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ACTA Vol. 196, 1 Sept. 2020, P295-303

Superalloys constitute an important class of materials that are heavily employed in turbines of aircraft engines and power plants. Vickers hardness is an important mechanical property for selection of a material. In this work, we develop an alternate approach, which uses the microstructures to estimate the hardness of a Co- and Ni- based superalloys. Advanced image processing techniques coupled with data-driven machine learning (ML) are used to predict the Vickers hardness of these superalloys. Complex image derived properties such as 2-point correlations and compositions of superalloys are utilized as a feature to develop highly accurate ML model. The ML model trained through Gaussian process regression (GPR) using microstructure and compositional features show unprecedented accuracy with root mean square error (RMSE) of 0.14 and R2 of 0.98. Further analysis of the model is done to establish a relationship between the Vickers hardness with microstructural and compositional parameters. Addition of certain compounds such as iron and titanium can in general lead to increase in Vickers hardness, while addition of elements such as aluminium, tantalum and hafnium negatively affect the Vickers hardness. Most importantly, the developed ML model is trained on experimental data, as opposed to simulated data, making our approach directly applicable for accurate prediction of Vickers hardness.

ACTA Vol. 196, 1 Sept. 2020, P304-312

英文摘要

ACTA Vol. 196, 1 Sept. 2020, P338-346

The first in situ quantitative synchrotron X-ray diffraction (XRD) study of plastic strain-induced phase transformation (PT) has been performed on α−ω PT in ultra-pure, strongly plastically predeformed Zr as an example, under different compression-shear pathways in rotational diamond anvil cell (RDAC). Radial distributions of pressure in each phase and in the mixture, and concentration of ω-Zr, all averaged over the sample thickness, as well as thickness profile were measured. The minimum pressure for the strain-induced α−ω PT, =1.2?GPa, is smaller than under hydrostatic loading by a factor of 4.5 and smaller than the phase equilibrium pressure by a factor of 3; it is independent of the compression-shear straining path. The theoretically predicted plastic strain-controlled kinetic equation was verified and quantified; it is independent of the pressure-plastic strain loading path and plastic deformation at pressures below p_ε^d. Thus, strain-induced PTs under compression in DAC and torsion in RDAC do not fundamentally differ. The yield strength of both phases is estimated using hardness and x-ray peak broadening; the yield strength in shear is not reached by the contact friction stress and cannot be evaluated using the pressure gradient. Obtained results open a new opportunity for quantitative study of strain-induced PTs and reactions with applications to material synthesis and processing, mechanochemistry, and geophysics.

ACTA Vol. 196, 1 Sept. 2020, P355-369

In this study, we describe an experimental setup and a new approach for operando investigation of structural evolution of materials during wear and friction. The setup is particularly suited for testing various friction pairs, including those in which both rubbing bodies are made of metals. The developed device allows circumventing the problems related to significant scattering of X-rays produced by metals and makes it possible using “real samples” in synchrotron beamlines operating in reflection mode. To demonstrate the capabilities of the device and the proposed new approach, an iron-based massive sample was subjected to thousands of friction cycles using a cemented carbide pin. The material was probed with synchrotron X-ray radiation within a few milliseconds after leaving the friction zone. The results of the microstructural and structural analysis, as well as results obtained from diverse mathematical models, allowed us to evaluate several features, including gradual accumulation of defects, microstructural refinement, dislocation density changes, surface layer oxidation, as well as several other phenomena caused by the dry sliding friction process. Mainly, it was possible to conclude that the process of wear occurred due to the cooperative action of oxidation and plastic deformation, which began during the first cycle of frictional interaction and was manifested in increasing the dislocation density, whose type was changed gradually during testing. The number of defects quickly reached a threshold value and subsequently fluctuated around it due to periodically repeated processes of defect accumulation and stress relaxation resulting from material wear. It was also observed that friction led to the quick formation of a mechanically mixed layer, consisting of the sample material and a mixture of two types of iron oxide – hematite and magnetite. The delamination of this layer was probably the primary wear mechanism.

