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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 246  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3159 journals]
  • Phase transformations in equiatomic CuZr shape memory thin films analyzed
           by differential nanocalorimetry
    • Abstract: Publication date: Available online 11 August 2018Source: Acta MaterialiaAuthor(s): Juanjuan Zheng, Yucong Miao, Haitao Zhang, Shi Chen, Dongwoo Lee, Raymundo Arroyave, Joost J. Vlassak We have investigated the phase transformations in sputtered CuZr shape memory thin films using a differential nanocalorimetry technique that is capable of making calorimetric measurement on thin-film samples with a sensitivity as small as 12 pJ/K. We first present a general procedure to accurately measure the heat capacity and enthalpies of transformation of a sample, even if there is a significant difference in the heat capacities of sample and reference. We then demonstrate the technique by analyzing the phase evolution of equiatomic CuZr thin films and explore the conditions for the formation of the martensitic phase responsible for the shape memory properties of this alloy. We show that fast, low-temperature cycling through the martensitic transformation increases the hysteresis, which we attribute to the accumulation of defects during the martensitic transformation. If the austenitic phase is given sufficient time at elevated temperature to annihilate these defects, the transformation is stable under thermal cycling conditions.Graphical abstractImage 1
       
  • Temporal Evolution of a Model Co-Al-W Superalloy Aged at 650 °C
           and 750 °C
    • Abstract: Publication date: Available online 11 August 2018Source: Acta MaterialiaAuthor(s): Peter J. Bocchini, Chantal K. Sudbrack, Ronald D. Noebe, David N. Seidman The temporal evolution of a γ(f.c.c.)/γ’ (L12) Co-8.8Al-7.3 W superalloy aged at 650 °C (10 min to 4096 h) and 750 °C (10 min to 256 h) is studied utilizing atom-probe tomography (APT), scanning electron microscopy, and Vickers microhardness testing. The evolution of the phase compositions, γ’ (L12) volume fraction, and mean precipitate radius, , are determined. Coarsening rate constants and temporal exponents are calculated for of the γ’ (L12)-nanoprecipitates. The temporal exponents are found to be generally close to 1/p = 1/3 as required for diffusion-limited coarsening. Tungsten solid-solubility is significantly reduced in the γ(f.c.c.)-matrix at 650 °C (0.54 ± 0.04 at. %) and 750 °C (1.35 ± 0.06 at. %) when compared with aging at 900 °C (5.5 at. %). The value of of the γ’ (L12)-nanoprecipitates increases with increasing aging time corresponding to an increase in the Vickers microhardness; the peak strength was not, however, achieved for the aging times investigated. The morphology of the γ’ (L12)-nanoprecipitates begins as spheroids but transitions to cuboids at longer aging times, with final the γ’ (L12) volume fractions for aging at 650 °C and 750 °C being φ = 53 % and 54 %, respectively. The effect of quench-rate (either furnace-cooled, air-cooled, oil quenched, or water quenched) from a supersolvus temperature of 1050 °C on the microstructure of the alloy is also investigated. Slow cooling (furnace and air-cooling) is shown to result in a uniform distribution of nanometer sized γ’ (L12)-nanoprecipitates, unlike Ni-based superalloys in which the γ’ (L12)-nanoprecipitates form in a non-uniform or multimodal distribution.Graphical abstractImage 1
       
  • On the globularization of the shapes associated with alpha-precipitate of
           two phase titanium alloys: Insights from phase-field simulations
    • Abstract: Publication date: Available online 10 August 2018Source: Acta MaterialiaAuthor(s): P.G. Kubendran Amos, Ephraim Schoof, Daniel Schneider, Britta Nestler In two-phase titanium alloys, α-precipitate exhibits plate morphology which spheroidizes during static globularization. The stability of the 3-dimensional plates to withstand perturbation is well-established. However, based on Rayleigh instabilities, the current understanding rendered by ‘the modified perturbation theory’ holds that the spheroidization of the plates is governed by the stable growth of the ridges. To address this counterintuitive view, a multiphase-field model, which recovers the sharp interface solutions, is employed to investigate the morphological transformation accompanying globularization of shapes that resemble α-precipitate. The present investigation unravels an alternate mechanism, wherein the plates globularize by the successive formation and decay, by lateral expansion, of multiple terminal perturbations. This unique mode of spheroidization, distinctly, characterizes the temporal evolution of the driving force which contradicts the hitherto assumed conventional monotonic-decrease in the curvature-difference. Particularly, it is identified that the ellipsoidal plates exhibit a step-wise change in the curvature-difference while non-monotonic series of peaks are observed during the evolution of the pancake structures. To aid the optimization of the thermal cycle involved in the globularization treatment, a simplified relation between the aspect ratio and spheroidization time, which encompasses the influence of the transformation mechanism, is derived.Graphical abstractImage 1
       
  • Anomalous effects of strain rate on the room-temperature ductility of a
           cast Mg-Gd-Y-Zr alloy
    • Abstract: Publication date: Available online 10 August 2018Source: Acta MaterialiaAuthor(s): J.L. Li, D. Wu, R.S. Chen, E.H. Han The loading rate significantly affects the mechanical properties of Mg alloys. However, the effects of strain rate on newly developed Mg–Gd–Y alloys at room temperature (RT) have rarely been investigated. Here, uniaxial tensile tests were conducted on a cast Mg-10Gd-3Y-0.5Zr (wt. %) alloy (GW103) in both as-aged and as-solutionized states with different grains sizes at RT and various strain rates. The results showed an anomalous positive strain-rate dependence of elongation in all the GW103 alloys investigated from 1 × 10−5 to 1 × 10−1 s−1. Careful microstructural characterization by slip trace analysis revealed that the exclusive deformation mechanism of pyramidal slip, non-Schmid behavior of basal slip, and slip transfer at 1 × 10−1 s−1; these induced the high RT ductility. The occurrence of pyramidal slip activity at high strain rates was suggested to be associated with rare earth solutes, fine precipitates, and local high stresses at grain boundaries due to dislocation pile-up. It was proposed that the increasing strain rate was approximately equivalent to decreases in grain size, enabling the generation of local stresses at grain boundaries, thus causing this anomalous phenomenon.Graphical abstractThe investigations of the anomalous strain rate effect on elongation in a cast Mg-Gd-Y-Zr alloy.Image 1
       
  • Compressive Behavior and Failure Mechanisms of Freestanding and Composite
           3D Graphitic Foams
    • Abstract: Publication date: Available online 9 August 2018Source: Acta MaterialiaAuthor(s): Kenichi Nakanishi, Adrianus I. Aria, Matthew Berwind, Robert S. Weatherup, Christoph Eberl, Stephan Hofmann, Norman A. Fleck Open-cell graphitic foams were fabricated by chemical vapor deposition using nickel templates and their compressive responses were measured over a range of relative densities. The mechanical response required an interpretation in terms of a hierarchical micromechanical model, spanning 3 distinct length scales. The power law scaling of elastic modulus and yield strength versus relative density suggests that the cell walls of the graphitic foam deform by bending. The length scale of the unit cell of the foam is set by the length of the struts comprising the cell wall, and is termed level I. The cell walls comprise hollow triangular tubes, and bending of these strut-like tubes involves axial stretching of the tube walls. This length scale is termed level II. In turn, the tube walls form a wavy stack of graphitic layers, and this waviness induces interlayer shear of the graphitic layers when the tube walls are subjected to axial stretch. The thickness of the tube wall defines the third length scale, termed level III. We show that the addition of a thin, flexible ceramic Al2O3 scaffold stiffens and strengthens the foam, yet preserves the power law scaling. The hierarchical model gives fresh insight into the mechanical properties of foams with cell walls made from emergent 2D layered solids.Graphical abstractImage 1
       
  • Application of the thermodynamic extremal principle to
           diffusion-controlled phase transformations in Fe-C-X alloys: Modeling and
           applications
    • Abstract: Publication date: Available online 9 August 2018Source: Acta MaterialiaAuthor(s): Wangwang Kuang, Haifeng Wang, Xin Li, Jianbao Zhang, Qing Zhou, Yuhong Zhao Diffusion-controlled phase transformations are of singular importance in controlling microstructures and mechanical properties but are difficult to model and calculate for Fe-C-X alloys because of the large difference in the diffusivities of the interstitial element C and the substitutional element X. In this work, the thermodynamic extremal principle was applied to propose a modified quasi-sharp-interface model that integrates trans-interface diffusion from the product phase to the interface, trans-interface diffusion from the interface to the parent phase, interface migration and bulk diffusion of C and X. Applications to isothermal and cyclic phase transformations showed that the model allows the arbitrary setting of the initial conditions. For isothermal phase transformations, three different kinds of characteristic phase transformation kinetics (i.e., a gradual transition from non-partition to partition, a sharp transition from non-partition to partition and solely partition) were found, and their dependence on the interface properties, the grain size and the isothermal temperature was discussed. For both isothermal and cyclic phase transformations, the important roles of trans-interface diffusion and interface migration were highlighted.Graphical abstractImage 1
       
  • A Machine Learning Approach for Engineering Bulk Metallic Glass Alloys
    • Abstract: Publication date: Available online 9 August 2018Source: Acta MaterialiaAuthor(s): Logan Ward, Stephanie C. O'Keeffe, Joseph Stevick, Glenton R. Jelbert, Muratahan Aykol, Chris Wolverton Bulk metallic glasses (BMGs) are a unique class of materials that are gaining traction in a wide variety of applications due to their attractive physical properties. One limitation to the wide-scale use of these materials is the lack of predictable tools for understanding the relationships between alloy composition and ideal properties. To address this issue, we developed a framework for designing metallic glasses using machine learning (ML) models that predict three Dmaxkey properties of candidate BMG compositions: ability to exist in an amorphous state, critical casting diameter (), and supercooled liquid range (ΔTx). Our models take only the composition of the alloy as input, and were created from a database of more than 8000 metallic glass experiments assembled from several dozen papers and handbooks. We employed these ML models to optimize the properties of existing commercial alloys and found, experimentally, several of our ML-predicted compositions can form glasses and exceed existing alloys in one of our two design variables, ΔTx.Graphical abstractImage 1
       
