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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 293  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3184 journals]
  • Editors for Acta Materialia
    • Abstract: Publication date: 1 August 2019Source: Acta Materialia, Volume 174Author(s):
  • Creep Resistance of Bulk Copper-Niobium Composites: an Inverse Effect of
           Multilayer Length Scale
    • Abstract: Publication date: Available online 22 June 2019Source: Acta MaterialiaAuthor(s): Jaclyn T. Avallone, Thomas J. Nizolek, Benjamin B. Bales, Tresa M. Pollock Metallic multilayer systems show promising performance in extreme environments, with high stability of bi-metal interfaces down to nanometer length scales. The creep behavior of bulk, accumulative roll bonded (ARB) Copper-Niobium (Cu-Nb) composites has been studied at 400 °C as a function of layer thickness, ranging from 2 μm to 65 nm. Similar to single phase metallic systems, three regimes are observed during creep: transient, steady-state and tertiary. The mechanism controlling minimum creep rate for all conditions tested has a strong dependence on stress, consistent with dislocation-dominated creep. Unlike the conventional effect of grain size on creep resistance, this study reveals that decreasing length scale increases creep resistance.Graphical abstractImage 1
  • Exploring Li-ion conductivity in cubic, tetragonal and mixed-phase
           Al-substituted Li7LaZr2O12using atomistic simulations and effective medium
    • Abstract: Publication date: Available online 22 June 2019Source: Acta MaterialiaAuthor(s): Mauricio R. Bonilla, Fabián A. García Daza, Javier Carrasco, Elena Akhmatskaya Garnet LiLaZrO (LLZO) is a promising solid electrolyte candidate for solid-state Li-ion batteries, but at room temperature it crystallizes in a poorly Li-ion conductive tetragonal phase. To this end, partial substitution of Li by Al ions is an effective way to stabilize the highly conductive cubic phase at room temperature. Yet, fundamental aspects regarding this aliovalent substitution remain poorly understood. In this work, we use molecular dynamics and advanced hybrid Monte Carlo methods for systematic study of the room temperature Li-ion diffusion in tetragonal and cubic LLZO to shed light on important open questions. We find that Al substitution in tetrahedral sites of the tetragonal LLZO allows previously inaccessible sites to become available, which enhances Li-ion conductivity. In contrast, in the cubic phase Li-ion diffusion paths become blocked in the vicinity of Al ions, resulting in a decrease of Li-ion conductivity. Moreover, combining the conductivities of individual phases through an effective medium approximation allowed us to estimate the conductivities of cubic/tetragonal phase mixtures that are in good agreement with those reported in several experimental works. This suggests that phase coexistence (due to phase equilibrium or gradients in Al content within a sample) could have a significant impact on the conductivity of Al-substituted LLZO, particularly at low contents of Al. Overall, by making a thorough comparison with reported experimental data, the theoretical study and simulations of this work advance our current understanding of Li-ion mobility in Al-substituted LLZO garnets and might guide future in-depth characterization experiments of this relevant energy storage material.Graphical abstractImage 1
  • Determination of single-crystal elasticity constants of the beta phase in
           a multiphase tungsten thin film using impulse excitation technique, X-ray
           diffraction and micro-mechanical modeling
    • Abstract: Publication date: Available online 21 June 2019Source: Acta MaterialiaAuthor(s): Mohamed Fares Slim, Akram Alhussein, Elia Zgheib, Manuel François The scope of this work is to propose a methodology allowing the determination of the single-crystal elasticity constants of a phase included in a multiphase thin film taking into account its microstructure (crystallographic and morphological texture, porosity and multiphase aspect). The methodology is based on the use of a macro-mechanical test, the impulse excitation technique, a micro-mechanical test, X-ray diffraction and the Kröner-Eshelby scale transition model. As a supporting example, it was applied to determine the single-crystal elasticity constants of the Wβ tungsten metastable phase embedded in a two phases (α+β) tungsten thin film deposited on a steel substrate by DC magnetron sputtering. The effects of the grain-shape, the crystallographic texture, the porosity and the Wβ volume fraction on the macroscopic elasticity constants were studied. Among all these effects, it was found that the effect of the Wβ volume fraction was the most pronounced. The effects of the crystallographic and morphological texture on the microscopic elastic behavior of the film were evaluated. No dominance of the crystallographic or morphological texture effect was observed and their contributions depend on the crystallographic plane and the measurement direction.Graphical abstractSchematic representation of the methodology used in the determination of the single-crystal elasticity constants.Image 1
  • Doping effects of point defects in shape memory alloys
    • Abstract: Publication date: Available online 21 June 2019Source: Acta MaterialiaAuthor(s): Yuanchao Yang, Dezhen Xue, Ruihao Yuan, Yumei Zhou, Turab Lookman, Xiangdong Ding, Xiaobing Ren, Jun Sun Doping point defects into shape memory alloys (SMAs) influences their transformation behavior and mechanical properties. We propose a general Landau free energy model to study doping effects, which only assume that point defects produce local dilatational stresses coupled to the non-order parameter volumetric strain. Different dopants can be represented by their range of interaction and potency of dilatational stress. Time-dependent simulations based on our model successfully reproduce experimentally observed doping effects in SMAs, including the elevation or suppression of the transformation temperature, the modification of mechanical properties, the appearance of a cross-hatched tweed structure and the emergence of a frozen glassy state with local strain order. We predict that the temperature range for superelasticity will be enhanced in the crossover regime between martensite and strain glass. In addition, an Elinvar effect appears most likely in alloys with dopants tending to increase the transformation temperature, which needs to be verified experimentally. Moreover, the two dopant parameters in the Landau model, the interaction range and potency of the dilatational stress, inspire us to identify three material descriptors with which we can construct an empirical machine learning model. The model predicts the transformation temperature, and the slope of the change in transformation temperature as a function of doping composition, enabling an effective search for doped SMAs with targeted properties via machine learning.Graphical abstractImage 1
  • High-temperature X-ray absorption spectroscopy study of thermochromic
           copper molybdate
    • Abstract: Publication date: Available online 21 June 2019Source: Acta MaterialiaAuthor(s): Inga Jonane, Andris Anspoks, Giuliana Aquilanti, Alexei Kuzmin X-ray absorption spectroscopy at the Cu and Mo K-edges was used to study the effect of heating on the local atomic structure and dynamics in copper molybdate (α-CuMoO4) in the temperature range from 296 to 973 K. The reverse Monte-Carlo (RMC) method was successfully employed to perform accurate simulations of EXAFS spectra at both absorption edges simultaneously. The method allowed us to determine structural models of α-CuMoO4 being consistent with the experimental EXAFS data. These models were further used to follow temperature dependencies of the local environment of copper and molybdenum atoms and to obtain the mean-square relative displacements for Cu–O and Mo–O atom pairs. Moreover, the same models were able to interpret strong temperature-dependence of the Cu K-edge XANES spectra. We found that the local environment of copper atoms is more affected by thermal disorder than that of molybdenum atoms. While the MoO4 tetrahedra behave mostly as the rigid units, a reduction of correlation in atomic motion between copper and axial oxygen atoms occurs upon heating. This dynamic effect seems to be the main responsible for the temperature-induced changes in the O2−→Cu2+ charge transfer processes and, thus, is the origin of the thermochromic properties of α-CuMoO4 upon heating above room temperature.Graphical abstractImage 1
  • High temperature strength of refractory complex concentrated alloys
    • Abstract: Publication date: Available online 20 June 2019Source: Acta MaterialiaAuthor(s): O.N. Senkov, S. Gorsse, D.B. MiracleABSTRACTThermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, Tm) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000°C and 1200°C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 Tm, so that the alloys with higher Tm retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000°C is probably due to dissolution of secondary phases.Graphical abstractImage 1
  • Incubation time for flash sintering as caused by internal reactions,
           exemplified for yttria stabilized zirconia
    • Abstract: Publication date: Available online 20 June 2019Source: Acta MaterialiaAuthor(s): Reiner Kirchheim Transient and steady state electrotransport and diffusion of ions being produced and annihilated by internal reactions are discussed and exemplified for Yttria Stabilized Zirconia (YSZ). These phenomena are important for understanding flash sintering. They will also play a role in solid oxide fuel cell (SOFC) and solid oxide electrolysis cells (SOEC), where current densities may exceed the reaction rates with gases at the porous electrodes. The characteristic time for attaining steady state transport contains two parts, one depending on the length of the sample and one depending on the field strength. This characteristic time is derived in this study for a linear increase of temperature for the first time. By assuming that the characteristic time is a measure of the onset of flash sintering, yields – without fitting parameters – incubation times or onset temperatures of flash sintering in good agreement with experimental results for YSZ. Thus concentration changes building up during the incubation period are driving forces for internal reactions generating or consuming holes and electrons. A concomitant increase of conductivity leads to Joule heating which further accelerates reaction rates and ion mobility.Graphical abstractImage 1
  • Real-time observation of stress-induced domain evolution in a [011]
           PIN-PMN-PT relaxor ferroelectric single crystal
    • Abstract: Publication date: Available online 20 June 2019Source: Acta MaterialiaAuthor(s): Ying Liu, Junhai Xia, Peter Finkel, Scott D. Moss, Xiaozhou Liao, Julie M. Cairney This paper reports on the real-time observation of mechanical stress-induced micro and nano domain evolution in a single crystal relaxor ferroelectric [011] poled 24PIN-PMN-PT “3-2 mode” nano-sized lamella. Mechanical loading in the [100] direction was applied to lamella in situ within a transmission electron microscope, with a [01¯1] viewing direction selected for the recording of real-time videos of the domain evolution within the lamella. The observed dominant behavior under loading is a reversible response composed of the movement of microdomains and disappearance of the nanodomains. Changes in the selected area diffraction pattern down the [01¯1] zone axis are reported, and provide evidence of a significant increase in crystal symmetry under compression. A qualitative insight into the behaviour of the lamella’s mechanical response is obtained via predictions of the non-uniform stress distribution within the lamella found using simple finite element analysis. The correlation between the observed morphology changes, diffraction pattern changes, and the known mechanical stress induced polydomain-rhombohedral to monodomain-orthorhombic phase transition of bulk [011] poled 24PIN-PMN-PT “3-2 mode” single crystal is discussed.Graphical abstractImage 1
  • Identification of active slip mode in a hexagonal material by correlative
           scanning electron microscopy
    • Abstract: Publication date: Available online 19 June 2019Source: Acta MaterialiaAuthor(s): X. Xu, D. Lunt, R. Thomas, R. Prasath Babu, A. Harte, M. Atkinson, J.Q. da Fonseca, M. Preuss Metals with a hexagonal close packed structure can deform by several different slip modes with different Critical Resolved Shear Stresses, which provides a great deal of complexity when considering mechanical performance of Mg, Ti and Zr alloys. Hence, an accurate but also statistically meaningful analysis of active slip systems and their contribution to plasticity is of great importance for the understanding of deformation mechanism. In the present study, a correlative scanning electron microscopy-based method of slip trace analysis has been utilised to provide statistical, accurate information of slip behaviour in a weakly textured Ti-6Al-4V alloy with a plastic strain of ∼2%. This is achieved through grain orientation mapping by Electron Backscatter Diffraction and strain mapping by High Resolution Digital Image Correlation. The initial identification of slip mode was performed by comparing the slip trace captured in the high-resolution effective shear strain map with all theoretical slip planes with an angle acceptance criterion of ±5°. Ambiguity in slip mode identification was further resolved using the Relative Displacement Ratio method, which enables the determination of the Burgers vector directly from the displacement data. The correctness of the identified slip modes has been confirmed by detailed dislocation analysis using Bright Field Scanning Transmission Electron Microscopy on thin foils extracted from specific grains employing Focused Ion Beam. This detailed investigation demonstrates the robustness of the slip trace analysis based on grain orientation and high-resolution strain mapping.Graphical abstractImage 1
  • Size dependent strengthening in high strength nanotwinned Al/Ti
