Tao Wei, Gregory Lumpkin, Yan Ma, Huijun Li The irradiation microstructure of a titanium aluminide (TiAl) alloy subjected to in situ transmission electron microscope (TEM) irradiation with 1 MeV Kr ions at the elevated temperature of 873K was investigated. Triangle and large hexagon shaped volume defects were observed within the γ-TiAl phase in the TEM images of the irradiated microstructure. High resolution TEM images and composition analyses revealed that the volume defects were vacancy-type stacking fault tetrahedra (SFTs). Molecular dynamic simulations showed that the increased diffusion coefficient at the elevated temperature promoted the movement and aggregation of vacancies, leading to the formation and growth of SFTs in the irradiated FCC γ phase. The lamellar interfaces in the irradiation microstructure were more effective for acting as strong sinks to absorb the primary point defects and defect clusters at the elevated temperature. The initial defects at the interfaces of the TiAl alloy enhanced the sink strength of the material and played an important role in refining SFTs near the lamellar interfaces.Graphical abstractImage 1
       
  • Effect of Stacking Fault Segregation and Local Phase Transformations on
           Creep Strength in Ni-base Superalloys
    • Abstract: Publication date: Available online 23 April 2019Source: Acta MaterialiaAuthor(s): T.M. Smith, B.S. Good, T.P. Gabb, B.D. Esser, A.J. Egan, L.J. Evans, D.W. McComb, M.J. Mills In this study, two similar, commercial polycrystalline Ni-based disk superalloys (LSHR and ME3) were creep tested at 760°C and 552MPa to approximately 0.3% plastic strain. LSHR consistently displayed superior creep properties at this stress/temperature regime even though the microstructural characteristics between the two alloys were comparable. High resolution structural and chemical analysis, however, revealed significant differences between the two alloys among active γ′ shearing modes involving superlattice intrinsic and extrinsic stacking faults. In ME3, Co and Cr segregation and Ni and Al depletion were observed along the intrinsic faults - revealing a γ′ to γ phase transformation. Conversely in LSHR, an alloy with a higher W content, Co and W segregation was observed along the intrinsic faults. This observation combined with scanning transmission electron microscopy (STEM) simulations confirm a γ′-to-D019 χ phase transformation along the intrinsic faults in LSHR. Using experimental observations and density functional theory calculations, a novel local phase transformation strengthening mechanism is proposed that could be further utilized to improve the high temperature creep capabilities of Ni-base disk alloys.Graphical abstractImage 1
       
  • Electric field distribution in porous piezoelectric materials during
           polarization
    • Abstract: Publication date: Available online 23 April 2019Source: Acta MaterialiaAuthor(s): Germán Martínez-Ayuso, Michael I. Friswell, Hamed Haddad Khodaparast, James I. Roscow, Christopher R. Bowen High piezoelectric coupling coefficients enable the harvesting of more energy or increase the sensitivity of sensors which work using the principle of piezoelectricity. These coefficients depend on the material properties, but the manufacturing process can have a significant impact on the resulting overall coefficients. During the manufacturing process, one of the main steps is the process of polarization where a poling electric field aligns the ferroelectric domains in a similar direction in order to create a transversely isotropic material able to generate electric fields or deformations. The degree of polarization depends on multiple factors and it can strongly influence the final piezoelectric coefficients. In this paper, a study on the electric field distribution on the sensitivity of the main piezoelectric and dielectric coefficients to the polarization process is performed, focusing on porous piezoelectric materials. Different inclusion geometries are considered, namely spherical, ellipsoidal and spheres with cracks. The electric field distribution at the micro scale within a representative volume element is modelled to determine the material polarization level using the finite element method. The results show that the electric field distribution is highly dependent on the inclusion geometries and cracks and it has a noticeable impact on the equivalent piezoelectric coefficients. These results are compared with experimental measurements from published literature. Good agreement is found between the ellipsoidal model and the experimental data.
       