  • New Insights into Martensite Strength and the Damage Behaviour of Dual
           Phase Steels
    • Abstract: Publication date: Available online 9 August 2018Source: Acta MaterialiaAuthor(s): C.P. Scott, B. Shalchi, I. Pushkareva, F. Fazeli, S.Y.P. Allain, H. Azizi A detailed investigation of martensite islands in ultra-high strength dual phase (DP) steels using TEM EELS carbon measurements, nano-indentation studies and micro-mechanical modelling has been carried out. EELS analysis showed that the dispersion in the martensite island-to-island carbon content increases at lower intercritical annealing temperatures due to the influence of undissolved cementite. In a coarse-grained DP alloy, the median martensite island nano-hardness values and those calculated from EELS carbon data were in excellent agreement. However, in a fine-grained (microalloyed) DP alloy significant and unexplained softening occurred that is not consistent with the measured martensite carbon content. In both steels, the dispersion in martensite nano-hardness was greater than that expected from the measured carbon variations. Micro-mechanical modelling using the continuous composite approach (CCA) method was employed to calculate the martensite flow stress distribution required to fit the bulk tensile response of the two materials. The median martensite nano-hardness values derived from the fitted CCA stress spectra were in good agreement with those measured by nano-indentation, corroborating the observed martensite softening. These results provide experimental support for the CCA approach and suggest that the physical origins of the martensite stress spectrum can be strongly influenced by mechanisms other than carbon segregation. Finally, these data explain why the beneficial effect of reducing the α'/α phase strength ratio (PSR) on DP damage properties is highly asymmetrical, depending on whether the ferrite is strengthened or the martensite is softened (by tempering).Graphical abstractImage 1
       
  • Vivid structural colors produced on stainless steel
    • Abstract: Publication date: Available online 9 August 2018Source: Acta MaterialiaAuthor(s): Minseok Seo, Myeongkyu Lee As the esthetic functions of metals have attracted increasing attention, their coloration is a significant issue in scientific and technological aspects. We here demonstrate that vivid structural colors can be produced on stainless steel. The structure consists of a SU-8 layer coated on the surface of bulk stainless steel that has a one-dimensional texture of 500 nm period. Polarization-dependent, diverse colors are produced simply by changing the thickness of the dielectric overlayer. The colors result from the surface plasmon resonance and guided mode resonance of incident light, which occur on the metal surface and inside the dielectric layer, respectively. Simulation based on the finite-difference time-domain method supports the experimental results, showing that the layer thickness influences the characteristic wavelengths of both resonances and the resulting colors. Color image patterns are also printable on the surface of stainless steel by irradiating a solution-coated SU-8 layer with a pulsed ultraviolet laser beam. The final thickness of the photopolymeric SU-8 layer is locally controlled by adjusting laser fluence. The current study provides a simple route to produce diverse structural colors on metals and may find many applications including surface decoration, product identification, and anti-counterfeiting.Graphical abstractImage 1
       
  • Corrigendum to “Topological changes in coarsening networks” [Acta
           Mater. 130 (2017) 147–154]
    • Abstract: Publication date: Available online 8 August 2018Source: Acta MaterialiaAuthor(s): D. Zöllner, P.R. Rios
       
  • Deconvolved intrinsic and extrinsic contributions to electrostrain in high
           performance, Nb-doped Pb(Zr x Ti1-x )O3 piezoceramics (0.50 ≤ x ≤
           0.56)
    • Abstract: Publication date: Available online 7 August 2018Source: Acta MaterialiaAuthor(s): Changhao Zhao, Dong Hou, Ching-Chang Chung, Hanhan Zhou, Antje Kynast, Eberhard Hennig, Wenfeng Liu, Shengtao Li, Jacob L. Jones Lead zirconate titanate (PZT) is the base compound for the highest performing piezoelectric compositions. When doped with Nb, PZT has superior electrostrain and piezoelectric properties. However, the origin of that electrostrain involves both intrinsic and extrinsic contributions which have been challenging to deconvolute. In the present work, we utilize high-energy, synchrotron X-ray diffraction (XRD) in combination with an area detector to measure the response of 1% Nb-doped PbZrxTi1-xO3 (PZT, 0.50 ≤ x ≤ 0.56) piezoceramics to electric fields. Using analysis involving micromechanics-based calculations and pair distribution functions (PDFs), it is found that both the intrinsic and extrinsic contributions are important for realization of high electrostrain. In the compositions nearest the morphotropic phase boundary (MPB), the relative contributions of the intrinsic response increase. The interdependence of crystal symmetry (tetragonal and rhombohedral), spontaneous strain, and the extent of non-180° domain switching are also elucidated. An orientation dependence in the field-induced lattice strain is observed and attributed to extrinsic effects, i.e., the intergranular interaction between domain switching and lattice strain. Finally, the PDFs suggest that a continuous rotation of the polarization vector occurs in the tetragonal phase samples due to piezoelectric distortion, being most obvious in the compositions near the MPB, but is not observed in the rhombohedral phase samples.Graphical abstractImage 1
       
  • High Ionic Conductivity of Mg2+-doped Non-stoichiometric Sodium
           Bismuth Titanate
    • Abstract: Publication date: Available online 7 August 2018Source: Acta MaterialiaAuthor(s): Rahul Bhattacharyya, Soumitra Das, Shobit Omar The influence of Mg-doping on the phase formation and conductivity of Na-excess Na0.5Bi0.5TiO3 (NBT) has been studied for its possible electrolyte applications in SOFCs. Dense ceramic bulk specimens are fabricated using the conventional solid oxide reaction route with the sintering temperature of 1000ºC. Phase analysis reveals the existence of a minor secondary phase along with the dominant perovskite phase. The secondary phases are identified to be based on sodium titanate in Na0.54Bi0.46TiO3-δ, and Na0.9Ti1.55Mg0.45O4 in Mg-doped compositions. A substantial enhancement in the grain conductivity is seen on substituting 1 mol.% Mg2+ for Ti present at the B-site of NBT. At 600ºC, the grain conductivity is observed to be 14.3 mS cm-1 which is ∼68% higher than the best reported NBT based composition. However, adding more than 1 mol.% Mg2+ leads to a decrease in the grain conductivity. A correlation between the conductivity and phase formation has been proposed which can explain the conductivity behaviour on Mg addition in Na-excess NBT.Graphical abstractArrhenius plots for the grain conductivity of several NBT based compositions indicating significantly higher oxygen-ion conductivity in Na0.54Bi0.46Ti0.98Mg0.02O3-δ.Image 1
       
  • Formation of helium-bubble networks in tungsten
    • Abstract: Publication date: Available online 6 August 2018Source: Acta MaterialiaAuthor(s): Luis Sandoval, Danny Perez, Blas P. Uberuaga, Arthur F. Voter The nucleation and subsequent growth of helium bubbles in bulk tungsten is investigated using molecular dynamics simulations. By considering a setting that includes the diffusion process of helium clusters, we study their attachment to existing bubbles and their interaction with tungsten crowdion structures generated in the bubble growth process. We find that incoming helium atoms, and especially small helium clusters, can become trapped in the crowdion structures, providing nucleation sites for new helium bubbles, and leading to a distributed network of bubbles rather than a single, growing bubble. The nature of this network depends on both the temperature and the implantation flux of helium. Our results indicate that the kinetic interaction of He with generated dislocations is a key factor dictating the evolution of bubble distributions in plasma-exposed tungsten.Graphical abstractImage 1
       
  • { 1 ¯ 012 } +Non-cozone+Twin-Twin+Interactions+in+Magnesium&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Structural Characteristics of { 1 ¯ 012 } Non-cozone Twin-Twin
           Interactions in Magnesium
    • Abstract: Publication date: Available online 6 August 2018Source: Acta MaterialiaAuthor(s): Mingyu Gong, Shun Xu, Yanyao Jiang, Yue Liu, Jian Wang Twin-twin interactions form twin-twin boundaries (TTBs) which can prevent twin propagation, inhibit direct twin transmission, retard detwinning, and facilitate secondary twins. The current work studies the microstructure and interaction mechanisms of non-cozone twin-twin junctions by combining electron back-scatter diffraction observations and atomistic simulations. Non-cozone twin-twin interactions are defined as the intersecting line of the two twins isn't parallel to one zone axis (a-axis) and include two types, Type II(a) (T2→T1) and Type II(b) (T3→T1), according to the crystallography of two interacting twins. For Type II(a) interaction, both statistical results of experimental observations and interfacial energy calculation confirm TTB formation on the obtuse side of the incoming twin instead of the acute side. However, for Type II(b) interaction, the growth of twins on both sides is impeded, although the TTB on the acute side possesses the lowest interfacial energy. Atomistic simulation demonstrates that, for Type II(a) twin-twin interactions, positive resolved shear stresses on the obtuse side favor T1 and T2 twinning, while negative resolved shear stresses on the acute side impede T1 and T2 twinning. For Type II(b), negative resolved shear stresses on both the acute and obtuse sides result in impediment of twinning on both sides. These results can be used in developing micro/macro-scale predictive models that deal with the role of multiple twins and twin variants during mechanical processing. The analytical and simulation methods can be generalized and applied to atomistic analysis in different material systems to further explain the hardening mechanisms associated with twin-twin interactions.Graphical abstractImage 1
       
  • Ultrahigh-strength titanium gyroid scaffolds manufactured by selective
           laser melting (SLM) for bone implant applications
    • Abstract: Publication date: Available online 4 August 2018Source: Acta MaterialiaAuthor(s): Arash Ataee, Yuncang Li, Milan Brandt, Cuie Wen Commercially pure titanium (CP-Ti) gyroid scaffolds with interconnected pores and high porosities in the range of 68–73% and three different unit cell sizes of 2, 2.5, and 3 mm were manufactured by selective laser melting (SLM) for bone implant applications. The microstructure and mechanical properties of the scaffolds with different unit cell sizes and sample orientations were evaluated. The microstructure of as-built struts was dominated by massive martensite and the average microhardness of the struts was 2.27 GPa, which is ∼50% higher than that of dense cast CP-Ti. The elastic modulus and yield strength of the as-built scaffolds ranged from 1465 to 2676 MPa and from 44.7 to 56.5 MPa, respectively, values which are close to the elastic modulus of trabecular bone and presumably strong enough to bear the physiological loading of implants. The as-built scaffolds exhibited excellent ductility up to 50% strain and no sign of fracture up to 20–30% strain under compression. The dominant compressive response of the scaffolds was observed by formation of a plastic hinge which led to rotation of the struts about the plastic hinges followed by development of local shear bands in struts in the long plateau region. These SLM-manufactured gyroid CP-Ti scaffolds with significantly enhanced hardness and compressive strength exhibited an elastic modulus close to that of trabecular bone and offer a promising improvement on CP-Ti scaffolds for bone implant applications.Graphical abstractImage 1
       