    • Abstract: Publication date: Available online 18 June 2019Source: Acta MaterialiaAuthor(s): Y.F. Zhang, S. Xue, Q. Li, Jin Li, Jie Ding, T.J. Niu, R. Su, H.Wang, X. Zhang Mechanical behavior of metallic multilayers has been intensively investigated. Here we report on the study of magnetron-sputtered highly textured Al/Ti multilayer films with various individual layer thicknesses (h = 1 - 90 nm). The hardness of Al/Ti multilayers increases monotonically with decreasing layer thickness without softening and exceeds 7 GPa, making it one of the strongest light-weight multilayer systems reported to date. High resolution transmission electron microscopy and X-ray diffraction pole figure analyses confirm the formation of high-density nanotwins and 9R phase in Al layers. The density of nanotwins and stacking faults scales inversely with individual layer thickness. In addition, there is an HCP-to-FCC phase transformation of Ti when h ≤ 4.5 nm. The high strength of Al/Ti multilayers primarily originates from incoherent interface, high-density twin boundaries, as well as stacking faults.Graphical abstractImage 1
  • Tunable first order transition in La(Fe,Cr,Si)13 compounds: retaining
           magnetocaloric response despite a magnetic moment reduction
    • Abstract: Publication date: Available online 17 June 2019Source: Acta MaterialiaAuthor(s): Luis M. Moreno-Ramírez, Carlos Romero-Muñiz, Jia Y. Law, Victorino Franco, Alejandro Conde, Iliya A. Radulov, Fernando Maccari, Konstantin P. Skokov, Oliver Gutfleisch Materials with a large magnetocaloric response require a large magnetic moment. However, we show in this paper that it is possible to retain both the magnetic entropy change and the adiabatic temperature change even using dopants that reduce the magnetic moment of the parent alloy, provided that the first order character of the transition is enhanced. In this work, a combination of first-principles calculations, experimental determination of the magnetocaloric response (direct and indirect) as well as a new criterion to determine the order of the phase transition are applied to Cr-doped La(Fe,Si)13 compounds. Despite a reduction in magnetic moment, the magnetocaloric response is retained up to x≈0.3 in LaFe11.6-xCrxSi1.4. Unlike other transition metal dopants, Cr occupy 8b sites and couple antiferromagnetically to Fe atoms. The cross-over of first to second order transition is achieved for a Cr content of x=0.53, larger in comparison to other dopants (e.g. Ni or Mn). A direct relation between the first order character and the hysteresis is observed.Graphical abstractImage 1
  • Multiphase-field and experimental study of solidification behavior in a
           nickel-based single crystal superalloy
    • Abstract: Publication date: Available online 17 June 2019Source: Acta MaterialiaAuthor(s): Cong Yang, Qingyan Xu, Xianglin Su, Baicheng Liu Understanding the solidification behavior in nickel-based single crystal superalloy solidification is essential for determining the following heat treatment process, while the experimental findings have led to contradictory results. On the one hand, the fine γ/γ' structure in the interdendritic zone suggests a eutectic reaction. On the other hand, the coarse γ' precipitates indicate a peritectic reaction. In this work, the CALPHAD-based multiphase-field model was used to investigate the superalloy solidification behavior and complex γ/γ' morphology generation. The fine γ/γ' and coarse γ' patterns were reproduced in the γ/γ' colony, which were compared with the interdendritic microstructure observed by experiments. And the eutectic and peritectic reactions were verified by analyzing the chemical driving force at the phase interfaces. The simulation results suggest that the coarsening of γ' in the γ/γ' colony was due to the fast decrease of chemical driving force at the γ/l and γ'/l interfaces. Besides, remelting of the γ dendrite was found near the bulk γ', which can be mainly ascribe to the increasing of Mo concentration rejected by the growing γ'. The simulated microsegregation patterns of the alloy components were also obtained and the results were in good agreement with the experimental findings.Graphical abstractImage 1
  • Random 3D-printed isotropic composites with high volume fraction of
           pore-like polydisperse inclusions and near-optimal elastic stiffness
    • Abstract: Publication date: Available online 14 June 2019Source: Acta MaterialiaAuthor(s): M.G. Tarantino, O. Zerhouni, K. Danas Highly porous materials with random closed-cell architecture combine isotropy with high stiffness. Yet in practice, the complexity of their manufacturing limits the experimental exploration of these materials, for which studies of the elastic response remain to date mainly theoretical. In this study, we measure experimentally the elastic moduli of random closed-cell porous-like composites fabricated by 3D-printing. These materials contain a high volume fraction (up to 82 vol pct) of non-overlapping, polydisperse void-like spherical inclusions, which are randomly dispersed in a homogeneous polymer matrix. We first generate the virtual microstructures of these materials using a random sequential adsorption (RSA) algorithm, and then use numerical homogenization to compute the size of the material representative volume element (RVE). The latter is used to assemble the test samples, whereby the void-like inclusions are 3D-printed using a gel-like polymer with mechanical properties that are in high contrast with those of the base polymer thus behaving mechanically as pores. Experiments reveal that the proposed isotropic random closed-cell porous materials have bulk and shear moduli that lie very close to the theoretical Hashin-Shtrikman upper bounds for an isotropic porous solid.Graphical abstractImage 1
  • Understanding the Role of Diffusion Induced Grain Boundary Migration on
           the Preferential Intergranular Oxidation Behaviour of Alloy 600 via
           Advanced Microstructural Characterization
    • Abstract: Publication date: Available online 14 June 2019Source: Acta MaterialiaAuthor(s): L. Volpe, M.G. Burke, F. Scenini This present work introduces a new understanding of the precursors events to Stress Corrosion Cracking (SCC) initiation in nuclear power plant components, exploring the role of grain boundary migration and Preferential Intergranular Oxidation (PIO). In this work, a systematic evaluation of Alloy 600 exposed in low pressure H2-steam over a range of oxidizing potentials in the vicinity of the Ni/NiO transition has revealed a notable decrease in PIO for conditions more oxidizing than the Ni/NiO transition, whilst local diffusion-induced grain boundary migration occurs irrespective of the oxidizing potential. Thus, grain boundary migration in itself is not a unique signature for PIO and SCC incubation, but it is necessary for the development of PIO and it represents a critical acceleration factor for more reducing conditions (such as those in pressurised water reactors) where oxide film instability allows for the initiation of PIO. It is also shown that Al/Ti enrichments that develop along the migrated GBs correlate with the occurrence of PIO when the alloy develops a non-protective external oxide.Graphical abstractImage 1
  • Development of high coercivity anisotropic Nd-Fe-B/Fe nanocomposite powder
           using hydrogenation disproportionation desorption recombination process
    • Abstract: Publication date: Available online 13 June 2019Source: Acta MaterialiaAuthor(s): H. Sepehri-Amin, I. Dirba, Xin Tang, T. Ohkubo, T. Schrefl, O. Gutfleisch, K. Hono Based on a modified hydrogenation disproportionation desorption recombination (HDDR) process we propose and realize a novel top-down processing route to synthesize anisotropic nano-composite magnet powders. Selection of alloy compositions with Nd content lower than the stoichiometry of Nd2Fe14B phase led to the formation of spherical shaped nano-sized α-Fe phase within the Nd2Fe14B matrix after HDDR process. Next anisotropic nanocomposite Nd-Fe-B/α-Fe powders with substantial coercivity were developed with optimized HDDR conditions and subsequent grain boundary engineering which remedied the initial lack of Nd-rich intergranular phase. Specifically, the infiltration of Nd70Cu30 alloy increased the coercivity from 0.0 to 0.85 T. Note that low coercivity and the absence of texture in nanocomposite magnets have been the main challenges to realize the high (BH)max postulated for many years for anisotropic nanocomposite magnets. We employed micromagnetic simulations for optimum microstructure design of a nanocomposite Nd2Fe14B/α-Fe magnet that gives a large maximum energy product, (BH)max. The simulations are then correlated with macroscopic hysteresis properties and high-resolution electron microscopy as well as atom probe tomography. Another very remarkable result is the observation that the formation of α-Fe phase with a size up to 200 nm within the matrix of Nd2Fe14B grains can still result in a significant coercivity of 0.85 T. This is in contrast to common understanding of exchange-coupled systems and we explain this observation with sharp and defect free α-Fe/Nd2Fe14B interface, the latter a result of the disproportionation and recombination reactions.Graphical abstractImage 1
  • Atomistic phase field chemomechanical modeling of
           dislocation-solute-precipitate interaction in Ni-Al-Co
    • Abstract: Publication date: Available online 13 June 2019Source: Acta MaterialiaAuthor(s): Jaber Rezaei Mianroodi, Pratheek Shanthraj, Paraskevas Kontis, Jonathan Cormier, Baptiste Gault, Bob Svendsen, Dierk Raabe Dislocation-precipitate interaction and solute segregation play important roles in controlling the mechanical behaviour of Ni-based superalloys at high temperature. In particular, the increased mobility of solutes at high temperature leads to increased dislocation-solute interaction. For example, atom probe tomography (APT) results [1] for single crystal MC2 superalloy indicate significant segregation of solute elements such as Co and Cr to dislocations and stacking faults in γ' precipitates. To gain further insight into solute segregation, dislocation-solute interaction, and its effect on the mechanical behavior in such Ni-superalloys, finite-deformation phase field chemomechanics [2] is applied in this work to develop a model for dislocation-solute-precipitate interaction in the two-phase γ-γ' Ni-based superalloy model system Ni-Al-Co. Identification and quantification of this model is based in particular on the corresponding Ni-Al-Co embedded atom method (EAM) potential [3]. Simulation results imply both Cottrell- and Suzuki-type segregation of Co in γ and γ'. Significant segregation of Co to dislocation cores and faults in γ' is also predicted, in agreement with APT results. Predicted as well is the drag of Co by γ dislocations entering and shearing γ'. Since solute elements such as Co generally prefer the γ phase, Co depletion in γ' could be reversed by such dislocation drag. The resulting change in precipitate chemistry may in turn affect its stability and play a role in precipitate coarsening and rafting.Graphical abstractImage 1
  • Kinetics of anticrossing between slip traces and vicinal steps on crystal
    • Abstract: Publication date: Available online 13 June 2019Source: Acta MaterialiaAuthor(s): C. Coupeau, D.M. Kazantsev, M. Drouet, V.L. Alperovich The interaction between vicinal atomic steps and slip traces – straight monatomic steps produced on a crystal surface by the emergence of dislocations – is experimentally investigated and compared to Monte-Carlo simulations. Near the point of apparent crossing between a vicinal step and a slip trace, a checkered three-level surface relief configuration is formed, with two new combinatory steps that borders the opposite highest and lowest terraces. This configuration is unstable with respect to an anticrossing effect which consists in the formation of a nanometer scale bridge that separates the regions with the highest and lowest levels and connects the opposite regions of equal level. It is shown that such an anticrossing effect is a general phenomenon observed on various crystal surfaces, from metals to semiconductors. The anticrossing kinetics was experimentally investigated on the Au(111) surface by scanning tunneling microscopy under ultra-high vacuum. It is observed that the bridge width increases with time according to the power law with exponent β = 0.45 ± 0.01, i.e. significantly smaller than for the single-particle diffusion (β = 0.5). Monte-Carlo simulations were performed in order to clarify the involved atomic diffusion mechanisms. In particular, the competition between two microscopic mechanisms of the bridge formation is discussed, i.e., the adatom diffusion along the combinatory steps versus across the bridge from the uppermost to the lowest terrace.Graphical abstractImage 1
  • Short-range ordered structure and phase stability of supersaturated
           nitrided layer on austenitic stainless steel
    • Abstract: Publication date: Available online 13 June 2019Source: Acta MaterialiaAuthor(s): Ke Tong, Fei Ye, Ya Kun Wang The structures of short-range ordered Fe-Cr-N clusters in face-centered cubic (f.c.c.) iron have been systematically studied by first-principles calculation to understand the atomic structure and phase stability of supersaturated nitrided layer on austenitic stainless steel. The clusters are formed by aggregation of Cr and N atoms due to their strong interaction. The unit of the clusters can be considered as a Fe6-nCrnN octahedral cluster, in which the N atom occupies an octahedral interstitial site of the f.c.c. lattice and the metal atoms are at the first nearest neighbor sites to the N atom. Moreover, the Cr atoms prefer to distribute in pairs around the N atom. When the N concentration increases, a larger cluster may be formed by combination of the octahedral clusters through edge-shared mode, and Cr atoms prefer the shared sites. The cluster structure is affected by both the local lattice distortion around the cluster and Coulombic interaction in the cluster. Then, the atomic structure of supersaturated nitrided layer can be described as the f.c.c. iron dispersively embedded with the short-range ordered clusters. Furthermore, the stabilization mechanism of the metastable phase was examined based on the structure model. It was suggested that the stabilization of the metastable phase is mainly a chemically-driven mechanism by Cr and N atoms forming short-range ordered clusters.Graphical abstractImage 1