ACTA Vol. 196, 1 Sept. 2020, P370-383

英文摘要

ACTA Vol. 196, 1 Sept. 2020, P384-395

An improved model based on thermally activated dislocation glide is proposed to describe the dislocation detachment-controlled creep of extruded and annealed GlidCop Al-15 and extruded GlidCop Al-60 at 973 K as well as the threshold behavior observed at lower stresses. Unlike the customary analysis using the Rösler-Arzt approach, both steady-state and strain-transient stress reduction creep tests were performed to obtain the activation energies and the obstacle strengths for creep. In contrast to pure copper, the constant-structure creep rates of GlidCop were greater than the subsequent steady-state creep rates in all tests. This result supports the concept that dislocation detachment from particles, rather than dislocation-dislocation interactions, is rate-controlling.In addition, the constant-structure creep rates at the same reduced stresses were found to be greater at higher initial stresses. This observation is interpreted as evidence that a forward internal stress arising from inter-dislocation interactions near the particles acts on the ready-to-detach dislocations and scales with the initial stress. This forward stress and an explicit treatment of the stress dependence of mobile dislocation density were incorporated in the improved model. Furthermore, the operational activation area for dislocation glide was determined at constant temperature and structure, and the values are consistent with the interparticle distances multiplied by the Burgers vector, which further supports the interpretation of detachment controlled glide. Finally, the true threshold stress for creep is modeled as originating from thermodynamic back jumps of just-detached dislocations due to the attractive nature of the particle/matrix interface.

ACTA Vol. 196, 1 Sept. 2020, P396-408

The chemistry,thermodynamics and mechanical properties of the L1₂-ordered Co₃(Al,W)γ’-phase are crucial for the understanding of γ(f.c.c.)/γ’(L1₂)cobalt-based superalloys. A single-phase γ’(L1₂) alloy with thecomposition Co-30Ni-11Al-5.5W-4Ti-2.5Ta-0.10B (at.%) and a γ’(L1₂)-solvustemperature of 1268 °C was recently identified using the Calphad-methodology.Scanning and transmission electron microscopy reveals that the single-phase microstructureis stable at 900 and 1000 °C for 1000 h and at 1100 °C for 168 h, without otherphases being observed, resulting in similar levels of microhardness for allannealing temperatures. Atom-probe tomography confirms the presence of asingle-phase γ’(L1₂)-microstructure with a composition of (Co,Ni)₃(Al,W,Ti,Ta).Grain boundaries exhibit depletion of Ni, W and Ta and enrichment of Co, Al andB. A remarkable yield stress anomaly is observed, with the yield strengthincreasing from ~ 300 to ~ 700 MPa from room temperature to 800 °C, which isstronger than Co₃(Al,W)(L1₂) and Ni₃Al(L1₂).The creep tests at 850 and 950 °C display power-law behavior with a stressexponent of n = ~ 3 and an activation energy of Q_n = 497 kJ•mol^-1for (Co,Ni)₃(Al,W,Ti,Ta), similar to that of single-phase Ni₃Al(L1₂)compound (Q_n = 406–421 kJ•mol^-1) .

ACTA Vol. 196, 1 Sept. 2020, P409-417

Materials with certain heterogeneous microstructures have been shown to hold a synergistic combination of strength and ductility. In this study, we demonstrate novel transformation pathways for creating such heterogeneous microstructure in Ti-alloys by integrating thermodynamic databases with phase field simulations. The results show that the concentration modulations at different length scales produced by (a) precursory spinodal decomposition in the parent phase and (b) interdiffusion in multi-layers having different alloy compositions can generate effectively hierarchical and gradient α + β two-phase microstructures, with a mixture of fine α precipitate regions and α precipitate-free-zones or coarse α precipitate regions. The novel microstructures produced include “inverted globular α” bi-modal microstructures and gradient microstructures with controlled spatial gradients in particle size and number density of α precipitates. This study may shed light on how to design novel hierarchical and gradient two-phase microstructures with tunable size and density of precipitates as well as the length scale of their spatial heterogeneity for desired properties.