  • Stable and large superelasticity and elastocaloric effect in
           nanocrystalline Ti-44Ni-5Cu-1Al (at%) alloy
    • Abstract: Publication date: Available online 4 August 2018Source: Acta MaterialiaAuthor(s): Hong Chen, Fei Xiao, Xiao Liang, Zhenxing Li, Xuejun Jin, Takashi Fukuda Superelastic behavior and elastocaloric effect were investigated in a Ti-44Ni-5Cu-1Al (at%) alloy subjected to various thermomechanical treatments. The specimen heat-treated at 673 K for 5 min after hot rolling and subsequent cold rolling exhibited excellent superelastic strain of 4.9% with a small stress hysteresis of 90 MPa when the maximum tensile stress was 500 MPa. This specimen also exhibited a large elastocaloric effect with a temperature decrease of 17 K when the stress of 500 MPa was removed adiabatically. No remarkable deterioration was observed for the superelastic strain and elastocaloric effect up to 5000 mechanical cycles. The maximum superelastic strain obtained was 6.8% under a tensile stress of 750 MPa. Transmission electron microscope observation and in-situ X-ray diffraction analysis under tensile stress revealed that the average grain size of the specimen is about 40 nm, and the specimen exhibits a successive B2-B19-B19’ transformation.Graphical abstractImage 1
       
  • Ferrite, martensite and supercritical iron: A coherent elastochemical
           theory of stress-induced carbon ordering in steel
    • Abstract: Publication date: Available online 4 August 2018Source: Acta MaterialiaAuthor(s): P. Maugis A mean-field model based on the elasticity theory of point defects has been developed to investigate the role of uniform stress fields on the long-range ordering of carbon atoms in bcc-iron. From an analysis of the thermodynamic equilibria, composition – temperature – stress state diagrams are derived. We demonstrate that ferrite, martensite and supercritical iron are various instances of the same bct-iron phase region. A coherent mapping of the phase transitions is drawn, identifying (i) continuous transitions such as ferrite ordering, martensite enhanced ordering and ferrite – martensite transformation, and (ii) discontinuous transitions such as temperature-induced martensite and stress-induced martensite. Our analysis is supported by rigid-lattice Monte Carlo simulations. Recently published experimental results on highly-drawn perlitic wires are re-interpreted in terms of supercritical iron, rather than strain-induced martensite. Novel low-temperature thermomechanical treatments of supersaturated ferrite are suggested, for improved nanostructure design of martensitic steels.Graphical abstractImage 1
       
  • Orientation dependent spall strength of tantalum single crystals
    • Abstract: Publication date: Available online 3 August 2018Source: Acta MaterialiaAuthor(s): Eric N. Hahn, Saryu J. Fensin, Timothy C. Germann, George T. Gray It is generally recognized that single crystals exhibit orientation-dependent elastic and plastic responses. However, it is less known how the dynamic tensile, or spall, strength depends on crystalline orientation, especially for BCC materials. It has been previously shown that the dynamic tensile strength of FCC materials is highly dependent on their plastic response under compression, with a direct correlation between plastic deformation and spall strength. In BCC materials like Ta, where the primary deformation mechanism at high strain rates is a combination of slip and twinning, the quantitative dependence of spall strength on these deformation mechanisms is less understood. To fill this gap in our knowledge, a series of non-equilibrium molecular dynamics simulations are completed for six tantalum single crystal orientations: , , , , , , to explore the role of directional anisotropy on ductile spallation. Our results show that due to the role of non-Schmid and release effects in BCC Ta, the evolution of plasticity follows a complex trajectory through shock compression, release, and tension. Orientations that contain residual twinning deformation have a reduced spall strength proportional to the amount of twinning present. Twins and their intersections function as regions of increased stress localization within the system due to compatibility requirements and their interactions with dislocations.Graphical abstractImage 1
       
  • Energetics of point defects in rocksalt structure transition metal
           nitrides: thermodynamic reasons for deviations from stoichiometry
    • Abstract: Publication date: Available online 3 August 2018Source: Acta MaterialiaAuthor(s): Karthik Balasubramanian, Sanjay V. Khare, Daniel Gall First principle calculations of point defect formation energies in group 3 - 6 transition metal (Me) nitrides MeNx are employed to explain the thermodynamic reasons for the large reported compositional range (typically x = 0.7-1.3) in the rocksalt structure. Both under-stoichiometric (x < 1) and over-stoichiometric (x> 1) compositions are due to relatively low vacancy formation energies that decrease from an average of 2.7 and 4.5 eV for nitrogen and cation vacancies in group 3 nitrides (ScN, YN, LaN) to -1.8 and -0.8 eV in group 6 nitrides (CrN, MoN, WN), indicating that they become thermodynamically stable at zero temperature for group 6 and for group 4 – 6 nitrides, respectively. Similarly, nitrogen and cation interstitials in tetragonal and 111- or 110-split configurations are unstable for groups 3-5 but become thermodynamically stable for group 6 nitrides, consistent with the mechanical instability of the latter compounds. All antisite defects possess high formation energies and are unlikely to form. The nitrogen chemical potential at finite temperatures and in equilibrium with a N2 gas is strongly affected by the vapor phase entropy, leading to shifts in the defect free energy of, for example, 1.2 eV at 1 Pa N2 at 800 K, causing an increasing likelihood for nitrogen vacancies and cation interstitials at elevated temperatures. In addition, the configurational entropy of point defects causes a correction of e.g. 0.4 eV for a 1% vacancy defect concentration at 800 K. Considering these entropy contributions leads to predicted temperature windows for stoichiometry of e.g. 200-1100 K for TiN, 500-1400 K for ZrN, and 1200-1400 K for HfN, while considerable cation and nitrogen vacancy concentrations are expected for temperatures below and above these ranges, respectively. Schottky pair defects are predicted in VN for T> 200 K and in NbN, TaN, and group 6 nitrides at all temperatures, independent of the N2 partial pressure. The overall results show that thermodynamic arguments (even in the absence of kinetic barriers) can explain many of the reported composition vs temperature and pressure relationships in rocksalt structure nitrides.Graphical abstractImage 1
       
  • Thermal Depolarization Regulation by Oxides Selection in Lead-Free
           BNT/oxides Piezoelectric Composites
    • Abstract: Publication date: Available online 1 August 2018Source: Acta MaterialiaAuthor(s): Jie Yin, Yangming Wang, Yuxing Zhang, Bo Wu, Jiagang Wu For the bismuth sodium titanate (BNT)-based materials, the thermal depolarization temperature (Td) is always an obstacle for practical applications. Recently, BNT/ZnO composite has provided one method to resist the thermal depolarization, and however, Td values just increase to a small extent and the conclusions derive from ZnO merely. A universal selection principle for the oxides will be helpful for us to choose the suitable oxides to effectively resist the thermal depolarization, which is desperately demanded but still lacks in BNT/oxide composites. Here, we report that the deferred thermal depolarization can be also obtained in piezoelectric Bi0.5(Na0.8K0.2)0.5TiO3: Al2O3(BNKT:Al2O3) composites. Td is deferred to the higher temperatures (from 116 °C to 227 °C) with increasing Al2O3 contents, as evidenced by the temperature dependence of dielectric, ferroelectric and piezoelectric properties. In addition, the piezoelectricity of BNKT:0.15Al2O3 remains stable at a high temperature (∼210 °C). And, the thermal deviatoric stress from the coefficients of thermal expansion (CTE) discrepancies between Al2O3 and BNKT matrix provides a stronger stabilization force than the ions diffusion-induced destabilization force, resulting in the ultimate deferred thermal depolarization and the significantly increased Td values. In particular, according to the results from the representative BNT/oxides (i.e., ZnO, Al2O3, ZrO2, HfO2) composite, the oxide selection principle (regulating several competing factors) is given to form the appropriate thermal resistant BNT/oxide composite, which may further open the door for piezoelectric BNT-based materials from the research and application scope.Graphical abstractImage 1
       
  • Revision of Ms. No. A-18-1156R1 How Evolving Multiaxial Stress States
           Affect the Kinetics of Rafting during Creep of Single Crystal Ni-base
           Superalloys
    • Abstract: Publication date: Available online 1 August 2018Source: Acta MaterialiaAuthor(s): L. Cao, P. Wollgramm, D. Bürger, A. Kostka, G. Cailletaud, G. Eggeler Miniature tensile creep specimens are used to investigate the effect of mild circular notches on microstructural evolution during [001] tensile creep of a Ni-base single crystal superalloy. Creep deformed material states from a uniaxial (950°C, uniaxial stress: 300 MPa) and a circular notched creep specimen (950°C, net section stress in notch root: 300 MPa) are compared. For both types of tests, creep experiments were interrupted after 81, 169 and 306 h. Quantitative scanning electron microscopy (SEM) is used to assess the evolution of the γ/γ’-microstructure from rafting to topological inversion. Scanning transmission electron microscopy (STEM) was applied to study the evolution of dislocation densities during creep. As a striking new result it is shown that in circular notched specimen, the microstructural evolution is well coupled to the kinetics of the stress redistribution during creep. Rafting, the directional coarsening of the γ’-phase, and the increase of γ-channel dislocation density, start in the notch root before the center of the specimen is affected. When stresses in the circular notched specimens are fully redistributed, the microstructural differences between the notch root and the center of the circular notched specimen disappear. The comparison of the mechanical data and the microstructural findings in uniaxial and circular notched specimens contribute to a better understanding of the role of mild notches, of stress multiaxiality and of strain accumulation in the microstructure evolution of single crystal Ni-base superalloys during creep. The results obtained in the present work are discussed in the light of previous work published in the literature.Graphical abstractImage 1
       