  • Intragranular localization induced by softening crystal plasticity:
           Analysis of slip and kink bands localization modes from high resolution
           FFT-simulations results
    • Abstract: Publication date: Available online 12 June 2019Source: Acta MaterialiaAuthor(s): Aldo Marano, Lionel Gélébart, Samuel Forest We investigate the ability of local continuum crystal plasticity theory to simulate intense slip localization at incipient plasticity observed experimentally in metals exhibiting softening mechanisms. A generic strain softening model is implemented within a massively parallel FFT solver framework to study intragranular strain localization throughout high resolution polycrystalline simulations. It is coupled to a systematic analysis strain localization modes: Equivalent plastic strain and lattice rotation fields are processed to create binary maps of slip and kink bands populations, estimate their volume fraction and mean strain level. High resolution simulations show the formation of an intragranular localization band network. The associated localization maps are used to identify accurately slip and kink bands populations and highlight the distinct evolution of kink bands, influenced by lattice rotation. Results highlight that the analysis of the nature of localization bands in numerical studies is fundamental to asses the validity of polycrystalline simulations. Indeed, it is evidenced that selection between slip or kink localization modes is only due to grain to grain incompatibilities as these two localization modes are equivalent in classical crystal plasticity models. As a result they predict the formation of a large amount of kink bands in contradiction with experimental observations of softening metals. We show that this holds for complex physics based models too. Hence, the use of classical crystal plasticity for strain localization simulation should be reconsidered in order to predict realistic localization modes.Graphical abstractImage 1
  • Prediction of Transformation Stresses in NiTi Shape Memory Alloy
    • Abstract: Publication date: Available online 12 June 2019Source: Acta MaterialiaAuthor(s): S. Alkan, H. Sehitoglu It is well known that interfaces play an important role in determining the mechanical response of materials. This paper focuses on the transforming shape memory alloy NiTi and is aimed towards a better understanding of austenite-martensite interface structure (steps and dislocation arrays) and the determination of transformation stress corresponding to the translation of this interface. In the present work, we characterize the defect content at the cubic-monoclinic interfaces via the Topological Model. The defect-induced displacement fields are generated within the framework of the Eshelby-Stroh formalism and further improved with Molecular Statics simulations accounting for interactions at the atomic level. The resulting defect core disregistry fields are employed as input to a modified Peierls-Nabarro framework for evaluating the transformation stress. We applied the proposed methodology to the particular case of NiTi alloy single crystals of specific orientations and predicted the transformation stress levels in close agreement with experiments. Moreover, the short-range interactions of dislocation core disregistry fields are shown to be responsible for the experimentally observed non-Schmid behavior of transformation stress levels. Overall, the paper represents an effort to improve our understanding of shape memory materials considering theory, computer simulation and experiment.Graphical abstractImage 1
  • Robust polarization switching in self-assembled BiFeO3 nanoislands with
           quad-domain structures
    • Abstract: Publication date: Available online 12 June 2019Source: Acta MaterialiaAuthor(s): Mingfeng Chen, Ji Ma, Renci Peng, Qinghua Zhang, Jing Wang, Yuhan Liang, Jialu Wu, Long-Qing Chen, Jing Ma, Ce-Wen Nan Miniaturizing ferroic oxides is essential for studying the fundamental physics of low-dimensional topological defects such as flux-closure vortex as well as the applications of future nanoelectronic devices. Here, we successfully synthesized self-assembled BiFeO3 nanoislands on LaAlO3 (001) substrate by pulsed laser deposition. Center-type quad-domains are spontaneously stabilized in these nanostructures, and the polarization vectors of each quarter can be independently switched with robust retention properties over time and elevated temperature. As a result, the long-sought exotic domain structures such as anti-vertex can be created in these nanoislands by selectively switching polarization state of certain quarters. Resistive switching effect is also confirmed in quarters of the nanoisland, indicating the great potential as memory cells in high density ferroelectric random access memory.Graphical abstractImage 1
  • Hot Carrier Transfer and Phonon Transport in Suspended nm WS2
    • Abstract: Publication date: Available online 12 June 2019Source: Acta MaterialiaAuthor(s): Hamidreza Zobeiri, Ridong Wang, Qianying Zhang, Guangjun Zhu, Xinwei Wang This work reports the first results on the conjugated hot carrier diffusivity (D) and thermal conductivity (κ) of suspended nm-thick WS2 structures. A novel nET-Raman technique is developed to distinguish and characterize these two properties by constructing steady and transient states of different laser heating and Raman probing sizes. The nET-Raman uses a nanosecond pulsed laser and a continuous wave laser for exciting Raman signals and heating samples. κ is found to increase from 15.1−0.4+0.3 to 38.8−2.4+2.6 W·m−1·K−1 when the sample’s thickness increases from 13 to 107 nm. This increase is attributed to the decreased effect of surface phonon scattering in thicker samples. Also, hot carrier diffusion length (ΔrHC) for these samples are measured without knowledge of hot carrier’s lifetime (τ). Measured D of these four samples are in close range (except the thickest sample). This is due to the fact that lattice scattering for all these samples is similar and there is no substrate effect on our suspended films. nET-Raman is very robust and has negligible effect from laser absorption depth, sample thickness, and laser spot drift during measurement.Graphical abstractImage 1
  • Low-energy grain boundaries in WC-Co cemented carbides
    • Abstract: Publication date: Available online 11 June 2019Source: Acta MaterialiaAuthor(s): Xingwei Liu, Xuemei Liu, Hao Lu, Haibin Wang, Chao Hou, Xiaoyan Song, Zuoren Nie The influencing factors, distribution characteristics and evolution mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were demonstrated. Based on characterizations from various angles, the correlations of grain shape aspect ratio, WC contiguity and distribution of low-energy grain boundaries were analyzed for cemented carbides with addition of different grain growth inhibitors. It was found that the ∑2 grain boundaries have higher fraction in the cemented carbides with stable complexions, finer grain size and larger WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbides with anisotropic surface energy by coalescence of WC platelets with large shape aspect ratio. The results suggest that the fraction and distribution of low-energy grain boundaries in the cemented carbides can be modulated by matching grain growth inhibitor and sintering temperature of the initial WC-Co composite powder.Graphical abstractThe features of low-energy grain boundaries in the WC-Co cemented carbides were studied considering the influencing factors, distribution laws and formation and evolution mechanisms. The correlations of the distribution and fraction of low-energy grain boundaries with the shape aspect ratio, size, and contiguity of WC grains were disclosed. The principles to modulate the low-energy grain boundaries in the cemented carbides through matching the composition and sintering temperature of the initial powder were proposed, which may be applicable to a large variety of powder metallurgical materials.Image 1
  • Ab initio phase stability and electronic conductivity of the
           doped-Li4Ti5O12 anode for Li-ion batteries
    • Abstract: Publication date: Available online 11 June 2019Source: Acta MaterialiaAuthor(s): Ping-chun Tsai, Ralph Nicolai Nasara, Yu-chen Shen, Chih-chao Liang, You-wen Chang, Wen-Dung Hsu, Ngoc Thanh Thuy Tran, Shih-kang Lin The Li4Ti5O12 (LTO) defect spinel is known for its excellent durability of “10,000” cycle counts and high level of safety as an anode material in lithium-ion batteries, but it shows an intrinsic insulating property and poor electrochemical kinetics. Doping is a direct approach to manipulate the electronic conductivity of LTO. However, doping may induce multiple effects influencing the overall electrochemical kinetics, e.g., changing the size of particles and the ionic and electronic conductivities. Here we systematically investigated the phase stability, electronic conductivity, and electrochemical kinetics of M-doped LTO (M = Na, K, Mg, Ca, Sr, Al, and Ga). With both ab initio calculations and experiments, the mechanism of electron transport within LTO is elucidated, the desired type of dopants for improving electronic conductivity of LTO is clarified, and the role of electronic conductivity in the electrochemical kinetics of LTO is revealed. These results provide an in-depth understanding of metal-doped LTO and would help the development of a variety of electrode materials.Graphical abstractImage 1
  • Influence of Si precipitates on fracture mechanisms of AlSi10Mg parts
           processed by Selective Laser Melting
    • Abstract: Publication date: Available online 11 June 2019Source: Acta MaterialiaAuthor(s): J. Delahaye, J. Tchoufang Tchuindjang, J. Lecomte-Beckers, O. Rigo, A.M. Habraken, A. Mertens While it is generally accepted that the rupture of SLM AlSi10Mg tensile specimens occurs at the melt pool boundary, the exact zone and microstructural features responsible for the rupture have not been clearly identified. In this study, the microstructures and local mechanical properties at the melt pool boundary are thus analyzed in details. The Si phase fraction and the Si precipitate spacing are measured by image analysis and SEM-EDS analysis. Hardness tests are performed by nanoindentation. Fracture features are observed on broken samples. It is found that the Heat Affected Zone (HAZ) exhibits low hardness due to coarse non-coherent Si precipitates. Void nucleation occurs at the interface between the coarse Si precipitates and the Al matrix by dislocations piling up. For that reason, the HAZ is found to be the preferential region where fracture is likely to occur. This analysis is confirmed by the matching of Si precipitate spacing within the HAZ with dimple spacing observed in fracture surfaces. Moreover, a simple analytical approach of the thermal history during manufacturing, using Rosenthal’s equation, allows elucidating the mechanisms by which the processing conditions affect the fracture behavior.Graphical abstractImage 1
  • Nano-sized MX carbonitrides contribute to the stability of mechanical
           properties of martensite ferritic steel in the later stages of long-term
    • Abstract: Publication date: Available online 10 June 2019Source: Acta MaterialiaAuthor(s): Yuantao Xu, Wei Li, Mingjia Wang, Xiying Zhang, Yun Wu, Na Min, Wenqing Liu, Xuejun Jin The coupling effects between MX carbonitrides, M2X phase, M23C6 carbides, and the Laves phase and their influences on the mechanical properties during long-term aging at 650 °C were studied. Before aging for 40500 h, the subgrain size is mainly stabilized by M23C6 carbides, whose pinning effect gradually weakens with the coarsening during long-term aging. However, after aging for 40500 h, the volume fraction of nano-sized MX carbonitrides gradually increases and compensates for the loss of pinning effect from M23C6 carbides, working together with the M23C6 carbides to hinder the coarsening of the subgrain. The increase in the volume fraction of nano-sized MX carbonitrides from 40500 h to 49500 h, mainly resulting from the gradual dissolution of M2(C,N) carbonitrides, which not only contributes to the stability of the strength through the pinning effect and precipitation strengthening, but also benefits the ductility by suppressing the rapid coarsening of the Laves phase through the strong competition for Si between the Laves phase and MX. An unusual combination of stable strength and stable ductility in the later stages of long-term aging at 650 °C (approximately 9000 h) was achieved.Graphical abstractImage 1
  • Counting electrons - a new approach to tailor the hydrogen sorption
           properties of high-entropy alloys
    • Abstract: Publication date: Available online 7 June 2019Source: Acta MaterialiaAuthor(s): Magnus Moe Nygård, Gustav Ek, Dennis Karlsson, Magnus H. Sørby, Martin Sahlberg, Bjørn C. Hauback We have investigated the structure and hydrogen storage properties of a series of quaternary and quintary high-entropy alloys related to the ternary system TiVNb with powder X-ray diffraction (PXD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and manometric measurements in a Sieverts apparatus. The alloys have body-centred cubic (bcc) crystal structures and form face-centred cubic (fcc) metal hydrides with hydrogen-to-metal ratios close to 2 by hydrogenation. The onset temperature for hydrogen desorption, Tonset, decreases linearly with the valence-electron concentration, VEC. Moreover, the volumetric expansion per metal atom from the bcc alloys to the fcc hydrides, [(V/Z)fcc−(V/Z)bcc]/(V/Z)bcc, increases linearly with the VEC. Therefore, it seems that a larger expansion of the lattice destabilizes the metal hydrides and that this effect can be tuned by altering the VEC. Kissinger analyses performed on the DSC measurements indicate that the destabilization is a thermodynamic rather than kinetic effect.Based upon these insights we have identified TiVCrNbH8 as a material with suitable thermodynamics for hydrogen storage in the solid state. This HEA-based hydride has a reversible hydrogen storage capacity of 1.96 wt.% H at room temperature and moderate H2-pressures. Moreover, it is not dependent on any elaborate activation procedure to absorb hydrogen.Graphical abstractImage 1