  • Structural and magnetic properties of Ce1−xSmxFe11−yTi1Vy
    • Abstract: Publication date: Available online 22 April 2019Source: Acta MaterialiaAuthor(s): D. Simon, H. Wuest, S. Hinderberger, T. Koehler, A. Marusczyk, S. Sawatzki, L.V.B. Diop, K. Skokov, F. Maccari, A. Senyshyn, H. Ehrenberg, O. Gutfleisch In order to find a promising trade-off permanent magnet material regarding a performance/cost-ratio, the Ce1−xSmxFe11−yTi1Vy-phase (x=0–1; y=0, 1) is analyzed in detail. In the first part, its existence range is studied (1000∘C) and the intrinsic magnetic properties are comprehensively determined. Diffraction experiments localize both structure-stabilizing transition metals on 8i-sites, explaining the measured reduction in saturation polarization as V is added. Curie temperatures increase upon SM-substitution with a negligible dependence on V. Annealings of nanocrystalline material produced via intensive milling and melt-spinning show that V especially raises the obtainable maximum coercivities for SM-rich phases (924 kA/m).In the second part, the promising magnetic properties of the nanocrystalline material are successfully transferred to the bulk state via hot-pressing. The isotropic Ce0.5Sm0.5Fe10Ti1V1-magnet (coercivity = 425 kA/m) is characterized by various means. Magnetic measurements, structural investigations and calculations of the elastic constants consider necessary factors for a successful texturing by die-upsetting (as accepted for Nd-Fe-B). The results are fundamental for further considerations in this active field of research.Graphical abstractImage 1
       
  • Effect of Al, Ti and Cr Additions on the γ-γ’ Microstructure of W-free
           Co-Ta-V-Based Superalloys
    • Abstract: Publication date: Available online 22 April 2019Source: Acta MaterialiaAuthor(s): Fernando L. Reyes Tirado, Spencer Taylor, David C. Dunand The recently-discovered metastable γ’-Co3(Ta0.76V0.24) phase formed on aging in a Co-6Ta-6V (at.%) ternary alloy is stabilized partial replacement of Ta and V with Al and Ti. In two alloys with composition Co-6Al-3Ta-3V and Co-5Al-3Ta-3V-1Ti with γ+γ’ microstructure, the γ’-precipitates remain stable for up to 168 h at 850 and 900 °C, with no precipitation of additional phases. Adding Ni and B and doubling the Ti concentration produces a γ/γ’ superalloy, Co-10Ni-5Al-3Ta-3V-2Ti-0.04B (at.%), with γ’ precipitates which are stable up to 6 weeks of aging at 850 °C, while slowly coarsening and coalescing from cuboidal to elongated shapes. After 1 day of aging at 850 °C, the γ’ nanoprecipitates have (Co0.87Ni0.17)3(Ta0.42Al0.23Ti0.19V0.15B0.01) composition, with Al and Ti replacing at the same rate both Ta and V in the original metastable Co3(Ta0.76V0.24) phase. To improve oxidation resistance, 4% Cr is added to the new superalloy, resulting in a somewhat higher volume fraction of finer cuboidal γ’ precipitates after one week of aging at 850 ºC, but no other deleterious phases. These W- and Mo-free γ/γ’ superalloys show good creep resistance at 850 °C, on par with two other recent Co-base γ/γ’ superalloys: (i) Co-9W-9Al-8Cr (at.%) which has higher density due to its high W content, and (ii) Co-30Ni-10Al-5Mo-2Nb (at.%) which has lower density (as it is W-free) but contains triple the Ni concentration.Graphical abstractImage 1
       
  • Designing high ductility in magnesium alloys
    • Abstract: Publication date: Available online 22 April 2019Source: Acta MaterialiaAuthor(s): Rasool Ahmad, Binglun Yin, Zhaoxuan Wu, W.A. Curtin The thermally activated pyramidal-to-basal (PB) transition of 〈c+a〉 dislocations, transforming glissile pyramidal dissociated core structures into sessile basal dissociated ones, lies at the origin of low ductility in pure magnesium (Mg). Solute-accelerated cross-slip and double cross-slip of pyramidal 〈c+a〉 dislocations have recently been proposed as a mechanism that can circumvent the deleterious effects of the PB transition by enabling rapid dislocation multiplication and isolating PB-transformed sessile segments. Here, the theory for solute-accelerated cross-slip is revisited with an explicit atomistic derivation, is extended to include multiple very dilute solute concentrations, and various aspects of the theory are demonstrated computationally. DFT inputs to the theory for a wide range of new alloying elements are presented. The theory is validated by comparing predicted ductility to literature experiments for a range of alloys. The theory is then applied to predict composition ranges for ductility in rare-earth free ternary and quaternary dilute alloys. The wide range of new alloys predicted to be ductile can serve as a guide to experimental development of new ductile Mg alloys.Graphical abstractImage 1
       