  • Junction growth in ultrasonic spot welding and ultrasonic additive
           manufacturing
    • Abstract: Publication date: Available online 31 July 2018Source: Acta MaterialiaAuthor(s): Austin A. Ward, Yibing Zhang, Zachary C. Cordero The processes of ultrasonic spot welding and ultrasonic additive manufacturing are modelled by approximating the weld interface as rough metallic surfaces in sliding contact. It is assumed that bonding is due to athermal plastic deformation of surface asperities and the associated growth of metallic junctions along the weld interface. To link the process variables and the extent of junction growth, an expression for the real contact area at the weld interface is combined with process-specific frictional heating models developed here. The resulting framework is validated by comparing its predictions of the weld strength with data from the ultrasonic welding literature. The close agreement between the framework's predictions and the experimental data demonstrates that thermal softening due to frictional heating is the dominant softening mechanism in ultrasonic welding, while acoustic softening is insignificant. The junction growth model is used to identify parameter sets for ultrasonic spot welding and ultrasonic additive manufacturing that maximize the weld strength while simultaneously minimizing the thermal excursion at the weld interface. It is found that in ultrasonic spot welding, certain processing conditions can cause interfacial melting although melting is not required to form strong bonds. It is also shown that in ultrasonic additive manufacturing, the deposition rate is highest when the positions of the peak temperature and complete interfacial bonding coincide underneath the sonotrode. If the position of complete interfacial bonding leads the position of the peak temperature, there is excessive heating of the build, and the sonotrode velocity can be increased without degrading bond quality.Graphical abstractImage 1
       
  • General trends between solute segregation tendency and grain boundary
           character in aluminum - an ab inito study
    • Abstract: Publication date: Available online 31 July 2018Source: Acta MaterialiaAuthor(s): Reza Mahjoub, Kevin J. Laws, Nikki Stanford, Michael Ferry Quantum mechanical calculations have been performed to establish general trends in propensity for the segregation of solutes across the periodic table at or in the neighborhood of an extended set of commonly observed special grain boundaries in face centered cubic aluminium. To this end, Al has been considered as the matrix and elements from 3d and 4d transition metals as well as those from group II, III and IV have been selected as solute atoms. For transition metal solutes, we find a concave-up parabolic-like dependency of segregation energy as a function of atomic number that is argued to be caused by the competition between chemical bonding and atomic size effects. The analysis is corroborated quantitatively by the computation of crystal orbital Hamiltonian population for solute-Al and Al-Al pairs as well as the Voronoi polyhedral surrounding solutes at a sample GB. The parabolic-like (concave-down trend) dependency of the cohesiveness of grain boundaries is explained by a similar trend in the bonding strength of Al-Al pairs at the segregated GBs. We extend this investigation to examine the stability of the solid solution polycrystalline state by comparing the calculated segregation energy against the combined energetic cost of grain boundary and intermetallic precipitate formation. The results may serve as a design tool for tailoring polycrystalline alloys with desired properties.Graphical abstractImage 1
       
  • Formation of eta carbide in ferrous martensite by room temperature aging
    • Abstract: Publication date: Available online 31 July 2018Source: Acta MaterialiaAuthor(s): W. Lu, M. Herbig, C.H. Liebscher, L. Morsdorf, R.K.W. Marceau, G. Dehm, D. Raabe For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of -25°C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α’) follows [001]α’//[001]η, (1¯10)α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100°C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years.Graphical abstractThe direct correlation of TEM (a) and APT (b) enables to identify the nm-scale, carbon-rich features forming after two years of room temperature aging in martensitic Fe-15Ni-1C wt% as eta carbides.Image 1
       
  • Uniaxial compression of silicon nanoparticles: An atomistic study on the
           shape and size effects
    • Abstract: Publication date: Available online 31 July 2018Source: Acta MaterialiaAuthor(s): D. Kilymis, C. Gérard, J. Amodeo, U.V. Waghmare, L. Pizzagalli Molecular dynamics simulations were carried out to investigate the mechanical properties of silicon nanoparticles during uniaxial compression by a flat-punch indenter. We considered a large set of systems, with dimensions in the range 10 nm–50 nm, and various shapes like cubic (perfect and blunt), spherical, truncated spherical, and Wulff-shaped, as well as two compression orientations and two temperatures. Thorough analyses of the simulations first revealed that the relation between nanoparticle size and strength, usually termed as ’smaller is stronger’, is critically dependent on the nanoparticle shape, at least for the investigated size range. For instance, a significant and size-dependent strength decrease is determined for facetted Wulff-like nanoparticles, but not for cubic or spherical systems for compression along . We also found that the nanoparticle shape greatly influences plasticity. Several original plasticity mechanisms are obtained, among which the nucleation of half-loop V-shaped dislocation contained in two different {111} planes, dislocations gliding in unusual {110} planes, or the nucleation of partial dislocations in shuffle {111} planes. Our investigations suggest that plasticity properties are mainly governed by the localization of shear stress build up during elastic loading, and the geometry of surfaces in contact with indenters, these two characteristics being intimately related to the nanoparticle shape.Graphical abstractImage 1
       
  • Grain size stabilization of mechanically alloyed nanocrystalline Fe-Zr
           alloys by forming highly dispersed coherent Fe-Zr-O nanoclusters
    • Abstract: Publication date: Available online 31 July 2018Source: Acta MaterialiaAuthor(s): Y.Z. Chen, K. Wang, G.B. Shan, A.V. Ceguerra, L.K. Huang, H. Dong, L.F. Cao, S.P. Ringer, F. Liu Grain size stabilization is crucial for the production and application of nanocrystalline (NC) materials. The mechanically alloyed (MA) NC Fe-Zr system is known as a very successful NC system as it exhibits excellent thermal stability at elevated temperatures. The grain size stabilization of this system has been previously ascribed to its reduced grain boundary (GB) energy by Zr segregation and Zener pinning of Zr-rich intermetallic precipitates. In this work, we report a different mechanism that significantly contributes to grain size stabilization of this NC alloy system using two MA-produced NC Fe-Zr alloys (Fe-1 at.% Zr and Fe-5 at.% Zr) as examples. We show by using atom probe tomography and Cs-corrected transmission electron microscopy that highly dispersed coherent Fe-Zr-O nanoclusters, with a number density up to 1024 m−3, form in ferrite matrix after annealing at certain temperatures. Our first-principles calculations indicate that the formation of these nanoclusters is caused by the ordering of Zr and O-impurity in ferrite matrix. We analyzed the underlying mechanism of grain size stabilization in terms of the experimental results and the Zener pinning theory, and suggest that the pinning effect exerted by these nanoclusters significantly contributes to grain size stabilization of the NC Fe-Zr alloys.Graphical abstractImage 1
       
  • Controlling the grain orientation during laser powder bed fusion to tailor
           the magnetic characteristics in a Ni-Fe based soft magnet
    • Abstract: Publication date: Available online 30 July 2018Source: Acta MaterialiaAuthor(s): Ji Zou, Y. Gaber, G. Voulazeris, S. Li, L. Vazquez, Lei-Feng Liu, M.-Y. Yao, Y.-J. Wang, M. Holynski, K. Bongs, M.M. Attallah is the favoured crystal growth direction during solidification of cubic metals. However, it is the hard magnetisation axis for Ni-rich soft magnetic materials. In this work, a strategy to enhance the magnetic shielding characteristics of laser powder bed processed soft magnetic alloy (permalloy-80) through the control of the crystallographic texture was developed. The strategy involves initially assessing the influence of the process parameters on the development of the (001) orientation within dense builds, then tilting the build orientation to achieve a crystallographic orientation along the magnetisation soft axis. Using this approach, dense cubic samples with the expected (001) texture were produced, with minimised cracking density. Thereafter, cubic samples tilted to achieve (111) and (110) textures were fabricated, as confirmed by X-ray diffraction and electron backscattered diffraction. Over 200 folds improvement in the magnetic susceptibility was found in the (111) textured build, compared with the (100) oriented build. The paper highlights the possibility to control the grain orientations to achieve improved magnetic properties in the build during laser powder bed processing.Graphical abstractImage 1
       
  • Reversion of natural ageing in Al-Mg-Si alloys
    • Abstract: Publication date: Available online 30 July 2018Source: Acta MaterialiaAuthor(s): Mazen Madanat, Meng Liu, John Banhart Al-0.59Mg-0.79Si (wt.%) alloy was naturally aged for 2 weeks after solutionising and quenching to produce a high number density of atomic clusters. This caused an increase of both hardness and electrical resistivity. Samples were then subjected to reversion ageing (RA) by applying temperature treatments at 250 °C of 1 s–5 min duration. The associated changes of hardness, resistivity, positron lifetime and the DSC dissolution signal were measured. Subsequent secondary ageing at 20 °C was studied in an analogous way. Ideas are developed how solute supersaturation and vacancy fraction vary during RA. It is found that the most reverted state still contains enough clusters to trap positrons efficiently and to contribute to hardness. Longer annealing at 250 °C leads to the formation of coherent and later incoherent precipitates. The differences between the evolution of resistivity and hardness indicate that beside the level of solute supersaturation the Mg:Si ratio in the matrix changes during reversion.Graphical abstractImage 1
       
  • Adiabatic shear instability is not necessary for adhesion in cold spray
    • Abstract: Publication date: Available online 30 July 2018Source: Acta MaterialiaAuthor(s): Mostafa Hassani-Gangaraj, David Veysset, Victor K. Champagne, Keith A. Nelson, Christopher A. Schuh When metallic microparticles impact substrates at high enough velocity, they bond cohesively. It has been widely argued that this critical adhesion velocity is associated with the impact velocity required to induce adiabatic shear instability. Here, we argue that the large interfacial strain needed to achieve bonding does not necessarily require adiabatic shear instability to trigger. Instead, we suggest that the interaction of strong pressure waves with the free surface at the particle edges—a natural dynamic effect of a sufficiently rapid impact—can cause hydrodynamic plasticity that effects bonding, without requiring shear instability. We proceed on this basis to postulate and confirm a proportionality between critical velocity and the bulk speed of sound, which supports the viewpoint that shear instability is not the mechanism of adhesion in cold spray.Graphical abstractImage 1
       
  • High thermoelectric performance of melt-spun CuxBi0.5Sb1.5Te3 by
           synergetic effect of carrier tuning and phonon engineering
    • Abstract: Publication date: Available online 30 July 2018Source: Acta MaterialiaAuthor(s): Jeong Seop Yoon, Jae Min Song, Jamil Ur Rahman, Soonil Lee, Won Seon Seo, Kyu Hyoung Lee, Seyun Kim, Hyun-Sik Kim, Sang-il Kim, Weon Ho Shin Bi-Te based materials have been used for near-room-temperature thermoelectric applications. However, their properties dramatically decrease at high temperatures (over 100 °C), limiting their use in power generation. In this study, we investigated the enhanced thermoelectric properties of Bi-Te based materials by Cu doping and employing the melt-spinning (MS) process that can be utilized especially at elevated temperatures. By changing the doping amount, we could modulate the temperature dependence of thermoelectric properties, where the maximum ZT temperature could be shifted from room temperature to 450 K. The highest ZT value, 1.34, was achieved at 400 K for 2% Cu-doped Bi0.5Sb1.5Te3, which is due to the enhancement in power factor and reduction in lattice thermal conductivity. The average ZT value between room temperature and 530 K was 1.17 for 2% Cu-doped Bi0.5Sb1.5Te3, which is 46% higher than that of pristine Bi0.5Sb1.5Te3. Consequently, the synergetic effect of MS process and Cu incorporation can be a promising method to widen the application of Bi-Te based thermoelectric materials for mid-temperature power generation.Graphical abstractThe combined effect of melt-spinning process and Cu incorporation gives to widen Bi-Te-based thermoelectric materials for mid-temperature power generation.Image 1
       