  • Room temperature plastic deformability in V-rich V-Si-B alloys
    • Abstract: Publication date: Available online 7 June 2019Source: Acta MaterialiaAuthor(s): G. Hasemann, C. Müller, D. Grüner, E. Wessel, M. Krüger In the present study compressive strength data and room temperature (RT) deformability of three V-rich V-Si-B alloys are reported. All alloys were taken from the VSS (V solid solution)-V3Si-V5SiB2 three-phase region of the respective phase diagram and differ in their primary solidification phase of either VSS, V3Si or V5SiB2. The RT yield stresses were determined by the 0.2 % offset method and the plastic strain was obtained by subtracting the combined compliance of the testing machine and the specimen from the individually measured load-displacement curves. The microstructures in the as-cast and deformed state were analyzed using SEM, EBSD and TEM. Dislocation activity was only observed in the VSS phase while the intermetallic phases were dislocation-free. Thus, the RT plasticity in V-rich V-Si-B alloys is mainly governed by the volume fraction of the VSS phase. Hence, the alloy containing primary VSS dendrites has the highest plastic strain compared to the V5SiB2 and V3Si primary crystallizing alloys. Investigations of the VSS phase on binary V-Si alloys reveal that silicon acts as solid solution strengthener, but ductility of the VSS phase is retained throughout the solubility range.The present results are quite astonishing since Mo-Si-B alloys containing similar amounts of Si and B are known to be fairly brittle at RT.Graphical abstractImage 1
  • Mechanical response of Ti-6Al-4V hierarchical architected metamaterials
    • Abstract: Publication date: Available online 7 June 2019Source: Acta MaterialiaAuthor(s): Liang Dong A snap-fit manufacturing technique has been designed for making hierarchically architected lattice structures from Ti-6Al-4V alloy sheets. By patterning the unit cell of a small-scale lattice along the struts of a self-similar unit cell of a large-scale lattice, fractal-like geometry with a hierarchy order of 2 was obtained. A subsequent vacuum brazing operation was performed to create fractal-like structures with an octahedron-of-octahedra half-cell geometry. Hierarchical architected lattices with a fractal number 4-15 and a relative density ranging from 0.7% to 12% have been manufactured by allowing a fixed small-scale cell size of approximately 10 mm and a varying large-scale cell size from 40 to 190 mm, and their mechanical responses under compression been experimentally examined. The measured compressive elastic moduli and strengths of the hierarchical lattices agreed well with micromechanical and finite element predictions. The Ti-6Al-4V hierarchical lattice materials exhibit very competitive mechanical properties when compared to other cellular materials, and provide a new solution for lightweight engineering materials for use at maximum service temperature up to 450oC.Graphical abstractImage 1
  • Solid-solution strengthening in refractory high entropy alloys
    • Abstract: Publication date: Available online 7 June 2019Source: Acta MaterialiaAuthor(s): Francisco Gil Coury, Michael Kaufman, Amy J. Clarke Compression stress-strain curves of a number of refractory high entropy alloys (RHEAs) were generated at temperatures ranging from room temperature to 1000 °C. It is shown that solid-solution strengthening in these alloys has both an athermal and a thermal component. Results from mechanical testing are combined with literature data to develop solid-solution strengthening models for both components that incorporate the particularities of single-phase body centered cubic (BCC) materials. The athermal component is affected by a combination of atomic size mismatch and elastic modulus mismatch, which depend upon average values from each alloy, thereby allowing this component to be estimated in a high-throughput fashion. On the other hand, the thermally-activated yield stress component does not correlate with any parameter that can be calculated by averaging pure elemental atomic properties and it is observed to be larger than the values found for pure BCC refractory metals and their dilute alloys. Overall, RHEAs are found to have larger thermal and athermal yield stress components compared to pure or conventional refractory alloys, which explains their relatively high strengths at room temperature.Graphical abstractImage 1
  • On the Measurement of Dislocations and Dislocation Structures using EBSD
           and HRSD Techniques
    • Abstract: Publication date: Available online 7 June 2019Source: Acta MaterialiaAuthor(s): O. Muránsky, L. Balogh, M. Tran, C.J. Hamelin, J.-S. Park, M.R. Daymond The accumulation of the dislocations and development of dislocation structures in plastically deformed Ni201 is examined using dedicated analyses of Electron Back-Scatter Diffraction (EBSD) acquired orientation maps, and High-Resolution Synchrotron Diffraction (HRSD) acquired patterns. The results show that the minimum detectable microstructure-averaged (bulk) total dislocation density (ρT) measured via HRSD is approximately 1E13 m-2, while the minimum GND density (ρG) measured via EBSD is approximately 2E12 m-2 – the EBSD technique being more sensitive at low plastic strain. This highlights complementarity of the two techniques when attempting to quantify amount of plastic deformation (damage) in a material via a measurement of present dislocations and their structures. Furthermore, a relationship between EBSD-measured ρG and the size of HRSD-measured Coherently Scattering Domains (CSDs) has been mathematically derived – this allows for an estimation of the size of CSDs from EBSD-acquired orientation maps, and conversely an estimation of ρG from HRSD-measured size of CSDs. The measured evolution of ρT, and ρG is compared with plasticity theory models – the current results suggest that Ashby’s single-slip model underestimates the amount of GNDs (ρG), while Taylor’s model is correctly predicting the total amount of dislocation (ρT) present in the material as a function of imparted plastic strain.Graphical abstractImage 1
  • The atomic local ordering of SBA-15 studied with pair distribution
           function analysis, and its relationship to porous structure and thermal
    • Abstract: Publication date: Available online 6 June 2019Source: Acta MaterialiaAuthor(s): Jimi Rantanen, Dorota Majda, Joakim Riikonen, Vesa-Pekka Lehto The atomic-scale structure of amorphous SBA-15 materials with different pore sizes (6.7–12.7 nm) was investigated using pair distribution functions calculated from the X-ray total scattering data. The data was collected with a laboratory X-ray powder diffraction instrument instead of synchrotron often considered necessary for pair distribution function analysis. A 3D model of the atomic structure within the pore walls was determined based on the pair distribution function analysis, and the degree of ordering was evaluated using a method based on autocorrelation. Nitrogen gas adsorption was utilized to determine the pore characteristics of the samples. X-ray diffraction and the information on pore diameter was utilized to estimate the wall thicknesses. It was observed that thicker walls possessed more micropores, and the wall structure was more disordered on the samples with lots of micropores. The thermal stability of the samples was investigated with thermogravimetric analysis, and the results showed that samples with higher degree of ordering and less microporosity possessed better thermal stability.Graphical abstractImage 1
  • Mesoscale modeling of polycrystalline light transmission
    • Abstract: Publication date: Available online 6 June 2019Source: Acta MaterialiaAuthor(s): Lukasz Kuna, John Mangeri, Edward P. Gorzkowski, James A. Wollmershauser, Serge Nakhmanson Recent advances in polycrystalline ceramic synthesis have established fabrication routes for producing dense ceramics with grain size on the order of few nanometers. Light transmission properties and, specifically, the high transparency of birefringent nanocrystalline ceramics cannot be captured by classical geometrical optics theory. In this investigation, we combine the Raman-Viswanathan wave-retardation theory with finite element method (FEM) based numerical simulations to develop an approach for predicting the refractive index variation within and real in-line transmission through relevant optical materials. This approach is validated on non-cubic (and, therefore birefringent) Al2O3 and MgF2 polycrystalline ceramics, by comparing the computed light transmission to experimental transmission measurements. For both of the considered ceramics systems, the developed numerical model effectively reproduces the experimentally measured transmission as a function of average grain size and incident light wavelength, showing improvement over the original approach of Raman and Viswanathan and a recent particle-scattering based adaptation of geometrical optics theory by Apetz and van Bruggen. The same modeling framework can also simulate the effects of applied elastic and electric fields, allowing for the design and predictive evaluation of functional optical properties in piezoelectric ceramics.Graphical abstractImage 1
  • Contrasting Thermal Behaviors in Σ3 Grain Boundary Motion in Nickel
    • Abstract: Publication date: Available online 5 June 2019Source: Acta MaterialiaAuthor(s): Jonathan Humberson, Ian Chesser, Elizabeth A. Holm Synthetic driving force molecular dynamics simulations were utilized to survey grain boundary mobility in three classes of incoherent Σ3 twin boundaries: , , and tilt boundaries. These boundaries are faceted on low energy planes, and step flow boundary motion occurs by glide of the triplets of partial dislocations that comprise the mobile facets. Systematic trends with inclination angle are identified and characterized. Observations of thermally activated, anti-thermal, and athermal motion are explained in terms of the orientation of the Shockley partial dislocations along close-packed and non-close-packed directions. Thermally activated tilt boundaries with {110} twin facets are found to have smaller energy barriers to motion than tilt boundaries with {112} twin facets. Thermally activated boundaries follow a compensation effect associated with a facet roughening transition. As for all faceting boundaries, system size and driving force must be chosen with care to prevent simulation artifacts.Graphical abstractImage 1
  • Short-Range Order Structure Motifs Learned from an Atomistic Model of a
           Zr50Cu45Al5 Metallic Glass
    • Abstract: Publication date: Available online 5 June 2019Source: Acta MaterialiaAuthor(s): Jason J. Maldonis, Arash Dehghan Banadaki, Srikanth Patala, Paul M. Voyles The structural motifs of a Zr50Cu45Al5 metallic glass were learned from atomistic models using a new structure analysis method called motif extraction that employs point-pattern matching and machine learning clustering techniques. The motifs are the nearest-neighbor building blocks of the glass and reveal a well-defined hierarchy of structures as a function of coordination number. Some of the motifs are icosahedral or quasi-icosahedral in structure, while others take on the structure of the most close-packed geometries for each coordination number. These results set the stage for developing clearer structure-property connections in metallic glasses. Motif extraction can be applied to any disordered material to identify its structural motifs without the need for human input.Graphical abstractImage 1
  • Combined phase-field crystal plasticity simulation of P- and N-type
           rafting in Co-based superalloys
    • Abstract: Publication date: Available online 4 June 2019Source: Acta MaterialiaAuthor(s): Chan Wang, Muhammad Adil Ali, Siwen Gao, Johannes V. Goerler, Ingo Steinbach We combine a phase-field model with a crystal plasticity model to simulate the microstructural evolution during creep in the Co-based superalloy ERBOCo-2Ta. Three-dimensional simulations of tensile and compressive creep tests in [100] direction were performed to study the rafting behavior in Co-based superalloys. The loss of coherency between γ matrix and γ' precipitate, which is essential for the understanding of rafted structures, is modeled in relation to the dislocation activity in the γ-channels. Special attention is given to the interplay between creep deformation and microstructure stability. Appropriate constitutive modeling is applied to simulate realistic microstructure evolution under creep conditions. Thus, with the removal of the misfit stress, γ′ precipitates lose their cuboidal shape and form rafts. During N-type rafting more γ′ precipitates coalesce than during P-type rafting. The γ′ volume fraction during rafting increases under tensile stress but decreases under compressive stress. The morphological evolution of γ′ precipitates under tensile and compressive stresses in Co-based superalloy is consistent with the rafting characteristics in experimental observations.Graphical abstractImage 1
  • Magnetic properties improvement of hot-deformed Nd–Fe–B permanent
           magnets by Pr-Cu eutectic pre-diffusion process