ACTA Vol. 196, 1 Sept. 2020, P430-443

A scale-independent model for the interaction between multivariant phase transformations (PTs) and discrete shear bands is advanced and utilized to simulate plastic strain-induced PTs at high pressure. The model includes a scale-free phase-field theory for martensitic PTs. The localized shear bands are introduced via a contact problem formulation. That is, the continuous distribution of sliding displacements along the prescribed slip surfaces is modeled to reproduce the plastic-strain-induced stress concentrators necessary for nucleation of a high-pressure phase (HPP). The strain-induced PTs in the bi/polycrystalline samples subjected to compression and shear are studied. The simulations show a severe reduction in the PT pressure by the plastic shear in comparison to a hydrostatic condition, even below the phase equilibrium pressure, like in known experiments. Transformation kinetics versus shear strain for each martensitic variant and the volume fraction of the HPP in individual grains and the entire aggregate are determined. The stationary volume fraction of the HPP is the same for polycrystals consisting of 13 and 38 grains, and a further shearing does not cause PT. The local phase equilibrium condition based on the transformation-work criterion is satisfied at almost all stationary phase interfaces. A similar phase equilibrium condition in terms of stresses averaged over the entire polycrystal or HPP is fulfilled. These results are important for the development of the microscale kinetic equations and modeling the sample behavior in traditional and rotational diamond anvils during the high-pressure torsion, ball milling, friction, and other deformation-transformation processes.

ACTA Vol. 196, 1 Sept. 2020, P456-469

Competing microstructural evolution mechanisms can exist simultaneously when duplex stainless steels are operating for several decades in a high temperature service environment. Such competition between different microstructural evolution pathways can be difficult to ascertain using simple model alloy systems necessitating detailed microstructural analysis of phase transformation mechanisms in complex alloys. Thus, duplex stainless steels with complex but well understood chemistries were used to investigate the relative importance of different heterogeneous nucleation sites – specifically, spinodal decomposition and Cu clustering – on Ni-Si-Mn precipitation during thermal aging. Precipitation of Ni-Si-Mn particles in ferrite-bearing steels during thermal aging and irradiation can greatly change mechanical properties. Using duplex stainless steels with custom-modified compositions along with advanced microstructural characterization and first-passage kinetic Monte Carlo simulations, it is revealed that while the interface between Cr and Fe formed during spinodal decomposition can be a pathway for solute diffusion, it is not a preferred site for Ni-Si-Mn precipitation. Instead, the presence of a higher concentration of Cu leads to the formation of small Cu-rich clusters with high energy interfaces that act as nucleation sites for Ni-Si-Mn particles. These results will inform predictive models for the use of precipitation-hardened alloys for extended operation at high temperatures.

ACTA Vol. 196, 1 Sept. 2020, P470-487

Metastable austenite strongly influences the mechanical properties of many advanced high strength steels (AHSS) and its formation kinetics during intercritical annealing strongly depend on the initial microstructure. In this contribution, we have performed detailed kinetic studies of austenite formation from cementite-ferrite aggregate in a range of Fe-C-Mn and Fe-C-Mn-Si/Al alloys via in situ neutron powder diffraction. Depending on the relative contribution of cementite dissolution in respect to migrating interface of austenite/ferrite, the incomplete dissolution of enveloped cementite limited by slow diffusion in austenite could result in austenite plateauing below equilibrium, while fast dissolution of matrix cementite could result in austenite plateau above equilibrium. Both contributions need to be considered and modelled to describe the austenite formation kinetics.

ACTA Vol. 196, 1 Sept. 2020, P488-504

In addition to conventional dislocation plasticity, mechanical twinning and structural phase transformations are another two important plasticity carriers. Although both are symmetry-breaking processes and theories to treat each of them individually have been well-established, the intrinsic coupling between the two has not been investigated. Here we employ a phase transition graph approach to analyze systematically deformation modes arising from the interplay between mechanical twinning and phase transformations. Using metastable β Ti-alloys as an example, we show that mechanical twinning and phase transformations are intrinsically coupled in the symmetry-breaking processes, which results in multiple interconnected transformation and non-transformation deformation pathways and characteristic twinning modes. This work not only reveals the physical origin of unique twinning modes (e.g., {3 3 2}, {5 8 11} and {3 9 10} twins) and extended core structures of twin boundaries (e.g., nested twins) observed in experiments, but also provides a new insight into the enhanced plasticity of metastable β Ti-alloys through coupled twinning and transformation pathway engineering.