  • Lattice and phase strain evolution during tensile loading of an
           intermetallic, multi-phase γ-TiAl based alloy
    • Abstract: Publication date: Available online 27 July 2018Source: Acta MaterialiaAuthor(s): Petra Erdely, Peter Staron, Emad Maawad, Norbert Schell, Helmut Clemens, Svea Mayer Intermetallic γ-TiAl based alloys are promising materials for lightweight high-temperature applications, but their limited room temperature ductility poses an obstacle to the exploitation of their full potential. Especially in the case of multi-phase TiAl alloys, such as the β-stabilised TNM alloy of a nominal chemical composition of Ti-43.5 A l-4Nb-1Mo-0.1 B (in at.%), an understanding of deformation and load partitioning mechanisms is required that works at all scales and encompasses all phases, including e.g. βo. In the present work, in situ high-energy X-ray diffraction measurements were conducted on a recent TNM sheet to study the load-bearing mechanisms and their sequential order upon tensile loading for the first time on the level of individual lattice planes and phases. Four specific stages of deformation were revealed. The direction-dependent analysis of the diffraction elastic moduli offered insights into the anisotropy of the individual phases and the initiation of intergranular and interphase stresses in the elastic regime. Plastic deformation was found to commence in the γ phase at applied stress levels of roughly 670-690 MPa. Load partitioning between differently oriented grains of the γ phase was observed, followed by a load transfer onto the α2 and βo phase. Further tensile loading entailed the onset of plasticity within favourably oriented α2 grains. The globular βo phase was found to deform elastically until failure. Differently oriented specimens of the weakly textured TNM sheet showed that the macroscopic mechanical properties can be assumed nearly isotropic.Graphical abstractImage 1
       
  • A low-alloy high-carbon martensite steel with 2.6 GPa tensile
           strength and good ductility
    • Abstract: Publication date: Available online 27 July 2018Source: Acta MaterialiaAuthor(s): Yingjun Wang, Junjie Sun, Tao Jiang, Yu Sun, Shengwu Guo, Yongning Liu A low-alloy and high-carbon martensite steel (0.66% C) with ultrafine grains is produced by combination of Tempforming (tempering and deforming of a quenched steel) and reheating followed by water quenching and low temperature tempering. The size of prior-austenite grains of the steel is reduced to 2.4 μm through this new technique, and its ultimate tensile strength of 2.6 GPa and elongation of 7% are obtained, which is the highest strength in low alloy high strength steels. The microstructure of the high carbon martensite consists of high density dislocations, undissolved spherical carbides, and dispersed nano-scale Fe3C and Fe5C2 phases precipitated at interior of martensitic after tempering. The strengthening mechanisms of the ultrafine grain martensitic steel are mainly dislocation strengthening and precipitation strengthening and also fine grain strengthening. The tensile strength and ductility of the steel are superior to that of existing maraging steels, such as C350, the highest strength in commercialized level, in which more than 20% precious alloy elements such as Co, Mo, Ni, and Ti are contained, and the cost of our materials is only about 1/50 of the C350. All above advantages are desirable for broad industrial applications at an economic cost.Graphical abstractImage 1
       
  • Nanoscale compositional segregation and suppression of polar coupling in a
           relaxor ferroelectric
    • Abstract: Publication date: Available online 27 July 2018Source: Acta MaterialiaAuthor(s): Teresa Roncal-Herrero, John Harrington, Aurang Zeb, Steven J. Milne, Andy P. Brown A number of relaxor ferroelectric ceramics have been demonstrated to possess a near stable value of relative permittivity over very wide temperature ranges. This can-not be explained by conventional theories of relaxors. One such system is based on the perovskite solid solution series: (1-x) (Ba0.8Ca0.2)TiO3-xBi(Mg0.5Ti0.5)O3, giving stable relative permittivity from 150 to 500 °C. We show by scanning transmission electron microscopy and electron energy loss spectroscopic elemental mapping that nanoscale compositional segregation occurs in the temperature stable relaxor composition (x = 0.55), with Ba/Ti clusters some 2–4 nm in extent, separated by Bi-rich regions of comparable size. This nanomosaic structure is consistent with phase separation into a ferroelectrically active BaTiO3 – type phase (Ba/Ti rich) and a weakly polar Bi/(Mg) rich perovskite solid solution. The possibility that nanophase segregation is the cause of weak dipole coupling and suppression of the dielectric relaxation peak is considered.Graphical abstractImage 1
       
  • Crystallographic texture can be rapidly determined by electrochemical
           surface analytics
    • Abstract: Publication date: Available online 26 July 2018Source: Acta MaterialiaAuthor(s): Alistair Speidel, Rong Su, Jonathon Mitchell-Smith, Paul Dryburgh, Ivan Bisterov, Don Pieris, Wenqi Li, Rikesh Patel, Matt Clark, Adam T. Clare Orientation affects application-defining properties of crystalline materials. Hence, information in this regard is highly-prized. We show that electrochemical jet processing (EJP), when coupled with accurate metrological appraisal, can characterise crystallographic texture. Implementation of this technique allows localised dissolution to be anisotropic and dependent on etch-rate selectivity, defined by the crystallography. EJP therefore, generates complex, but characteristic topographies. Through rapid surface processing and analysis, textural information can be elucidated. In this study, samples of polycrystalline Al and Ni have been subjected to EJP, and the resulting surfaces analysed to generate three-color orientation contrast maps. Comparison of raw data acquired through our method with prior electron back-scatter diffraction data shows broad correlation and assignment (68% on a pixel-by-pixel basis), showcasing rapid large-area analysis at high efficiency.Graphical abstractImage 1
       
  • The influence of anisotropic surface stresses and bulk stresses on defect
           thermodynamics in LiCoO2 nanoparticles
    • Abstract: Publication date: Available online 26 July 2018Source: Acta MaterialiaAuthor(s): Peter Stein, Ashkan Moradabadi, Manuel Diehm, Bai-Xiang Xu, Karsten Albe The demand for higher specific capacity and rate capability has led to the adoption of nanostructured electrodes for lithium-ion batteries. At these length scales, surface effects gain an appreciable impact not only on the electrochemical and mechanical behavior of the electrode material, but also on defect thermodynamics. The focus of this study is the distribution of surface-induced bulk stresses in a LiCoO2 nanoparticle and their impact on the migration of Li-vacancies. LiCoO2 is prototypical cathode material, the deintercalation is initially mediated by the vacancy mechanism.For this investigation, elastic parameters and anisotropic surface stress components are computed using Density Functional Theory calculations. They are incorporated into a surface-enhanced continuum model, implemented by means of the Finite Element method. The particle geometry is derived from a Wulff construction, and changes in the formation energy and migration barriers of a Li vacancy are determined using the defect dipole tensor concept.Within the considered nanoparticle, the surface stresses result in a highly heterogeneous bulk stress distribution with a vortex-like transition region between the tensile particle core and its non-uniformly stressed boundaries. Both the center and the exterior of the particle show enhanced formation energy and migration barriers for of a Li vacancy. These experience a reduction in the transition region in the particle, culminating in a peak increase in vacancy diffusivity and ionic conductivity by circa 10% each. For a particle at a length-scale of 10nm, this yields an overall increase in ionic conductivity by a mere 0.8%. This surface stress-enhanced conductivity decays rapidly with increasing particle size.Graphical abstractImage 1
       
  • Bake-hardenable Mg–Al–Zn–Mn–Ca sheet alloy
           processed by twin-roll casting
    • Abstract: Publication date: Available online 26 July 2018Source: Acta MaterialiaAuthor(s): M.Z. Bian, T.T. Sasaki, T. Nakata, Y. Yoshida, N. Kawabe, S. Kamado, K. Hono Mg–1.3Al–0.8Zn–0.7Mn–0.5Ca (wt.%) alloy (AZMX1110) sheet produced by low-cost twin-roll casting process exhibits excellent room temperature formability with a large Index Erichsen (I.E.) value of 7.8 mm at room temperature in a solution treated condition (T4). The large I.E. value is associated with a weak basal texture in the T4 condition. This alloy shows rapid age-hardening response at 170 °C, resulting in a significant increase of the yield strength from 177 MPa to 238 MPa within 20 min; such bake hardenability (BH) has never been explored in magnesium sheet alloys before. The microstructure of the bake-hardened sample, characterized by a correlative transmission electron microscopy and atom probe tomography (TEM-APT), reveals that Al, Zn and Ca atoms are segregated to basal dislocations and contribute to the strengthening by pinning dislocation motions, along with the co-clustering of these atoms. This first BH Mg sheet alloy composed of only ubiquitous elements is highly attractive for application to automotive bodies.Graphical abstractImage 1
       
  • A new framework for rotationally invariant two-point spatial correlations
           in microstructure datasets
    • Abstract: Publication date: Available online 26 July 2018Source: Acta MaterialiaAuthor(s): Ahmet Cecen, Yuksel C. Yabansu, Surya R. Kalidindi Quantification of the material internal structure (i.e., microstructure) is central to establishing the highly sought-after process-structure-property (PSP) relationships central to any materials design effort. In recent years, two-point spatial correlations (a subset of the n-point spatial correlations) have garnered significant attention because of their tremendous potential in arriving at practically useful PSP linkages. The central advantage of the two-point spatial correlations is that they capture an exceedingly large number of directionally resolved microstructure statistics. However, they are sensitive to the selection of the observer reference frame. In a number of practical applications, there is a critical need to establish the directionally resolved microstructure statistics, while attaining invariance to the observer reference frame (i.e., the statistics extracted are independent of the selection of the observer frame). A framework for defining and computing such observer-frame invariant 2-point spatial correlations does not exist at the present time. This paper addresses this gap by introducing a new form of two-point spatial correlations, hereafter called rotationally invariant two-point spatial correlations. The theoretical framework for these new rotationally invariant 2-point spatial correlations is introduced in this paper, and demonstrated through a comprehensive case study.Graphical abstractImage 1
       