    • Abstract: Publication date: Available online 1 June 2019Source: Acta MaterialiaAuthor(s): Tingting Song, Xu Tang, Wenzong Yin, Jingyun Ju, Zexuan Wang, Qiaobo Liu, Yang Tang, Renjie Chen, Aru Yan Pre-diffusion grain boundary strategy was proposed to fabricate die-upset Nd-Fe-B magnets with excellent magnetic properties. Microstructure modification of die-upset magnets played a key role in determining the magnetic properties. The effective coercivity enhancement of die-upset magnets through pre-diffusion process was enhanced from 0.076 T/wt%Pr to 0.114 T/wt%Pr with a slight remanence loss. Driven by heat, the Pr-Cu eutectic alloys were diffused into melt-spun ribbons with weakened exchange couple of matrix phase, which led to enhanced coercivity. The uniform distribution of intergranular phase was obtained in die-upset magnet with pre-diffusion process and the remanence reduction was limited with a high squareness factor. Microstructure analysis confirmed that the pre-diffusion process suppressed the longitudinal/lateral ratio of platelet-shaped grains and grain growth in die-upset process. Simultaneously, the pre-diffusion process facilitated the formation of continuous and uniform grain boundaries (GBs). The grain size was distributed over a narrow range of value with similar local critical nucleation field, which resulted in the improved squareness. The thick and continuous intergranular phase strengthened the magnetic isolation between neighboring Nd2Fe14B phases and offered more “pinning” sites for domain wall shift. The modified structure hindered the nucleation and spread of reverse domains in a low magnetic field in favor of the coercivity enhancement. The view-direct time-dependent behavior of reverse magnetic domains indicated that the coercivity mechanism for die-upset magnet was a combination of “pinning” effect and nucleation model.Graphical abstractImage 1
  • Scaling features of conductivity spectra reveal complexities in ionic,
           polaronic and mixed ionic-polaronic conduction in phosphate glasses
    • Abstract: Publication date: Available online 1 June 2019Source: Acta MaterialiaAuthor(s): Ana Šantić, Juraj Nikolić, Luka Pavić, Radha D. Banhatti, Petr Mošner, Ladislav Koudelka, Andrea Moguš-Milanković The scaling behaviour of six series of mixed ion-polaron glasses with the composition xWO3/MoO3-(30-0.5x)Li2O/Na2O/Ag2O-(30-0.5x)ZnO-40P2O5, 0≤x≤60 mol%, has been studied using Summerfield and Sidebottom scaling procedures that are model free. The validity of the Sidebottom scaling procedure for each individual glass confirms that all glasses obey time-temperature superposition principle implying that the conduction mechanism does not change with temperature. On the other hand, Summerfield scaling is not found valid for all glasses. First, this deviation is observed in all glasses containing Li2O up to 20 mol% of MoO3/WO3. We relate this result to a non-typical interaction of Li+ ion with the compact zinc phosphate network resulting either in change of its hopping distance or in available conduction pathways for it as a function of temperature. Second, non-Summerfield scaling was also observed for glasses containing Li2O/Na2O/Ag2O with about 30 to 40 mol% of WO3. Thus, the origin of this deviation lies in the significant amounts of both types of charge carriers - ions and polarons, and their differently thermally activated mobilities.Graphical abstractUnusual Li+ dynamics and mixed ionic-polaronic conduction in zinc phosphate glasses detected by scaling conductivity spectra.Image 1
  • Phase-field study of grain growth in porous polycrystals
    • Abstract: Publication date: Available online 1 June 2019Source: Acta MaterialiaAuthor(s): Veronika Rehn, Johannes Hötzer, Wolfgang Rheinheimer, Marco Seiz, Christopher Serr, Britta Nestler During sintering, a multitude of mechanisms act in different ways resulting in densification and coarsening. Since the material properties depend on the microstructure, the manufacturing of advanced ceramics requires a deep understanding of the sintering process. The present study focuses on the occurrence of grain growth during final stage sintering. In a porous microstructure, the pores exert a dragging force to grain boundary motion retarding the grain growth. However, as soon as the porosity becomes low enough, the grain growth exceeds the dragging force and the microstructure starts to coarsen. This interdependency of densification and grain growth is still not fully understood. The present study uses a 3D phase-field model to investigate grain growth in porous microstructures during final stage sintering. A model extension treats pore dynamics under consideration of pressure stability as well as pore coalescence. To account for the size effect of pores on its dragging force, a surface diffusion based mobility approache is incorporated. Large-scale 3D grain growth simulations are conducted to analyze the effect of different porosities and pore sizes with statistical relevance comparable to real microstructures. Depending on the porosity, two dominating effects on the reduction of grain growth rate are found. For low porosities, the growth rate depends on the number of pores whereas for higher porosities the pore size and their mobility dominate the process. The results show the need to consider the effects of the pore size and their distribution during final stage sintering.Graphical abstractImage 1
  • Role of Co on the magnetic properties of Ce-substituted Nd-Fe-B
           hot-deformed magnets
    • Abstract: Publication date: Available online 1 June 2019Source: Acta MaterialiaAuthor(s): Xin Tang, H. Sepehri-Amin, M. Matsumoto, T. Ohkubo, K. Hono In this paper, we report for the first time improved hard magnetic properties of Ce-substituted Nd-Fe-B based anisotropic permanent magnet despite the expected reduction of intrinsic hard magnetic properties of (Nd1-xCex)2Fe14B compound for x>0. With increasing Ce substitution, x, from 0 to 0.1, the coercivity (μ0Hc) increased from 1.36 T to 1.44 T while the remanent magnetization (Jr) remains to be 1.49 T when the magnets contain Co. Further increase in x decreases the remanent magnetization and coercivity. The ab initio calculations show that the magnetization would not be degraded if the Ce substitutes Nd at 4g site in the Co-containing magnets, which is found to be the case by atom-resolved STEM-EDS mapping. The enhancement of coercivity of the sample with x=0.1 originates from higher rare earth concentration in the grain boundary phase, resulting stronger pinning force against reversed domain wall motion. However, the improvement of hard magnetic properties with Ce substitution for Nd was not found in Co-free compounds.Graphical abstractImage 1
  • Giant reversible magnetocaloric effect in MnNiGe-based materials:
           Minimizing thermal hysteresis via crystallographic compatibility
    • Abstract: Publication date: Available online 1 June 2019Source: Acta MaterialiaAuthor(s): Jun Liu, Yuanyuan Gong, Yurong You, Xinmin You, Bowei Huang, Xuefei Miao, Guizhou Xu, Feng Xu, Ekkes Brück MnMX (M = Co or Ni, X = Si or Ge) alloys with strong magnetostructural coupling exhibit giant magnetic entropy change and are currently extensively studied. However, large thermal hysteresis results in serious irreversibility of the magnetocaloric effect in this well-known system. In this work, we report a low thermal hysteresis and large reversible magnetocaloric effect in a MnNiGe-based system. The introduction of Fe into both Ni and Mn sites can establish stable magnetostructural transitions from paramagnetic hexagonal to ferromagnetic orthorhombic phases. Fascinatingly, a low thermal hysteresis of 5.2 K is achieved in Mn0·9Fe0·2Ni0.9Ge alloy with a large magnetization difference of 62.1 A m2/kg between the two phases. These optimized parameters lead to a partially reversible phase transformation under a magnetic stimulus and bring about a large reversible magnetic entropy change of −18.6 J kg−1K−1 under the field variation of 0–5 T, which is the largest value reported in MnMX system up to now. Moreover, this low-hysteresis magnetostructural transformation and large reversible magnetocaloric effect can be tuned by doping with Si in a wide temperature range covering room temperature. We also introduce geometrically nonlinear theory to discuss the origin of low hysteresis in MnMX alloys. A strong relation is found between thermal hysteresis and the change of c axis in the orthorhombic structure during the transition. Our work greatly develops the potential of MnMX alloys as magnetocaloric materials and is meaningful to seek or design a MnMX system with low thermal hysteresis.Graphical abstractImage 1
  • Influence of strain rate on subsolvus dynamic and post-dynamic
           recrystallization kinetics of Inconel 718
    • Abstract: Publication date: Available online 31 May 2019Source: Acta MaterialiaAuthor(s): A. Nicolaÿ, G. Fiorucci, J.M. Franchet, J. Cormier, N. Bozzolo Influence of strain rate on dynamic and post-dynamic recrystallization kinetics of Inconel 718 is investigated by performing hot compression tests at constant strain rate in the range [0.001;1]s−1 in the δ -subsolvus domain, with or without post-deformation holding at the deformation temperature. Dynamically and post-dynamically recrystallized grains are distinguished based on their internal misorientations, using EBSD data with enhanced angular resolution. For the applied deformation conditions (T=980°C and ε=0.7), dynamic recrystallization is inhibited at ε˙>0.1s−1. On the other hand, very fast post-dynamic recrystallization is promoted by high strain rates, with characteristic times which can be as short as few seconds to achieve full recrystallization. Most of previous works on the effect of strain rate on dynamic recrystallization kinetics were done by quenching samples right after deformation, without discriminating dynamically and post-dynamically recrystallized grains. Those works led to the conclusion that increasing strain rate beyond a critical value leads to an increase in dynamic recrystallization kinetics. Experimental quenching delays cannot be shorter that few seconds, which is shown here to be sufficient to get a significant increase in recrystallized fraction by post-dynamic mechanisms. Based on the present work, post-dynamic evolutions are actually very likely to be responsible for the apparent increase in dynamic recrystallization kinetics at high strain rates which has often been reported previously.Graphical abstractImage 1
  • Exploration of the Microstructure Space in TiAlZrN Ultra-Hard
           Nanostructured Coatings
    • Abstract: Publication date: Available online 31 May 2019Source: Acta MaterialiaAuthor(s): Vahid Attari, Aitor Cruzado, Raymundo Arroyave Ti1−x−yAlxZryN cubic alloys within the 25-70% Al composition range have high age-hardening capabilities due to metastable phase transition pathways at high temperatures. They are thus ideal candidates for ultra-hard nano-coating materials. There is growing evidence that this effect is associated with the elasto-chemical field-induced phase separation into compositionally-segregated nanocrystaline nitride phases. Here, we studied the microstructural evolution in this pseudo-ternary system within spinodal regions at 1200°C by using an elasto-chemical phase field model. Our simulations indicate that elastic interactions between nitride nano-domains greatly affect not only the morphology of the microstructure but also the local chemical phase equilibria. In Al-rich regions of the composition space we further observe the onset of the transformation of AlN-rich phases into their equilibrium wurtzite crystal structure. This work points to a wide palette of microstructures potentially accessible to these nitride systems and their tailoring is likely to result in significant improvements in the performance of transition metal nitride-based coating materials.Graphical abstractImage 1
  • Temperature dependent fracture toughness of KNN-based lead-free
           piezoelectric ceramics
    • Abstract: Publication date: Available online 30 May 2019Source: Acta MaterialiaAuthor(s): Yingwei Li, Yixuan Liu, Paul-Erich Öchsner, Daniel Isaia, Yichi Zhang, Ke Wang, Kyle G. Webber, Jing-Feng Li, Jürgen Rödel The fracture toughness of unpoled and electrically poled lead-free KNN-based piezoelectric ceramics with the composition of 0.92KNN-0.02Bi0.5Li0.5TiO3-0.06BaZrO3 was investigated. Results reveal that at room temperature, the intrinsic fracture toughness (KI0) of the unpoled samples, evaluated by the near-tip crack opening displacement (COD) technique, is the lowest with a value of 0.70 MPa⋅m0.5; the long (through-thickness) crack fracture toughness (KIvnb), obtained by the single edge V-notch beam (SEVNB) technique, is the highest, with a value of 0.95 MPa⋅m0.5; intermediate short surface crack fracture toughness (KIsc) of 0.86 MPa⋅m0.5 was determined by the surface crack in flexure (SCF) technique. These results were rationalized by the toughening behavior of the material combined with the crack geometry-dependent stress intensity evolution during crack propagation. With increasing temperature, KIvnb and KIsc decrease, and become nearly identical at 350 °C, suggesting an absence of toughening. For electrically poled samples, their room temperature fracture toughness was characterized by both SCF and SEVNB techniques, with values of 0.88 MPa⋅m0.5 and 0.99 MPa⋅m0.5, respectively, slightly larger than the values measured for unpoled samples. Nonlinear electric field-strain and stress-strain analysis of the material was also employed during electric field loading, mechanical compression and four-point bending in order to quantify crack tip shielding by domain switching and the actual stress at the point of instable crack propagation.Graphical abstract(a) Comparison between the intrinsic fracture toughness and fracture toughness of KNN-BLT-6BZ measured by SCF and SEVNB technique. For comparison, the calculated fracture toughness and the KIsc obtained with the nonlinear stress-strain behavior considered were also plotted. (b) Crack growth resistance KIR curves with toughening effect and the evolution of the stress intensity factor as a function of crack length. The red tangent dots represent the point at which unstable crack growth begins. The part of the KIR curve with red color denotes the stage in which stable crack growth will happen with increasing applied load.Image 1
  • Microstructural Evolution and High-Temperature Strength of a
           γ(f.c.c.)/γ’(L12) Co-Al-W-Ti-B Superalloy