ACTA Vol. 196, 1 Sept. 2020, P505-515

MAB phases are a new class of layered ternary materials that have already shown a number of outstanding properties. Here, we investigate defect evolution and radiation tolerance of two MAB phases, MoAlB and Fe₂AlB₂, using a combination of experimental characterization and first-principles calculations. We find that Fe₂AlB₂ is more tolerant to radiation-induced amorphization than MoAlB, both at 150 °C and at 300 °C. The results can be explained by the fact that the Mo Frenkel pair is unstable in MoAlB and as a result, irradiated MoAlB is expected to have a significant concentration of MoAl antisites, which are difficult to anneal even at 300 °C. We find that the tolerance to radiation-induced amorphization of MAB phases is lower than in MAX phases, but it is comparable to that of SiC. However, MAB phases do not show radiation-induced cracking which is observed in MAX phases under the same irradiation conditions. This study suggests that MAB phases might be a promising class of materials for applications that involve radiation.

ACTA Vol. 196, 1 Sept. 2020, P516-527

The hydrogen trapping behaviors of dual nanometer-sized ε-copper and TiC carbide precipitates have been investigated in a low-carbon steel. The precipitation fashions were controlled by quenching and tempering at different austenization temperatures. The nanostructures of the dual precipitates were characterized by high-resolution transmission electron microscopy and atom probe tomography. Hydrogen desorption was studied through thermal desorption analysis of steels electrochemically charged with hydrogen. The hydrogen trapping capability of the precipitates was investigated through the release process. The precipitation routes of TiC carbides influenced the behavior of hydrogen trapping and the co-precipitation of copper and TiC carbides significantly increased the hydrogen trapping capability of the steels. These heat treatments can therefore be used to tailor the resistance to hydrogen embrittlement in tempered martensitic steels. Importantly, co-precipitated TiC and ε-copper particles in tempered martensitic steels show an enhanced capacity for hydrogen trapping and provide a range of trapping strengths. Co-precipitation is therefore proposed as a new prospect for designing steels with the capacity for substantial hydrogen trapping.

ACTA Vol. 196, 1 Sept. 2020, P565-575

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Lüders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

ACTA Vol. 196, 1 Sept. 2020, P584-594

To understand work hardening behavior during low-cycle loading, ductile cast iron containing 10.1 vol% spheroidal graphite, 19.6 vol% pearlite, and ferrite matrix was investigated in an in situ neutron diffraction study of up to four cycles of tensile–compressive loading with applied strains of ±0.01. The amplitudes of applied stress, Bauschinger stress, and Bauschinger strain were found to increase with increasing cycle number, indicating work hardening as cyclic loading progressed. Absolute values of ferrite lattice strain at maximum and minimum applied strains increased with increasing cycle number, indicating an increase in ferrite strength. Consequently, the stress contribution to the strength from ferrite increased as cyclic loading progressed. Cementite embedded in pearlite behaved as the hardest phase and maintained elastic deformation, but its stress contribution to strength was limited because the volume fraction was only about 2.2%. Meanwhile, graphite accommodated little stress. The increase in ferrite strength, caused by dislocation accumulation in ferrite during cyclic loading, played an important role in the work hardening of the ductile cast iron.

ACTA Vol. 196, 1 Sept. 2020, P595-608

An Al–10Ce-8Mn (wt%) alloy was designed and fabricated by laser powder bed fusion additive manufacturing (AM). The rapid cooling rates of the AM process produced a refined microstructure with a large fraction of reinforcing intermetallic phases. The tensile properties of the alloy were characterized in the as-fabricated state and following thermal exposure. The properties of the as-fabricated microstructure showed exceptional high-temperature performance and strength retention at elevated temperatures up to 400 °C relative to benchmark wrought Al and AM Al alloy properties. Characterization of the microstructure and thermodynamic modeling of the ternary Al–Ce–Mn system rationalized the solidification and solid-state phase transformations. Analysis of the relevant strengthening mechanisms for both the as-fabricated and thermally exposed conditions was performed.