  • In situ Synchrotron X-Ray Diffraction study of high-temperature stress
           relaxation in chromia scales containing the reactive element yttrium
    • Abstract: Publication date: Available online 24 July 2018Source: Acta MaterialiaAuthor(s): F. Rakotovao, B. Panicaud, J.L. Grosseau-Poussard, Z. Tao, G. Geandier, P.O. Renault, P. Girault, P. Goudeau, N. Blanc, N. Boudet, G. Bonnet The viscoplastic behaviour of the model yttria-coated Ni28Cr alloy was studied under high temperature oxidation conditions for various small quantities of reactive element (RE). The main purpose was to determine the way the RE acts on the non-destructive stress relaxation mechanisms (creep) occurring in thermally grown chromia scales. After building of an initial chromia microstructure by 3 h oxidizing at 1273 K, temperature jumps were applied to generate thermal stress loading, the chromia scale microstructure being considered as stable. The generated stress was then released along 3 h isothermal periods and X-ray Synchrotron diffraction was used to follow in situ its evolution. The experimental curves were then simulated in order to calculate the creep coefficients, to evaluate the activation energy of the diffusion-creep mechanism and determine the diffusion coefficients and the elementary mechanisms associated to the viscoplastic behaviour of chromia scales in relation with the introduced RE quantity.Graphical abstractImage 1
       
  • Compressive Creep Behavior of Hot-Pressed GeTe based TAGS-85 and Effect of
           Creep on Thermoelectric Properties
    • Abstract: Publication date: Available online 24 July 2018Source: Acta MaterialiaAuthor(s): M.C. Chang, M.T. Agne, R.A. Michi, D.C. Dunand, G.J. Snyder Thermoelectric materials often operate at high homologous temperatures where they can be subjected to sizeable internal and external stresses, making them prone to creep deformation over long periods of use. Hot-pressed (GeTe)85(AgSbTe2)15 (TAGS-85) exhibits a fine grain size averaging ∼8 μm – of concern for creep resistance - with 70 % of the grains below 10 μm and 30 % between 10 and 150 μm; second phase Ag8GeTe6 particles, ∼2 μm in size, are also present at grain boundaries. Strain rates of TAGS-85 under a series of stresses are measured at 375-425 °C. By fitting the creep data to a power law creep equation, the stress exponent n and activation energy Q for creep are determined to be n = 2.6 ± 0.1 (12 MPa applied stress) and Q = 157 ± 2 kJ/mol, respectively. Thermoelectric measurements show 15% lower electrical conductivity and ∼30% higher lattice thermal conductivity after creeping to 10% strain. The evolution of ambient-temperature electrical conductivity and thermal conductivity for a single sample crept at intervals of 2% strain is consistent with the expectation that dislocations accumulate during primary creep while annealing effects may reduce subgrain boundaries that scatter phonons.Graphical abstractImage 1
       
  • Rational design of a lean magnesium-based alloy with high age-hardening
           response
    • Abstract: Publication date: Available online 24 July 2018Source: Acta MaterialiaAuthor(s): M. Cihova, R. Schäublin, L.B. Hauser, S.S.A. Gerstl, C. Simson, P.J. Uggowitzer, J.F. Löffler Magnesium-based alloys that allow fast processing, easy formability and subsequent age hardening to their final strength are highly desirable for lightweight structural applications. In this study, a lean age-hardenable wrought Mg–Al–Ca–Mn alloy, which combines precipitation hardening and grain refinement by secondary-phase pinning, was designed via thermodynamic calculation. The resulting alloy, AXM100, with a nominal composition Mg-Al0.6-Ca0.28-Mn0.25 (in wt.%), shows a remarkable improvement in tensile yield strength of 70 and 100 MPa upon artificial aging from the as-extruded state (T5) and the solution-heat-treated state (T6), respectively, reaching 253 MPa for the latter. A multi-scale microstructural analysis, combining light microscopy, transmission electron microscopy and atom probe tomography, was performed. It revealed a fine dispersion of Al–Mn precipitates with a β-Mn structure and Al–Ca-rich Guinier–Preston (G.P.) zones, which have an Al-to-Ca ratio of about 2. The former are responsible for impeding grain growth and the latter for age hardening. In addition, a fine dispersion of nanometric Ca-rich clusters preceding the G.P.-zone formation were identified which may contribute to strength. While the microstructural analysis, in terms of volume fraction and composition of the phases, reveals the limitation of the calculations, the latter successfully predict the elements contained in the various phases that play a key role in the mechanical properties, thereby proving them to be an invaluable tool for alloy design. In fact, the alloy designed in this study shows, despite its leanness, an age-hardening potential of 87 MPa and 118 MPa per 1 at.% total alloying content for the T5 and T6 condition, respectively, which is the highest among the compositions known for this type of alloys.Graphical abstractImage 1
       
  • Analysis of the interaction between moving α/γ interfaces and interphase
           precipitated carbides during cyclic phase transformations in a
           Nb-containing Fe-C-Mn alloy
    • Abstract: Publication date: Available online 24 July 2018Source: Acta MaterialiaAuthor(s): Haokai Dong, Hao Chen, Wei Wang, Yongjie Zhang, Goro Miyamoto, Tadashi Furuhara, Chi Zhang, Zhigang Yang, Sybrand van der Zwaag The interaction between moving α/γ interfaces and interphase precipitated (IPd) carbides during the austenite (γ) to ferrite (α) and the ferrite (α) to austenite (γ) transformation has been systematically investigated through cyclic phase transformation experiments for a 0.1C-1.5Mn alloy containing 0.1wt% Niobium (Nb) and its Nb-free counterpart. Shifts in the critical reaction temperatures during continuous heating and cooling are observed, which are attributed to the pinning force (PF) originating from the IPd carbides present. By applying the Gibbs energy balance (GEB) model to analyze experimental results, the PF was derived to be about 15 J/mol for the α→γ transformation and about 5 J/mol for the γ→α transformation, respectively, both of which are quite small compared to chemical driving force of phase transformations. Moreover, various modified Zener pinning equations have also been used to predict the PF, and it was found that these values are comparable with those obtained from experiments, which suggests that the classical Zener theory still has promising potential for carbide-interface interaction analysis.Graphical abstractImage 1
       
  • Current progress and future challenges in rare-earth-free permanent
           magnets
    • Abstract: Publication date: Available online 24 July 2018Source: Acta MaterialiaAuthor(s): Jun Cui, Matt Kramer, Lin Zhou, Fei Liu, Alexander Gabay, George Hadjipanayis, Balamurugan Balasubramanian, David Sellmyer Permanent magnets (PM) are critical components for electric motors and power generators. Key properties of permanent magnets, especially coercivity and remanent magnetization, are strongly dependent on microstructure. Understanding metallurgical processing, phase stability and microstructural changes are essential for designing and improving permanent magnets. The widely used PM for the traction motor in electric vehicles and for the power generator in wind turbines contain rare earth elements Nd and Dy due to their high maximum energy product. Dy is used to sustain NdFeB's coercivity at higher temperature. Due to the high supply risk of rare earth elements (REE) such as Dy and Nd, these elements are listed as critical materials by the U.S. Department of Energy and other international institutes. Other than Dy, finer grain size is also found to have effect on sustaining coercivity at higher temperature. A proper control of phase stability and microstructures has direct impact on mitigating REE supply risk. Compared to rare earth PMs, non-rare earth (non-RE) PMs typically have lower maximum energy products, however, given their small supply risks and low cost, they are being intensively investigated for less-demanding applications. The general goal for the development of non-RE PMs is to fill in the gap between the most cost-effective but low performing hard ferrite magnet and the most expensive but high performing RE PMs. In the past five years great progress has been made toward improving the microstructure and physical properties of non-RE PMs. Several new candidate materials systems were investigated, and some have showed realistic potential for replacing RE PMs for some applications. In this article, we review the science and technology of various types of non-RE materials for PM applications. These materials systems include Mn based, high magnetocrystalline anisotropy alloys (MnBi, MnAl compounds), spinodally decomposing alloys (Alnico), high-coercivity tetrataenite L10 phase (FeNi or FeCo), and nitride/carbide systems (such as α" based, high saturation magnetization Fe16N2 type phase and Cobalt carbide acicular particle phase). The current status, challenges, potentials as well as the future directions for these candidates non-RE magnet materials are discussed.Graphical abstractImage 1
       
  • Tensorial nature of γ′-rafting evolution in nickel-based single
           crystal superalloys
    • Abstract: Publication date: Available online 23 July 2018Source: Acta MaterialiaAuthor(s): V. Caccuri, R. Desmorat, J. Cormier Our work aims at providing a tensorial analysis of directional γ′ rafting state in a nickel based single crystal superalloy, for several orientations. According to a visco-plasticity modeling accounting for microstructure evolution during creep (including rafting), different —even— order fabric tensor representations are provided, respectively for γ channel width, primary γ′ cuboidal particles and microstructure periodicity λ. Several image processing algorithms are presented and used to best measure these quantities, particularly the AutoCorrelation Method (ACM) and the Rotational Intercept Method (RIM). Each method is developed for a specific purpose: the AutoCorrelation Method yields information about principal directions and pattern periodicity, while RIM method provides a polar distribution of a physical quantity. A methodology is proposed and applied to measure the Rose diagrams and the corresponding fabric tensors of order 2, 4, 6 and 8, starting by the best fitting Fourier series coefficients of the experimental data. When RIM method is possible to apply, fabric tensors up to order 8 are considered. In any case a 2ndorder tensor representation of single crystals γ′ rafting state is provided.Graphical abstractImage 1
       
  • The search for high entropy alloys: a high-throughput ab-initio
           approach
    • Abstract: Publication date: Available online 23 July 2018Source: Acta MaterialiaAuthor(s): Yoav Lederer, Cormac Toher, Kenneth S. Vecchio, Stefano Curtarolo While the ongoing search to discover new high-entropy systems is slowly expanding beyond metals, a rational and effective method for predicting “in silico” the solid solution forming ability of multi-component systems remains yet to be developed. In this article, we propose a novel high-throughput approach, called “ LTVC”, for estimating the transition temperature of a solid solution: ab-initio energies are incorporated into a mean field statistical mechanical model where an order parameter follows the evolution of disorder. The LTVC method is corroborated by Monte Carlo simulations and the results from the current most reliable data for binary, ternary, quaternary and quinary systems (96.6%; 90.7%; 100% and 100%, of correct solid solution predictions, respectively). By scanning through the many thousands of systems available in the AFLOW consortium repository, it is possible to predict a plethora of previously unknown potential quaternary and quinary solid solutions for future experimental validation.Graphical abstractImage 1
       