    • Abstract: Publication date: Available online 30 May 2019Source: Acta MaterialiaAuthor(s): Daniel J. Sauza, David C. Dunand, David N. Seidman We characterized the microstructural features and mechanical performance of a model Co-5.6Al-5.8W-6.6Ti-0.12 B (at.%) alloy consisting of γ(L12)-precipitates in a γ(f.c.c.)-matrix. Scanning electron microscopy (SEM) was used to follow the isothermal aging of the microstructure at 900 and 1000 °C for 256 h, and 950 °C for 1000 h. The compositions of the γ(L12)-precipitates and a γ(f.c.c.)-matrix were evaluated by atom-probe tomography (APT) for solution-treated and air-cooled conditions, as well as in specimens aged at 950 °C for 16 and 100 h. Boron was shown to partition preferentially to the γ(L12)-precipitates, and profiles taken across the γ(f.c.c.)-matrix channels in both aged specimens revealed confined segregation of Al at one of the two γ(f.c.c.)/γ’(L12) heterophase interfaces. After aging at 950 °C for 16 h, Co-5.6Al-5.8W-6.6Ti-0.12B (at.%) exhibited anomalous flow-strength behavior in the range 625-900 °C with a peak yield stress of 822 MPa between 800 and 825 °C. Compressive creep tests performed at 850 °C demonstrated a creep strength comparable to archival literature results for Co-9Al-9W-0.12B (at.%), despite a smaller γ’(L12)-volume fraction and lack of strengthening borides along the grain boundaries (GBs). The activation energy for creep in the temperature range 800-900 °C was 606 kJ mol-1. The post-creep microstructure consists of rafted γ’(L12)-precipitates perpendicular to the compression axis, confirming the positive γ(f.c.c.)/γ’(L12) lattice parameter misfit character of this class of alloys. Failure could occur due to GB embrittlement caused by deleterious Ti-rich (L21 or B2) and D019 phases formed at the GBs during creep. (246 words)Graphical abstractImage 1
  • Nucleation kinetics in a supercooled metallic glass former
    • Abstract: Publication date: Available online 30 May 2019Source: Acta MaterialiaAuthor(s): F. Puosi, A. Pasturel We use molecular dynamics simulations to shed light on the mechanism underlying crystal nucleation in a supercooled metallic glass former characterized by a concurring crystal and amorphous local order based on icosahedral symmetry. At a crossover temperature, well below the melting point, we find that the supercooled phase exhibits glassy dynamics which includes a breakdown of the Stokes-Einstein relation and the emergence of spatially heterogeneous dynamics. In addition, we show that the origin of these phenomena can be related to a structural heterogeneity caused by the increase of icosahedral symmetry upon cooling. We also reveal that crystal nucleation occurs close to the glass transition and takes place in regions of high icosahedral symmetry, which can be interpreted by a strong reduction of the crystal-liquid interfacial energy. This scenario provides a framework for testing the estimation of the nucleation time according to the classical nucleation theory. More specifically, our findings allow to quantify how the heterogeneous character of the supercooled phase affects the predictions of this theory.Graphical abstractImage 1
  • Sol-gel derived alumina glass: Mechanistic study of its structural
    • Abstract: Publication date: Available online 30 May 2019Source: Acta MaterialiaAuthor(s): Jin He, David Avnir, Long Zhang Single-component Al2O3 atomic-level glass free of some degree of crystallinity cannot be obtained by conventional melt quenching methodology. So far, glassy amorphous alumina was obtained only by far from equilibrium conditions, which limit their dimensions to thin film or nanoparticles. In 2007 we reported a sol-gel Al-lactate route to amorphous alumina (L. Zhang et al, J. Non-Cryst. Solid, 353, 1255), which recently was proven to be glassy on the atomistic level. This, to the best of our knowledge, is the first time that a real alumina glass has been proven. Therefore there is much interest in understanding the mechanistic details of this aqueous sol-gel process which leads to homogeneous Al2O3 glass, as a guideline for the formation of other non-trivial ionic-oxide based glasses. Detailed 27Al NMR analysis supported by several other analytical methods, indicate the formation of a metal coordination network based on aluminum cations-lactate (citrate) ligand interactions, which occurs during the sol-gel transition. This water soluble metal coordination network demonstrated a facile reversible sol-gel-sol transition based on concentration-driven formation and breaking of the intramolecular non-covalent interactions between the monomers. Subsequent annealing of the xerogel by gradual heating to 500-700 oC promotes the formation of the interconnecting Al-O-Al bonds, concomitant with the removal of the lactate ligands upon heating, resulting in pure alumina glass as the final product. A glass transition temperature (Tg) of Al2O3 glass was observed at ∼670 oC. Similar observations are reported here also for aluminum citrate, which indicates that the Al-chelating approach to alumina glass is general.Graphical abstractImage 1
  • Deformation of Ni-Mn-Ga 7M modulated martensite through
           detwinning/twinning and forward/reverse intermartensitic transformation
           studied by in-situ neutron diffraction and interrupted in-situ EBSD
    • Abstract: Publication date: Available online 30 May 2019Source: Acta MaterialiaAuthor(s): Naifu Zou, Zongbin Li, Yudong Zhang, Weimin Gan, Bo Yang, Xiang Zhao, Claude Esling, Michael Hofmann, Liang Zuo Shape memory alloys, especially the newly developed Ni-Mn-based heusler-type intermetallic compounds, exhibit specific mechanical responses to mechanical loading. Although the deformation behaviors have been studied for reducing the number of martensite variants, the mechanisms are not fully revealed. Thus in this work the compression process of twin-related 7M modulated martensite of Ni-Mn-Ga intermetallic compound was studied by in-situ neutron diffraction at macroscopic scale and by interrupted in-situ EBSD at microscopic scale. It is revealed that the mechanical response of the 7M martensite is featured by three states: a linear elastic-plastic state, a steady plastic state, and a second linear plastic state. The plastic deformation is initiated by the detwinning of the existing variants in the first linear state. It proceeds to the steady state by intensive detwinning of the these variants and by twinning of the remaining variants that result in the disappearance of the existing variants and the appearance of new variants, then by intermartensitic transformation to form non-modulated martensite (NM). These three shear processes are highly coordinated and compatible with the annihilation of the local incompatible strains by reverse intermartensitic transformation, which allows a steady progress of deformation and a continuous reorientation of the variants. The reorientation produces new twins with unfavorable orientations and limited deformation capacity, leading to a stress increase for further deformation. The present work provides comprehensive information on deformation mechanisms of Ni-Mn-Ga 7M martensite at each characteristic deformation step that is useful for mechanical simulation of deformation behaviors of intermetallic compounds.Graphical abstractImage 1
  • Electron-configuration stabilized (W,Al)B2 solid solutions
    • Abstract: Publication date: Available online 29 May 2019Source: Acta MaterialiaAuthor(s): Rainer Hahn, Vincent Moraes, Andreas Limbeck, Peter Polcik, Paul H. Mayrhofer, Holger Euchner By combining experimental and theoretical methods, we have conducted a detailed study of the ternary diboride system (W1-xAlx)1-yB2(1-z). Tungsten rich solid solutions of (W1-xAlx)1-yB2(1-z) were synthesized by physical vapor deposition and subsequently investigated for structure, mechanical properties and thermal stability. All crystalline films show hardness values above 35 GPa, while the highest thermal stability was found for low Al contents. In this context, the impact of point defects on the stabilization of the AlB2 structure type is investigated, by means of ab initio methods. Most notably, we are able to show that vacancies on the boron sublattice are detrimental for the formation of Al-rich (W1-xAlx)1-yB2(1-z), thus providing an explanation why only tungsten rich phases are crystalline.Graphical abstractImage 1
  • Structural transformation and embrittlement during lithiation and
           delithiation cycles in an amorphous silicon electrode
    • Abstract: Publication date: Available online 29 May 2019Source: Acta MaterialiaAuthor(s): Swastik Basu, Nikhil Koratkar, Yunfeng Shi Silicon shows potential as an anode material in lithium ion batteries due to its high specific capacity, yet its considerable volume expansion during lithiation leads to fracture and pulverization. Unfortunately, neither the atomic-level structural evolution, nor the mechanical behaviors of the anode during lithiation and delithiation cycles is well understood. Interestingly, the lithiation process of a-Si provides an interesting continuum from open-structured network glass to densely-packed atomic glass, which could be used to obtain useful insights regarding commonalities in glasses. Here atomic level simulation has been used to investigate one cycle of lithiation and delithiation of amorphous silicon electrode, using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The atomic level structural transformation and damage accumulation of the anode during cycling has been systematically analyzed, as well as their mechanical responses in compact tension tests. There appears to be a ductile-brittle-ductile transition for the amorphous silicon anode during both the lithiation and delithiation cycle. In other words, amorphous silicon is particularly vulnerable at intermediate lithiation. The fracture behavior of lithiated silicon was found to correlate to the Poisson’s ratio, due to variations in bond covalency and structural disorder.Graphical abstractImage 1
  • Role of Microstructure on the Actuation Fatigue Performance of Ni-Rich
           NiTiHf High Temperature Shape Memory Alloys
    • Abstract: Publication date: Available online 29 May 2019Source: Acta MaterialiaAuthor(s): O. Karakoc, C. Hayrettin, A. Evirgen, R. Santamarta, D. Canadinc, R.W. Wheeler, S.J. Wang, D.C. Lagoudas, I. Karaman The focus of the present study was to systematically investigate the influence of microstructure on the actuation fatigue performance of a Ni-rich NiTiHf high temperature shape memory alloy (HTSMA). Different aging heat treatments led to the formation of H-phase nano-precipitates with different sizes and morphology within the matrix, enhancing thermo-mechanical stability and enabling control of transformation temperatures. The actuation fatigue testing of specimens was performed until failure through thermally-induced reversible martensitic transformation under a constant stress (300 MPa) with two distinct upper cycle temperatures (UCT) of 300°C and 350°C. Consequently, the specimens heated to 700°C and furnace cooled to 100°C in 48h, with relatively large precipitates failed at average fatigue life of 15,500 cycles and exhibited 1.0% average actuation strain in the 300°C UCT experiments, while those aged at 550°C for 3h, with the precipitate sizes less than 20 nm, attained an average fatigue life of 10,800 cycles and an actuation strain of 2.5%. Samples with intermediate precipitate sizes after aging at 600°C for 10h failed at the shortest average fatigue life of 8,200 cycles with an intermediate average actuation strain of 2.0%. Furthermore, increase in UCT decreased the fatigue life and resulted in larger average actuation strains for all samples. Overall, the current findings constitute the first systematic results demonstrating the microstructure dependence of the evolution of actuation strain, irrecoverable strain, and transformation temperatures during actuation fatigue, and actuation fatigue life of the Ni-rich Ni50.3Ti29.7Hf20 HTSMA, demonstrating the importance of controlling the H-phase precipitate size.Graphical abstractImage 1
  • The heterogenous distribution of white etching matter (WEM) around
           subsurface cracks in bearing steels
    • Abstract: Publication date: Available online 28 May 2019Source: Acta MaterialiaAuthor(s): M.E. Curd, T.L. Burnett, J. Fellowes, J. Donoghue, P. Yan, P.J. Withers White etching cracks (WECs) initiate subsurface in bearings and can propagate to cause premature failure. These cracks are bordered by an altered microstructure known as white etching matter (WEM), which is thought to form via a crack-rubbing mechanism. However, WEM is often observed bordering a single side of a crack. In search of a microstructural difference to justify the observed WEM asymmetry, regions of untransformed material, adjacent to cracking in a bearing inner ring, which had undergone hydrogen charging prior to testing, were studied using correlative electron microscopy (EM), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA) and nano-indentation techniques. The investigations found no significant differences between the untransformed material neighbouring the cracks and the parent material; both were found to have similar: grain size and shape; crystallographic texture; carbon concentration; carbide population and hardness, which questions why only one rubbed surface of the crack has formed WEM. The initiation of WEM at the tip of the crack is suggested, however more investigations are needed to build another WEM formation model. EPMA characterisation revealed evidence of carbide dissolution in the WEM. Despite this, significant variation in the carbon concentration of the WEM was found; ranging from +13% enrichment to 42% depletion (in counts), relative to the parent material.Graphical abstractImage 1
  • Heterophase interface-mediated formation of nanotwins and 9R phase in
           Aluminum: Underlying mechanisms and strengthening effect
    • Abstract: Publication date: Available online 28 May 2019Source: Acta MaterialiaAuthor(s): J.D. Zuo, C. He, M. Cheng, K. Wu, Y.Q. Wang, J.Y. Zhang, G. Liu, J. Sun Nanostructured crystalline Al/amorphous AlN multilayer films with a wide layer thickness (h) range from ∼10 nm up to ∼200 nm were prepared by using magnetron sputtering. Nanotwins and 9R phase were substantially observed in the Al layers, showing a strong thickness dependence. The 9R phase predominantly penetrated through the Al layer in the II regime of h ≤ ∼ 20 nm, while mainly terminated within the layer interior in the I regime of h> ∼ 20 nm. On the contrary, the coherent nanotwins were boosted when h> ∼ 20 nm and the percentage of twined Al grains was greatly increased. The formation mechanisms of 9R phase and coherent nanotwins were discussed in terms of the interfacial chemistry/physics modulated by the amorphous AlN layers, which displayed gradient characteristics and hence was sensitive to the layer thickness. A significant thickness dependence of hardness was also evident that the hardness monotonically increased with reducing h in the I regime, while reached a peak value and hold almost unchanged in the II regime. The hardness in the II regime is about 1 GPa greater than the predictions from an interfacial barrier crossing model. This discrepancy is mainly contributed by the layer-penetrating 9R phase rather than the nanotwins. This study provides a new perspective on fabricating nanotwinned Al by utilizing heterophase interfaces.Graphical abstractImage 1