  • Ab initio study of phosphorus effect on vacancy-mediated process in nickel
           alloys – an insight into Ni2Cr ordering
    • Abstract: Publication date: Available online 21 April 2019Source: Acta MaterialiaAuthor(s): Jia-Hong Ke, George A. Young, Julie D. Tucker The development of long range order in nickel-chromium alloys is of great technological interest but the kinetics and mechanisms of the transformation are poorly understood. The present research utilizes a combined computational and experimental approach to elucidate the mechanism by which phosphorus accelerates the ordering rate of stoichiometric Ni2Cr in Ni-Cr alloys. A series of Ni-33%Cr-x%P samples (in atomic percent) were fabricated with phosphorus concentrations, x =
       
  • A Method to Predict the Glass Transition Temperature in Metallic Glasses
           from Properties of the Equilibrium Liquid
    • Abstract: Publication date: Available online 20 April 2019Source: Acta MaterialiaAuthor(s): R. Dai, A.K. Gangopadhyay, R.J. Chang, K.F. Kelton The glass transition temperature, Tg, is important for predicting glass formation and stability and for designing processing steps for tailoring the glass to specific applications. It is conventionally determined from measurements of the shear viscosity or specific heat. This requires that the glass first be made, significantly limiting the use of this parameter for the prediction of glass formation. Further, viscosity or specific heat measurements can only be made for glasses that have a high stability against crystallization at Tg. It is demonstrated here that it is possible to accurately predict Tg from measurements of the shear viscosity and thermal expansion coefficient of the high temperature equilibrium liquid. In addition to the practical usefulness of this, the connection between the glass transition and the properties of the equilibrium liquid may shed light into the processes that determine glass formation in metallic alloys and of the nature of the glass transition.Graphical abstractImage 1
       
  • Microstructure and coercivity of grain boundary diffusion processed
           Dy-free and Dy-containing Nd-Fe-B sintered magnets
    • Abstract: Publication date: Available online 20 April 2019Source: Acta MaterialiaAuthor(s): T.-H. Kim, T.T. Sasaki, T. Ohkubo, Y. Takada, A. Kato, Y. Kaneko, K. Hono Dy distributions in Dy-free and Dy-containing Nd-Fe-B sintered magnets in the course of the Dy-vapor grain boundary diffusion (GBD) process have been investigated in order to understand the origin of the high coercivity of 3.0 T that is reachable only when the initial magnet is alloyed with Dy. We have discovered the formation of a secondary Dy-rich shell within the well-known primary Dy-rich shell, which is a key contributor to the 3.0 T coercivity. The coercivity increment of the Dy-containing magnet after the GBD treatment was only 0.08 T, much lower than 0.87 T for the Dy-free magnet; however, it was substantially enhanced by a post-diffusion annealing. Compared to the Dy-free magnet, a larger amount of Dy atoms were diffused from the Nd-rich grain boundary (GB) phase to the primary Dy-rich shell in the Dy-containing magnet after the annealing, resulting in the formation of a secondary Dy-rich shell with a higher Dy-concentration at the GB phase/secondary shell interfaces.Graphical abstractImage 1
       
  • Toughness Enhancement in TiN/WN Superlattice Coatings
    • Abstract: Publication date: Available online 17 April 2019Source: Acta MaterialiaAuthor(s): Julian Buchinger, Nikola Koutná, Zhuo Chen, Zaoli Zhang, Paul Heinz Mayrhofer, David Holec, Matthias Bartosik Transition metal nitride thin films traditionally possess a low intrinsic fracture toughness. Motivated by the recently discovered fracture toughness enhancing superlattice (SL) effect, as well as the remarkably high potential for toughness predicted by theoretical studies for TiN/WN superlattices, we synthesise a series of these materials by DC reactive magnetron sputtering. The SL coatings demonstrate a vacancy-stabilised cubic configuration throughout, as well as a marginal lattice mismatch between the TiN and WN layers. All investigated mechanical properties produced a distinct dependence on the bilayer period, featuring a hardness peak of 36.7 ± 0.2 GPa and a minimum of the indentation modulus of 387 ± 2 GPa. The toughness-related quantities of the SLs in particular show a significant enhancement compared to monolithic TiN and WN, including a tripling of the fracture energy. The fracture toughness is raised from 2.8 ± 0.1 (TiN) and 3.1 ± 0.1 (WN) to 4.6 ± 0.2 MPa√m by the SL arrangement. We relate this maximisation to the vastly disparate elastic moduli and compositional fluctuations. To complement our experimental data, we present Density Functional Theory-based models to disentangle the conspicuous trends observed for TiN/WN superlattices.Graphical abstractImage 1
       