  • Spall strength dependence on grain size and strain rate in tantalum
    • Abstract: Publication date: Available online 23 July 2018Source: Acta MaterialiaAuthor(s): T.P. Remington, E.N. Hahn, S. Zhao, R. Flanagan, J.C.E. Mertens, S. Sabbaghianrad, T.G. Langdon, C.E. Wehrenberg, B.R. Maddox, D.C. Swift, B.A. Remington, N. Chawla, M.A. Meyers We examine the effect of grain size on the dynamic failure tantalum during laser-shock compression and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples; thus, monocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultra-fine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra and intergranular in the poly and ultra-fine-grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length geometrically-necessary dislocations required to form a grain-boundary (intergranular) void is lower than that of grain-interior (intragranular) void with the same maximum diameter; thus, the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall strength with strain rate from 6x106 to 5x107 s-1. Molecular dynamics calculations agree with the experimental results and also predict grain-boundary separation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals.Graphical abstractImage 1
       
  • Shock-induced amorphization in silicon carbide
    • Abstract: Publication date: Available online 23 July 2018Source: Acta MaterialiaAuthor(s): S. Zhao, R. Flanagan, E.N. Hahn, B. Kad, B.A. Remington, C.E. Wehrenberg, R. Cauble, K. More, M.A. Meyers While silicon carbide (SiC) has been predicted to undergo pressure-induced amorphization, the microstructural evidence of such a drastic phase change is absent as its brittleness usually prevents its successful recovery from high pressure experiments. Here we report on the observation of amorphous SiC recovered from laser-ablation-driven shock compression with a peak stress of approximately 50 GPa. Transmission electron microscopy reveals that the amorphous regions are extremely localized, forming bands as narrow as a few nanometers. In addition to these amorphous bands, planar stacking faults are observed. Large-scale non-equilibrium molecular dynamic simulations elucidate the process and suggest that the planar stacking faults serve as the precursors to amorphization. Our results suggest that the amorphous phase produced is a high-density form, which enhances its thermodynamically stability under the high pressures combined with the shear stresses generated by the uniaxial strain state in shock compression.Graphical abstractImage 1
       
  • The effect of multiple precipitate types and texture on yield asymmetry in
           Mg-Sn-Zn(-Al-Na-Ca) alloys
    • Abstract: Publication date: Available online 23 July 2018Source: Acta MaterialiaAuthor(s): A.E. Davis, J.D. Robson, M. Turski Strong textures produced during thermomechanical processing cause high levels of mechanical asymmetry in wrought Mg alloys. Through careful choice of precipitate type, asymmetry can be reduced by age hardening. In this work various Mg-Sn-Zn(-Al-Na-Ca) alloys were extruded then aged in the direct-aged and solution-treated conditions at 150°C and 200°C to investigate the capability of the combination of basal lath and c-axis rod precipitates to reduce asymmetry. Precipitate populations were found to be sensitive to ageing temperature as ageing at 150°C produced basal ellipsoid precipitates instead of higher aspect ratio basal laths as was expected. In addition, basal ellipsoids and c-axis rods were found to nucleate epitaxially reducing their strengthening effect. A Mg-8.8Sn-4.0Zn-0.9Al-0.3Na (TZA941+0.3Na) (wt.%) extruded alloy was shown to exhibit extrusion-direction tensile and compressive yield stresses of 360 MPa and 233 MPa in the solution treated condition with a compression to tension ratio of 0.65 after ageing at 150°C. It was also demonstrated that increasing the Sn content in the alloys reduced the time to peak hardness when ageing at 200°C where TZA941+0.3Na reached peak hardness within 12 h, as opposed to 168 h when ageing at 150°C. In addition, increasing Na additions served to strengthen extrusion textures, and Ca additions were shown to be ineffective at weakening extrusion textures in these alloys.Graphical abstractImage 1
       
  • Crystal structure, energetics, and phase stability of strengthening
           precipitates in Mg alloys: A first-principles study
    • Abstract: Publication date: Available online 20 July 2018Source: Acta MaterialiaAuthor(s): Dongshu Wang, Maximilian Amsler, Vinay I. Hegde, James E. Saal, Ahmed Issa, Bi-Cheng Zhou, Xiaoqin Zeng, Chris Wolverton Magnesium alloys have attracted increasing interest due to their potential use as light-weight structural materials but their application is limited by their low strength compared to conventional alloys. Age hardening is commonly employed to form strengthening precipitates in such alloys, which impedes the motion of dislocations, and leads to improved strength. However, the exact composition, crystal structure, and energetics of many of these strengthening precipitates are either unknown or not clearly resolved, making the precise engineering and design of these alloys difficult. Toward this end, we use first-principles density functional theory calculations to elucidate the crystal structures and energetics of a very large set of precipitates in magnesium alloys. For cases where the precipitate crystal structure is not known, we comprehensively search over decorations of many prototype structures, including hcp superstructures, and in addition, perform global structural optimization using the Minima Hopping Method to predict suitable crystal structures. For all the strengthening precipitates, we calculate the formation energies, construct the respective zero temperature convex hulls, and analyze their stabilities. We show that the bulk formation energies per solute atom (essentially, the solute chemical potentials) decrease along the observed sequences of precipitation, validating our calculations in Mg-{Nd, Gd, Y, Y-Nd, Nd-Zn, Gd-Zn, Y-Zn, Al, Zn, Sn, Al-Ca, Ca-Zn} alloy systems. In addition, we construct a monolayer model for the Guinier-Preston zones (GP zones) observed in the Mg-Nd-Zn system during early stages of age hardening, and thereby explain the formation of the γ'' (Mg5(Nd,Zn)) phase from the GP zones, as observed in experiments.Graphical abstractImage 1
       
  • Elucidating the contribution of mobile hydrogen-deformation interactions
           to hydrogen-induced intergranular cracking in polycrystalline nickel
    • Abstract: Publication date: Available online 20 July 2018Source: Acta MaterialiaAuthor(s): Zachary D. Harris, Samantha K. Lawrence, Douglas L. Medlin, Gael Guetard, James T. Burns, Brian P. Somerday Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile hydrogen (H)-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for hydrogen-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results.Graphical abstractImage 1
       
  • Cross-slip of long dislocations in FCC solid solutions
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): Wolfram Georg Nöhring, W.A. Curtin Cross-slip of screw dislocations is a dislocation process involved in dislocation structuring, work hardening, and fatigue. Cross-slip nucleation in FCC solid solution alloys has recently been shown to be strongly influenced by local fluctuations in spatial arrangement of solutes, leading to a statistical distribution of cross-slip nucleation barriers. For cross-slip to be effective macroscopically, however, small cross-slip nuclei (∼40b) must expand across the entire length of typical dislocation segments (102–103b). Here, a model is developed to compute the relevant activation energy distribution for cross-slip in a random FCC alloy over arbitrary lengths and under non-zero Escaig and Schmid stresses. The model considers cross-slip as a random walk of successive flips of adjacent 1b segments, with each flip having an energy consisting of a deterministic contribution due to constriction formation and stress effects, plus a stochastic contribution. The corresponding distribution is computed analytically from solute-dislocation and solute-solute binding energies. At zero stress, the probability of high activation energies increases with dislocation length. However, at stresses of just a few MPa, these barriers are eliminated and lower barriers are dominant. For increasing segment length, the effective energy barrier decreases according to a weak-link scaling relationship and good analytic predictions can be made using only known material properties. Overall, these results show that the effective cross-slip barrier in a random alloy is significantly lower than estimates based on average elastic and stacking fault properties of the alloy.Graphical abstractImage 1
       
  • { 10 1 ¯ 2 } +twin-dominated+deformation+of+Mg+pillars:+Twinning+mechanism,+size+effects+and+rate+dependency&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">In-situ TEM observation of { 10 1 ¯ 2 } twin-dominated deformation of
           Mg pillars: Twinning mechanism, size effects and rate dependency
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): Jiwon Jeong, Markus Alfreider, Ruth Konetschnik, Daniel Kiener, Sang Ho Oh To investigate the mechanism of {101¯2} twinning in magnesium (Mg) single crystal and its influence on mechanical size effects and strain rate sensitivity, in-situ microcompression of Mg [21¯1¯0]pillars of various sizes from 0.5 μm to 4 μm was carried out in a scanning electron microscope (SEM) and also in a transmission electron microscope (TEM) with covering the strain rates from 10−4 to 10−2 s−1. The in-situ observations directly showed that the pile-up of prismatic dislocations acts as local stress concentration for the twin nucleation. Preceding the twin nucleation, the lead dislocation in the pile-up cross-slips to the basal plane and dissociates into partial dislocations, one of which trails a stacking fault (SF) behind. A twin nucleus of a finite size formed at the junction between prismatic dislocations and basal SFs and subsequently propagated rapidly across the pillar. The present in-situ observations reveal that not only the dislocation pile-up but also the dissociation reaction of dislocations play critical roles in the nucleation of {101¯2} twins. Furthermore, the {101¯2} twinning exhibits a relatively strong size effect in terms of the twin nucleation stress (size exponent, n = 0.7). This pronounced size effect may arise from the fact that the precursor to twin nucleation, namely dislocation pile-up and junction formation, depends more strongly on the crystal size than the ordinary dislocation source operation. Moreover, a noticeable effect of the strain rate is that a higher rate (10−2 s−1) promotes the activation of basal slip within {101¯2} twin. While the twin nucleation occurs more easily at a high strain rate, here the twin growth rate cannot cope with the applied strain rate, so that strain energy accumulation increases with applied strain. When the twin grows to reach the required twin thickness for basal slip, the basal slip promptly activates within the twinned region to release the accumulated strain energy and plastic deformation swiftly catches up with the applied strain rate.Graphical abstractImage 1
       