  • Screw dislocation driven martensitic nucleation: A step toward consilience
           of deformation scenario in fcc materials
    • Abstract: Publication date: Available online 28 May 2019Source: Acta MaterialiaAuthor(s): Tae-Ho Lee, Sung-Dae Kim, Heon-Young Ha, Jae Hoon Jang, Joonoh Moon, Jun-Yun Kang, Chang-Hoon Lee, Seong-Jun Park, Wanchuck Woo, Jong-Ho Shin, Jong-Wook Lee, Dong-Woo Suh, Hyun-Uk Hong Martensitic transformation (MT), constituting an essential part of deformation scenario, plays a key role in plasticity and thermoelasticity of face-centered cubic (fcc) materials. Despite being an area of intense research, discrepancies remain about the essential parameter dictating nucleation of hexagonal close-packed (hcp) martensite and about the dislocation activities governing plasticity. Here, we show that screw dislocation induces the torsional flow of close-packed atomic planes of fcc matrix, characterized by Eshelby twist, and its dissociation provides a self-perpetuating step to bring forth Frank partial dislocation acting as a critical component to accomplish the atomic periodicity for fcc-to-hcp MT. The critical condition to initiate fcc-to-hcp MT was estimated from the Eshelby twist angle measured from high-angle annular dark field scanning transmission electron microscopy (HAAD STEM). Once the critical condition is satisfied, the trajectory of screw dislocation can span two atomic planes, its dissociation proceeds by forming both Frank partial sessile to {111} and Shockley partial glissile along {111} plane. Based on our dislocation model for MT, we demonstrate how dissociation route of perfect dislocation can be exploited to determine deformation mechanism (MT, twinning, slip). By incorporating dislocation dissociation model into the concept of stacking fault energy, we suggest a synthesized concept of deformation scenario that can provide fundamental and predictive insight into plasticity and transformability of fcc material.Graphical abstractImage 1
  • Simulation and experimental study of the particle size distribution and
           pore effect on the crystallization of glass powders
    • Abstract: Publication date: Available online 28 May 2019Source: Acta MaterialiaAuthor(s): Roger G. Fernandes, Raphael M.C.V. Reis, Raúl R. Tobar, Edgar D. Zanotto, Eduardo B. Ferreira Surface nucleation is a frequent phenomenon and plays an essential role in the crystallization of most glasses, especially glass powders used in sintered glass-ceramics, for example. A technique often employed to study the crystallization kinetics of glassy powders is Differential Scanning Calorimetry (DSC). However, the shape of the crystallization peak profile is usually very complex. For instance, in certain cases of surface nucleation, the crystal growth dimensionality may change during crystallization after crystal impingement, and a large density difference between the parent glass and the resulting crystalline phases may produce cavitation pores in the particle volume. Finally, crushed glass particles are not regularly shaped, as assumed in most models. Hence, to evaluate the crystallization kinetics of glass powders by DSC experiments, in this work, we modified and tested a particle crystallization model using the DSC crystallization peaks of jagged powders of a non-stoichiometric diopside (0.9CaO.0.7MgO.2SiO2) glass having different granulometries and a large crystal/glass density difference. The Reis-Zanotto model was adapted to include two complex variables: particle size distribution and crystallization-induced porosity. As predicted by the modified model, we confirmed the radical change of the DSC peak shape for some particle sizes as a result of crystal impingement and pore formation. This combined experimental-simulation study demonstrates that the adapted model can describe this type of complex and yet frequent crystallization case.Graphical abstractImage 1
  • On the compositional partitioning during phase transformation in a binary
           ferromagnetic MnAl alloy
    • Abstract: Publication date: Available online 25 May 2019Source: Acta MaterialiaAuthor(s): D. Palanisamy, D. Raabe, B. Gault We introduce a new perspective on the classical massive mode of solid-state phase transformation enabled by the correlative use of atomic scale electron microscopy and atom probe tomography. This is demonstrated in a binary MnAl alloy which has Heusler-like characteristics. In this system, the τ phase formed by a massive transformation from the high-temperature ε phase is metastable and ferromagnetic. The transformation results in a high density of micro-twins inside the massively grown τ phase. Atomic-scale compositional analysis across the interface boundaries and atomic structure of the micro-twins reveals the involvement of both structural modification and also the compositional partitioning during massive growth of the τ phase. This is assisted by the migrating τ/ε interface boundary during transformation. Further, the role of micro-twins on nucleating the equilibrium phases are discussed.Graphical abstractImage 1
  • Hierarchical analysis of alloying element effects on gas nitriding rate of
           Fe alloys: A DFT, microkinetic and kMC study
    • Abstract: Publication date: Available online 24 May 2019Source: Acta MaterialiaAuthor(s): Ku Kang, Soonho Kwon, Changsoo Lee, Doosun Hong, Hyuck Mo Lee Nitriding is the most widely employed thermochemical surface treatment to enhance the mechanical properties of steel. Specifically, gas nitriding, which is a low-temperature process for efficiently producing high-performance steels, has a disadvantage in that it consumes a large amount of time. To enhance the nitriding rate, we studied the surface alloying of iron (Fe) and its effect on ammonia (NH3) nitriding of Fe using a hierarchical protocol with density functional theory (DFT)-based microkinetics and real-time simulations. First, we considered the NH3 decomposition and nitrogen (N) diffusion mechanism on clean and alloyed (Fe-X) Fe (100) surfaces using DFT. In this study, the alloying elements including transition metals and period III to VI elements in the periodic table were considered for DFT-based computational screening. For the candidate Fe-X systems selected to improve the nitriding rate in the previous step, we calculated all the energy barriers for every elementary reaction step by varying the alloying elements and performed microkinetic analysis using those kinetic energy barriers to determine their influence on the nitriding rate. After adding consideration of thermodynamic factors, selected candidate alloys were subjected to detailed DFT calculations of the nitriding mechanism with N coverage, and based on these results, a kinetic Monte Carlo (kMC) simulation was performed to reconfirm the results under the actual nitriding process conditions. Through a hierarchical protocol, we performed a theoretical analysis and simulation of the effects of alloying elements on the nitriding rate that were not explained experimentally and suggested the best alloying element with the improved nitriding rate.Graphical abstractImage 1
  • Maximum N content in a-CNx by ab-initio simulations
    • Abstract: Publication date: Available online 24 May 2019Source: Acta MaterialiaAuthor(s): Jiri Houska Structures of amorphous CNx materials are predicted by extensive ab-initio molecular-dynamics simulations (more than 800 trajectories) in a wide range of compositions and densities. The main attention is paid to the formation of N2 molecules, with the aim to predict and explain the maximum N content in stable CNx networks. The results show that the maximum N content is of ≈42 at.%. From the kinetics point of view, higher N contents lead to steeply increasing rate of N2 formation during materials formation. From the thermodynamics point of view, higher N contents in a network may be temporarily stabilized by N2 molecules sitting in voids around the network, but a subsequent N2 diffusion into the atmosphere makes them unstable. The results are important for the design of CNx (and other nitride) materials and pathways for their preparation for various technological applications.Graphical abstractImage 1
  • Reversible displacive transformation with continuous transition interface
           in a metastable β titanium alloy
    • Abstract: Publication date: Available online 23 May 2019Source: Acta MaterialiaAuthor(s): Lu Qi, Chunjin Chen, Huichao Duan, Suyun He, Yulin Hao, Hengqiang Ye, Rui Yang, Kui Du Super-elasticity and shape memory of materials are typically associated with reversible phase transformations. The reversible phase transformations and their governing factors thus have long been research interests of materials scientists and physicists. Here, a novel reversible ω transformation has been observed in a metastable β-Ti alloy during tensile deformation with in situ aberration corrected transmission electron microscopy. We reveal that the reversible transformation is attributed to a trigonal crystal structure of the ω phase. Moreover, continuous transition interfaces with no interfacial defects are formed between the ω and β phases, and they contribute essentially to the occurrence of the reverse transformation. This reversible transformation has great potential for developing super-elasticity and shape memory in materials.Graphical abstractImage 1
  • Kinetics of zircon formation in yttria partially stabilized zirconia as a
           result of oxidation of embedded molybdenum disilicide
    • Abstract: Publication date: Available online 23 May 2019Source: Acta MaterialiaAuthor(s): F. Nozahic, A.L. Carabat, W. Mao, D. Monceau, C. Estournes, C. Kwakernaak, S. van der Zwaag, W.G. Sloof Recently MoSi2 sacrificial particles embedded in yttria partially stabilized zirconia (YPSZ) have been proposed as attractive healing agents to realize significant extension of the lifetime of the thermally loaded structures. Upon local fracture of the YPSZ, the embedded healing particles in the path and in the vicinity of the crack react with the oxygen atoms transported via the crack and first fill the crack with a viscous glassy silica phase (SiO2). The subsequent reaction between this freshly formed SiO2 and the existing tetragonal ZrO2 of the YPSZ leads to the formation of rigid crystalline zircon (ZrSiO4), which is key in the crack-healing mechanism of YPSZ based materials. The isothermal kinetics of the self-healing reaction and the mechanism of zircon formation from the decomposing MoSi2 and the surrounding YPSZ were assessed via X-ray diffraction (XRD). The obtained results revealed that at 1100 °C the reaction between amorphous SiO2 and YPSZ is completed after about 10 hours. For a more accurate determination of the kinetics of the self-healing reaction, bilayer samples of YPSZ – MoSi2 (with and without boron addition) were annealed in air over a temperature range of 1100 to 1300 °C. This led to the formation of a MoSi2/amorphous (boro)silica/zircon/YPSZ multi-layer, which was investigated with scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA). Kinetic modeling of the growth of zircon and silica or borosilicate layers showed that zircon growth was dominated by the diffusion of Si4+ in zircon whereas the growth of the silica or borosilicate layer was controlled by oxygen diffusion. Moreover, a significant increase in the rate of ZrSiO4 formation was observed due to the presence of B in the MoSi2 particles.Graphical abstractImage 1
  • Thermal stability of nanolamellar fcc-Ti1-xAlxN grown by chemical vapor
    • Abstract: Publication date: Available online 23 May 2019Source: Acta MaterialiaAuthor(s): Michael Tkadletz, Christina Hofer, Christina Wüstefeld, Nina Schalk, Mykhaylo Motylenko, David Rafaja, Helga Holzschuh, Werner Bürgin, Bernhard Sartory, Christian Mitterer, Christoph Czettl In recent years, nanolamellar aluminum-rich face-centered cubic (fcc) Ti1-xAlxN coatings with x as high as 0.8-0.9 grown by thermal chemical vapor deposition (CVD) have been investigated extensively. However, detailed information about their microstructure characteristics, local chemical composition and phase stability at elevated temperatures is still missing. Thus, within the present work, the temperature-induced microstructural changes of a nanolamellar fcc-Ti0.2Al0.8N coating, synthesized by thermal CVD at ∼790 °C, were studied up to temperatures of 1300 °C. In situ high-temperature X-ray powder diffraction and differential scanning calorimetry were employed to follow the phase evolution at elevated temperatures. Scanning electron microscopy and electron backscatter diffraction, carried out ex situ for six different microstructural states after isothermal annealing, revealed the distribution of individual phases and morphology of different phase regions. Complementary atom probe tomography as well as transmission electron microscopy experiments were performed on the as-deposited, an intermediate and the final decomposed and transformed state. The results provided 3D elemental information as well as detailed morphology, phase and orientation information of the respective samples. In the as-deposited state, the coating was characterized by columnar, relatively large fcc grains exhibiting a nanolamellar microstructure. Decomposition of initially supersaturated fcc-Ti1-xAlxN was detected at temperatures of ∼900-1000 °C. Transformation of metastable Al-rich fcc regions into the thermodynamically stable wurtzitic modification started at ∼1000 °C and persisted up to ∼1175 °C. In this temperature range, intact nanolamellar fcc areas coexisted with fully decomposed and transformed regions, leading to a constant reduction of the fcc fraction with increasing temperature. At temperatures above ∼1175 °C, the coating was fully decomposed and transformed into fcc-TiN and w-AlN. The obtained findings provide a detailed description of the decomposition and transformation behavior of nanolamellar CVD fcc-Ti1-xAlxN coatings, which significantly contributes to the fundamental understanding of this complex coating system.Graphical abstractImage 1