  • Sustainable and simple processing technique for n-type skutterudites with
           high ZT and their analysis
    • Abstract: Publication date: Available online 16 April 2019Source: Acta MaterialiaAuthor(s): Gerda Rogl, Kunio Yubuta, Michael Kerber, Andriy Grytsiv, Michael Zehetbauer, Ernst Bauer, Peter Rogl As various branches of industry are interested in good thermoelectric generators, there is a serious demand for an easy, fast, cheap and sustainable production route of excellent thermoelectric materials. In this paper we demonstrate how we could achieve a high-ZT (ZT ∼ 1.45 at 823 K) n-type bulk skutterudite, (Mm,Sm)yCo4Sb12, by processing the industrially produced raw powder using a modified high-pressure torsion (HPT) equipment at elevated temperature under inert gas. For this HPT processed sample as well as for the annealed one, structural properties, especially focused on grain size and dislocation density, (backed up by electron probe microanalysis, transmission electron microscopy investigations as well as XPD profile analysis), physical (from 300 - 823 K and for the electrical resistivity in addition from 4.2 – 300 K) and mechanical properties (elastic moduli, hardness, fracture resistance) were measured and evaluated and compared with the values of a traditionally prepared bulk material, using hot pressing (HP). As a consequence of severe plastic deformation, we found a change from metallic to semiconducting behavior during measurement induced annealing of the HPT processed skutterudite, which could be explained via the change of lattice distortion and its influence on the band gap. A positive ZT net effect occurred because although the electrical resistivity was enhanced the thermal conductivity was decreased. It turned out that the elastic moduli of the HPT samples were not much different from those of the HP skutterudite, however, that the hardness was significantly increased.Graphical abstractImage 1
       
  • Effect of powder characteristics and oxygen content on modifications to
           the microstructural topology during hot isostatic pressing of an
           austenitic steel
    • Abstract: Publication date: Available online 5 April 2019Source: Acta MaterialiaAuthor(s): S. Irukuvarghula, H. Hassanin, C. Cayron, M. Aristizabal, M.M. Attallah, M. Preuss The effect of powder size distribution and oxygen content on the extent of multiple twinning and spatial distribution of oxide inclusions in hot isostatic pressed (HIPed) 316L steels was investigated using powders with different characteristics. Modifications to, and differences in their microstructural topology, were tracked quantitatively by evaluating the metrics related to twin related domains (TRDs) on specimens produced by interrupting the HIPing process at various points in time. Results revealed that powder size distribution has a strong effect on the extent of multiple twinning in the fully HIPed microstructure, with specimens produced using narrow distribution showing better statistics (i.e., homogeneously recrystallized) than the ones produced using broad size distribution. The oxide inclusion density in fully HIPed microstructures increased with the amount of oxygen content in the powders while prior particle boundaries (PPBs) were only observed in the specimens that were HIPed using broad powder distribution. More importantly, results clearly revealed that the spatial distribution of the inclusions was strongly affected by the homogeneity of recrystallization. Implications of the results are further discussed in a broader context, emphasizing the importance of utilizing the occurrence of solid state phase transformations during HIPing for controlling the microstructure evolution.Graphical abstractImage 1
       
  • Analysis of the Partial Molar Excess Entropy of Dilute Hydrogen in Liquid
           Metals and Its Change at the Solid-Liquid Transition
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): Andrew H. Caldwell, Antoine Allanore A systematic change in the partial molar enthalpy of mixing (Δh¯Hmix) and partial molar excess entropy (Δs¯Hex) for dilute hydrogen-metal systems at the solid-liquid transition is reported. Expressions for Δh¯Hmixand Δs¯Hexare derived from the Fowler model of hydrogen solubility, and the change in Δs¯Hexat melting is bounded. The theoretical bound is in agreement with measured data. A connection is made between the change in Δs¯Hexand short range order in the metal-hydrogen system.Graphical abstractImage 1
       
 
 
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