  • On the Bulk Glass Formation in the Ternary Pd-Ni-S System
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): Alexander Kuball, Benedikt Bochtler, Oliver Gross, Victor Pacheco, Moritz Stolpe, Simon Hechler, Ralf Busch We report on the formation of bulk metallic glasses in the ternary Pd-Ni-S system. In a large compositional range, glass formation is observed by copper mold casting with a glass forming ability of up to 2 mm in diameter for the composition Pd37Ni37S26. The best compromise of thermal stability upon heating from the as-cast state and glass forming ability was found for Pd31Ni42S27, having a critical diameter of 1.5 mm and an extension of the supercooled liquid region of 27.2 K (ΔTx = Tx – Tg). Differential scanning calorimetry and X-ray diffraction experiments were conducted in order to study the influence of the composition on the glass forming ability and thermal stability. The primary precipitating crystalline phases Ni3S2 and Pd4S are identified by in-situ high energy synchrotron X-ray scattering experiments upon heating from the glassy state as well as upon cooling from the equilibrium liquid. Finally, the origin of the bulk glass formation in this novel system is discussed regarding thermodynamics and kinetics and compared to current models for the prediction of the glass forming ability. Furthermore, the mechanical properties are investigated and discussed with respect to the rather fragile kinetic behavior. All in all, we gain new insights in the process of glass formation in this novel alloying system and give conclusions about the determining contributions for the glass forming ability and glass forming range.Graphical abstractImage 1
       
  • Direct electrical switching of ferroelectric vortices by a sweeping biased
           tip
    • Abstract: Publication date: Available online 17 July 2018Source: Acta MaterialiaAuthor(s): L.L. Ma, Ye Ji, W.J. Chen, J.Y. Liu, Y.L. Liu, Biao Wang, Yue Zheng The precise manipulation of ferroelectric vortices is one of the most important issues concerning its potential applications in functional electronic devices such as non-volatile memory. Despite investigations focusing on the switching of toroidization of vortex domain structure performed in recent years, there is still a lack of a simple and general method to realize vortex switching. In this work, we propose a direct electrical method to switch ferroelectric vortices by sweeping a biased tip, which is easy-to-operate in practice and is demonstrated to be feasible for various ferroelectric structures. It is the electric field gradient generated by the tip bias in conjunction with the time-reversal asymmetric tip-sweeping operation that induces the vortex switching. The influencing factors of this method, e.g., field profile, tip size, tip bias, tip displacement and surface screening conditions, etc., are systematically studied. As an implementation, we put forward a nanowire vortex memory system in which the information stored by vortex chirality can be successfully manipulated by the tip-sweeping method. The effects of temperature and nanowire structure on the feasibility of vortex switching are analyzed. Our findings provide an efficient control strategy on ferroelectric vortices and suggest broad device opportunities exploiting vortex domain structures.
       
  • Influence of deformation induced nanoscale twinning and FCC-HCP
           transformation on hardening and texture development in medium-entropy
           CrCoNi alloy
    • Abstract: Publication date: Available online 12 July 2018Source: Acta MaterialiaAuthor(s): C.E. Slone, S. Chakraborty, J. Miao, E.P. George, M.J. Mills, S.R. Niezgoda Texture evolution during room-temperature tensile testing of recrystallized equimolar CrCoNi was studied using electron backscatter diffraction and electron channeling contrast imaging on specimens from interrupted tests. Dominant deformation mechanisms included slip at low strains and deformation twinning at larger strains, which were accompanied by the development of a strong texture parallel to the tensile axis. Highly deformed material also contained nanotwin/hcp lamellae, which have previously been hypothesized to act as potent barriers for non-coplanar dislocations. To examine this hypothesis, mean-field modeling was performed using the viscoplastic self-consistent framework with varying ratios for hardening by slip and twinning. In the optimal model, twinning produced approximately three times as much non-coplanar hardening as slip, which is larger than previous observations in other twinning-induced plasticity materials that do not form twin/hcp lamellae. Additional full-field elasto-viscoplastic simulations were performed using the fast Fourier transform (EVP-FFT) method to examine intragranular rotation and the effect of initial grain orientation on the deformation mode. Grains with initial orientations near had the greatest propensity for deformation twinning while grains near were more likely to deform by slip even at large strains. Excellent quantitative agreement was obtained between the experiments and EVP-FFT model.Graphical abstractImage 1
       
  • Recursive alloy Hamiltonian construction and its application to the
           Ni-Al-Cr system
    • Abstract: Publication date: Available online 4 July 2018Source: Acta MaterialiaAuthor(s): Jon Gabriel Goiri, Anton Van der Ven Navigating the high dimensional composition spaces of multi-principle element alloys, also referred to as high entropy alloys, will require new methods to construct first-principles alloy Hamiltonians. We introduce a recursive approach to parameterizing multi-component alloy Hamiltonians using interaction parameters from simpler subsystems as Bayesian informative priors. We applied this approach to perform a first-principles statistical mechanics study of the Ni-Al-Cr system. Ternary cluster expansions for the Ni-Al-Cr alloy were constructed by building on optimized Ni-Al and Ni-Cr binary cluster expansions. Monte Carlo simulations predict a sizable Cr solubility in the Ni-rich FCC based γ and γ′ phases. The L12 -ordered γ′ phase is predicted to dissolve Cr primarily on its Al-sublattice. We also identify a family of hierarchical long-period super structure orderings as groundstates in the Ni-Cr and Al-Cr binaries. The recursive approach to parameterizing alloy Hamiltonians opens the door to rigorous first-principles treatments of the elevated temperature thermodynamics of alloys in high dimensional composition spaces.Graphical abstractImage 1
       
  • Electron radiation-induced material diffusion and nanocrystallization in
           nanostructured amorphous CoFeB thin film
    • Abstract: Publication date: Available online 18 June 2018Source: Acta MaterialiaAuthor(s): Binghai Liu, Taiebeh Tahmasebi, Kenny Ong, Hanwei Teo, Zhiqiang Mo, Jeffrey Lam, Pik Kee Tan, Yuzhe Zhao, Zhili Dong, Dimitri Houssameddine, Jacob Wang, Junming Xue, Zhihong Mai Transmission electron microscopy (TEM) is widely used for physical characterization of CoFeB -based magnetic tunneling junctions (MTJ) with its atomic-scale resolution. However, highly energetic electron radiation during TEM analysis may cause phase and microstructure modification of CoFeB and its associated MTJ layers. It is the intention of this work to address the issues of the electron-beam sensitivity of CoFeB material. With in-situ TEM, we investigated the electron beam radiation-induced material diffusion and the nanocrystallization behaviors in nanostructured amorphous CowFexByOz/Co60Fe20B20/SiO2 thin films. It was found that electron radiation with different electron dose led to massive diffusion of Co, Fe, B and O atoms across the whole thin film layers, which directly resulted in the modification of the phase and composition of the thin film layers, i.e. the oxidation of Co, Fe, B with O diffusion and the formation of pure Si phase from SiO2. Two stages of material diffusion were observed. While Stage-I material diffusion proceeded with a high diffusion speed, Stage-II had a relatively low diffusion rate accompanying with the nanocrystallization at the bottom of the CoFeB layer. A detailed kinetic study by in-situ TEM revealed the electron-beam radiation induced massive diffusion was a non-thermal process, and the underlying driving force arose from radiation-enhanced diffusion (RED) effects. Nanocrystallization during Stage-II electron-radiation experiment showed unique phase transformation phenomena, repeated nanocrystallization, amorphization, and nanocrystallization processes in the sequence before a stable grain growth could be achieved. A detailed TEM analysis revealed that RED-enhanced B diffusion was responsible for such unique repeated phase transformation processes. B diffusion and the associated structure distortion and the local short-range re-ordering may also account for the phase transformation from fcc-CoxFe23-xB6 to B-rich orthorhombic- CoxFe3-xB phase.
       
  • The role of the interface stiffness tensor on grain boundary dynamics
    • Abstract: Publication date: Available online 18 June 2018Source: Acta MaterialiaAuthor(s): Fadi Abdeljawad, Stephen M. Foiles, Alex Moore, Adam R. Hinkle, Christopher Barr, Nathan M. Heckman, Khalid Hattar, Brad L. Boyce Grain boundary (GB) properties and associated anisotropies due to the boundary's geometric degrees of freedom (DOF) greatly influence many of the salient features of polycrystalline aggregates, and as a consequence the observable properties of the material. Through theoretical analysis and atomistic simulations, we show that when considering the GB plane normal DOF, the GB interface stiffness tensor plays a paramount role in a wide range of GB dynamical processes. As a demonstration, we examine the interface stiffness tensor of Σ3 GBs in nickel and show that the stiffness can be much larger in magnitude and more anisotropic than the GB energy itself. Moreover, it is found that a wide range of inclinations exhibit negative stiffness values, a signature of a structural instability. This is the first study to consider the complete spatial description of the GB stiffness tensor, where both angular variations describing the interface plane normal are explored. In broad terms, our results highlight the integral role that the stiffness tensor plays in GB interfacial phenomena, such as curvature-driven flow and faceting instabilities.Graphical abstractImage 1
       
  • Hierarchical microstructure design of a bimodal grained twinning-induced
           plasticity steel with excellent cryogenic mechanical properties
    • Abstract: Publication date: Available online 14 June 2018Source: Acta MaterialiaAuthor(s): Yu Li, Yufei Lu, Wei Li, Mahmoud Khedr, Huibin Liu, Xuejun Jin Combined nanoprecipitation and grain refinement were introduced in a bimodal grained (BG) twinning-induced plasticity (TWIP) high manganese steel to achieve high strength-ductility combinations. Hierarchical microstructural characteristics of heterogeneous grain size (0.2 μm - 4 μm) and precipitates (κ-carbides and Nb-rich carbides) distribution was obtained by a controlled thermo-mechanical treatment. Compared with the as-received states, the BG-TWIP steels showed a significant improvement in yield strength (YS) with little loss in plasticity whether at room temperature (RT) or liquid nitrogen temperature (LNT). The multiple strengthening contributions to YS mainly originate from the combination of solid solution and grain refinement strengthening. When deformed at RT, some deformation twins nucleated in the coarse grains (CG) of the BG-TWIP steels while only numerous stacking faults formed in the fine grains (FG) at a true strain of 0.14. The twin density remained nearly unchanged with progressive deformation and the dislocation strengthening dominated in the later deformation stage. In the early deformation stage at LNT, the twin amount of the BG-TWIP steel was still small. However, the volume fraction of twins increased greatly in both the FGs and CGs when deformed to a true strain of 0.26. The occurrence of high density of nano-twins in the later deformation stage at LNT not only contributes to a strength increment of 220 MPa, but also largely increases the geometrically necessary dislocations (GND) density to enhance the forest hardening effect, corresponding to a significant higher back stress hardening at large strains.Graphical abstractImage 1
       
 
 
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