  • An efficient scheme to tailor the magnetostructural transitions by staged
           quenching and cyclical ageing in hexagonal martensitic alloys
    • Abstract: Publication date: Available online 22 May 2019Source: Acta MaterialiaAuthor(s): Yong Li, Qingqi Zeng, Zhiyang Wei, Enke Liu, Xiaolei Han, Zhiwei Du, Lingwei Li, Xuekui Xi, Wenhong Wang, Shouguo Wang, Guangheng Wu The hexagonal MM'X alloys with giant magnetocaloric effects have become an important magnetic phase-transition alloy family. Among the studies on this family, various methods, including composition engineering, have been developed to tailor the magnetostructural transitions. In this study, we present an alternative scheme to tailor the magnetostructural transitions by applying high-temperature staged quenching and low-temperature cyclical ageing on the hexagonal MnCoGe-based alloys with unchanged compositions. The experimental results show that the martensitic transition temperature decreases due to the increasing number of thermal vacancies produced in the increasing annealing temperature, which indicates the martensitic transition is strongly dependent on the staged quenching from high temperatures that can tune the concentration of equilibrious thermal vacancies in alloys. As a result, a Curie temperature window is established, in which the coupling of magnetostructural transitions is successfully realized, showing tunable, large magnetocaloric effects. By further applying cyclical ageing at low-temperature region, the magnetostructural transitions and magnetocaloric effects can be tuned to higher temperatures again due to the release of the trapped thermal vacancies, showing a high tunability of the magnetostructural transitions. During these heat treatments, furthermore, the transitions are robust against the high temperatures up to 550 ºC, with no decoupling of the structural and magnetic transitions. The present work provides an important, efficient scheme to tailor the magnetostructural transitions, multiple magnetic/structural phase states, and the related functional properties of hexagonal phase-transition alloy family, without changing the chemical composition of the materials.Graphical abstractImage 1
  • An atomic scale structural investigation of nanometre-sized η
           precipitates in the 7050 aluminium alloy
    • Abstract: Publication date: Available online 22 May 2019Source: Acta MaterialiaAuthor(s): Tsai-Fu Chung, Yo-Lun Yang, Makoto Shiojiri, Chien-Nan Hsiao, Wei-Chih Li, Cheng-Si Tsao, Zhusheng Shi, Jianguo Lin, Jer-Ren Yang Using high-angle-annular-dark-field (HAADF) scanning-transmission-electron microscopy (STEM), we have investigated η-precipitates in the Al-Zn-Mg-Cu (AA7050) aluminium alloy. The HAADF STEM images taken along the zone axes of [101¯0]η, [12¯10]η, and [0001]η illustrated the projected atomic-scale configurations of η-MgZn2 crystal. The precipitates developed in layer-by-layer growth, supplied with precursors such as Zn, Cu, and Mg, which were solute atoms segregated around the η/Al interfaces due to the higher lattice strain energy. Stacking faults and defect layers composed of flattened hexagons were frequently observed along the zone axes of [12¯10]η and [101¯0]η, respectively, and their formation was elucidated, similarly taking into account the layer-by-layer growth. Occasional coalescence between two precipitates yielded a complicated boundary or a twin-like boundary. Based on the differences in orientation relationships between η-types and the Al matrix reported to date, two new types of η precipitates have been recognized and named η4' and η12.Graphical abstractImage 1
  • Super-high-strength and formable medium Mn steel manufactured by warm
           rolling process
    • Abstract: Publication date: Available online 22 May 2019Source: Acta MaterialiaAuthor(s): Bin Hu, BinBin He, GuanJu Cheng, HungWei Yen, MingXin Huang, Hai Wen Luo In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy could produce not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby–Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in the ultrafine RA grains. The best combination of strength and ductility included 1.5 GPa ultimate tensile strength and 28% total elongation. This was achieved due to the sustainable TRIP and TWIP effects in the later straining stage resulting from the ultrafine austenite grains, the latter were reversely transformed from the recrystallized ferrite grains.Graphical abstractImage 1
  • Non-Arrhenius grain growth in strontium titanate: Quantification of
           bimodal grain growth
    • Abstract: Publication date: Available online 22 May 2019Source: Acta MaterialiaAuthor(s): Wolfgang Rheinheimer, Ephraim Schoof, Michael Selzer, Britta Nestler, Michael J. Hoffmann Strontium titanate is well-known for its non-Arrhenius grain growth, where grain growth coefficients decrease by orders of magnitude between 1350 °C and 1425 °C. This transition is assumed to be caused by the existence and coexistence of two grain boundary types and results in the formation of bimodal microstructures. So far, no quantified data on the transition behavior was available. The present study uses a comparison of experimental microstructures for various heating times and temperatures with simulated microstructures from phase-field simulations considering various fractions of fast-growing grains. The microstructures are compared by means of their grain size distributions. It is found that the fraction of fast-growing grains follows an anti-Arrhenius behavior. Evaluating the present findings with respective literature data, the grain growth transition could be related to a space charge transition where the fast and slow grain boundaries are associated with strong and weak space charge and segregation. Overall, the present study sheds light on general grain growth transitions observed in several perovskite ceramics.Graphical abstractImage 1
  • Small scale testing approach to reveal specific features of slip behavior
           in BCC metals
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): Nousha Kheradmand, Bjørn Rune Rogne, Stéphane Dumoulin, Yun Deng, Roy Johnsen, Afrooz Barnoush Plastic behavior of Fe–3%Si was investigated with a novel methodology at microscopic scale and higher stress resolution beyond the existing macroscopic test methods. Activation of single slip system in micropillars under compression provides perfect shear and excludes the lattice rotation. Crystallographic orientation dependency of critical resolved shear stress (CRSS) was observed. Different scenarios regarding activated slip planes are discussed to explain this orientation dependency. Schmid’s law approach, which only considers the stress component parallel to the Burgers vector, fails to explain this orientation dependency of CRSS. It is shown that the non-glide stress components should be taken into account in order to precisely define the plasticity in BCC metals.
  • Sequential obstacle interactions with dislocations in a planar array
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): Shuozhi Xu, David L. McDowell, Irene J. Beyerlein The strengthening of metals by nano-scale obstacles is mainly attributed to the impediment to glide dislocations offered by these obstacles. It is important to understand the mechanisms for dislocation bypass of obstacles having nano-scale dimension, including the atomic-scale structure changes sustained by both obstacles and dislocations after the bypass process. Recently, atomic-scale modeling has provided much insight into obstacle interactions involving a single dislocation. However, the more naturally occurring scenarios involving a sequence of encounters with arrays of moving dislocations are not as well understood owing to prohibitively large length scale requirements for atomistic models. In this study, we utilize a novel multiscale concurrent atomistic-continuum method to simulate a sequence of interactions between glide dislocations in an array with a spherical nano-obstacle (either a void or an impenetrable precipitate) in Al. In the case of a void, the bypassing array of dislocations progressively weakens the void until it splits the originally spherical void into two hemispheres. We present an analytical model for the depinning stress for the first dislocation in the array. In the case of a large impenetrable precipitate, sequential dislocations in the array bypass via alternating mechanisms of Orowan looping and Hirsch looping. The residual dislocation loop created around the precipitate by the bypass of the first dislocation is completely removed by the passage of the subsequent dislocation. These mechanisms can benefit the design of materials that are reinforced with nanophase inhomogeneities to achieve ultra high strength.Graphical abstractImage 1
  • High-cycle-fatigue induced continuous grain growth in ultrafine-grained
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): P. Zhao, B. Chen, J. Kelleher, G. Yuan, B. Guan, X. Zhang, S. Tu The cyclic deformation behaviour and microstructural stability of severe plastic deformation processed bulk nanostructured (ultrafine-grained, UFG) commercially pure cp-Ti were investigated by using in situ neutron diffraction combined with R=-1 high-cycle-fatigue (HCF) loading at room and cryogenic temperatures. The UFG microstructure was created by equal channel angular pressing (ECAP) and multi-direction forging (MDF). A considerable continuous grain growth was revealed by neutron diffraction for MDF cp-Ti fatigued at 25 °C, as opposed to that at -200 °C. The same HCF fatigue loading at 25 °C only caused very limited grain growth for ECAP cp-Ti. Transmission electron microscopy confirmed the grain growth. Further confirmation of the room-temperature HCF fatigue-induced grain growth was obtained by transmission Kikuchi diffraction based analysis. Novel insights into fatigue induced grain growth mechanism in UFG cp-Ti are thus provided: (i) the thermally activated process plays an important role in grain growth during the room-temperature HCF fatigue; (ii) Continuous dynamic recrystallisation is responsible for the grain growth and dislocation slip or twinning is not essential to trigger such a grain growth; (iii) the anisotropic grain growth behaviour in {0002} grain family can be reconciled by accepting that these grains accumulated highly stored energy during initial severe plastic deformation and the subsequent recrystallisation nucleation occurred at these highly deformed regions.Graphical abstractImage 1
  • In situ real-time annealing of ultrathin vertical Fe nanowires grown by
           focused electron beam induced deposition
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): Javier Pablo-Navarro, Robert Winkler, Georg Haberfehlner, César Magén, Harald Plank, José María de Teresa Focused Electron Beam Induced Deposition is a consolidated technique for the growth of three-dimensional (3D) nanostructures. However, this single-step nanofabrication method requires further efforts to optimize simultaneously dimensional and compositional properties, in particular for deposits with a high aspect ratio. More specifically, ferromagnetic 3D nanowires (NWs) with diameters in the sub-50 nm regime and high metallic contents up to 95 at. % attract great interest to improve the final performance of magnetic nanodevices such as magnetic tips for scanning probe microscopy. In this work, we report on real-time monitoring during chemical purification and structural crystallization processes of ultra-narrow 3D Fe NWs (
  • Development of Ni-free Mn-stabilised maraging steels using
           Fe2SiTi precipitates
    • Abstract: Publication date: Available online 21 May 2019Source: Acta MaterialiaAuthor(s): Alexander.J. Knowles, Peng Gong, Khandaker.M. Rahman, W.Mark Rainforth, David Dye, Enrique.I. Galindo-Nava Computational alloy design has been used to develop a new maraging steel system with low cost, using Mn for austenite reversion and Heusler Fe2SiTi nm-scale precipitates to strengthen the martensite, avoiding high cost alloying elements such as Ni and Co. A pronounced ageing response was obtained, of over 100 HV, associated with the formation of 2-30nm Fe2SiTi precipitates alongside the development of ∼10% Mn rich austenite, at the martensite boundaries with the Kurdjumov-Sachs orientation relationship. The precipitates took on different orientation relationships, depending on the size scale and ageing time, with fine ∼5nm precipitates possessing an L21//α orientation relationship, compared to larger ∼20nm precipitates with L21//α. Computational alloy design has been used for the development and demonstration of an alloy design concept having multiple constraints. Whilst in this case computational design lacked the fidelity to completely replace experimental optimisation, it identifies the importance of embedding Thermodynamic and kinetic modelling within each experimental iteration, and vice versa, training the model between experimental iterations. In this approach, the model would guide targeted experiments, the experimental results would then be taken into future modelling to greatly accelerate the rate of alloy development.Graphical abstractImage 1
  • Direct observation of growth and stability of Al-Cu-Fe quasicrystal thin
    • 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
    • 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
  • 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
  • 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
  • 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.
  • 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
  • Emergence of shallow energy levels in B-doped Q-carbon: A high-temperature
    • 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
  • 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
  • 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
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