ACTA Vol. 196, 1 Sept. 2020, P609-625

Metallic materials produced by additive manufacturing experience complex stress and thermal gyrations along the build direction. This has the potential to produce complicated heterogeneous microstructures that may exhibit a wide variety of mechanical properties. There remains a paucity of studies on the nature and the formation mechanisms of the microstructural heterogeneity and this limits our capability for microstructural design in additively manufactured metallic materials. Here, we present an electron microscopy-based investigation of a CrMnFeCoNi high-entropy alloy produced by selective laser melting. We have focussed on a systematic investigation of the microstructural evolution along the build direction. Our results reveal a remarkable hierarchy of microstructures, including the formation of nanocrystalline grains, elemental segregation and precipitation, cellular dislocation structures, deformation twinning, and deformation-induced phase transformation. Our research clarifies the relationships amongst different features, and provides guidance for future structural manipulation of materials produced by additive manufacturing.

ACTA Vol. 196, 1 Sept. 2020, P635-650

Yielding in pure BCC metals and dilute substitutional alloys occurs by double-kink nucleation and propagation along screw dislocations. At low temperatures, the yield stress is controlled by double-kink nucleation. Here, an analytical statistical model is presented to predict the stress- and length-dependent double-kink nucleation barrier in dilute BCC alloys solely in terms of the double-kink process in the pure metal and the solute/screw-dislocation interaction energies in the dilute alloy. Consistent with early literature, dilute alloying always reduces the double-kink nucleation barrier (softening) independent of solutes or matrix. The model is extensively validated via simulations in model Fe-Si alloys described by interatomic potentials. The model is then compared to experiments on real Fe-Si, W-Ta, and W-Re alloys, showing qualitative agreement consistent with the accuracy of the inputs. A cross-over from the dilute limit to the non-dilute limit, where there is hardening, is analyzed using the present theory and the non-dilute theory of Maresca et al. The analysis for Fe-Si is consistent with a cross-over at Si, as observed experimentally, and qualitatively consistent with W-Ta and W-Re. The present theory plus the recent theory of Maresca et al. together provide a coherent predictive framework for strengthening of screw dislocations over the full range of concentrations from extremely dilute (?1 at.%), to dilute (up to a few at.%) and non-dilute alloys including High Entropy Alloys.

ACTA Vol. 196, 1 Sept. 2020, P651-659

We demonstrate that the early-stage oxidation of a CoCrFeNi multi-principal element alloy depends upon a competition between kinetic and thermodynamic factors involving the relative diffusion rate of cations and oxygen, and how this couples to inward or outward oxide growth. The microstructures of oxide layers formed at temperatures from 500 to 800 °C for 0.5 h, as well as their chemical compositions, were investigated by transmission electron microscopy. A triple layer microstructure with an outer Fe-rich spinel oxide, an intermediate Cr-rich corundum structure, and a Ni-rich (Fe, Co, and Cr depleted) dealloyed region at the metal/oxide interface was observed. The dominant oxygen transport in corundum at 800 °C and below led to an inward growth of the corundum phase; the spinel oxide growth was dominated by cation diffusion, so it grew outward. The chromium was sequestered in the corundum layer, thereby favoring the formation of the chromium-free, Fe-rich spinel oxides with Co and Ni dopants. Since nickel cannot readily diffuse through corundum, it tends to remain in the alloy phase leading to the Ni-rich dealloyed region at the metal/oxide interface. Beyond the microstructure results, we exploit secondary electron image contrast to show the doping nature of the oxides, a p-type spinel and a n-type corundum growing on the metal surface.

ACTA Vol. 196, 1 Sept. 2020, P660-668

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that body-centered cubic (bcc) crystal structure of as-quenched martensite contained elastic distortions leading to small tetragonality, even in martensitic steels without solute carbon, and that the extended [001]bcc of martensite tended to be parallel to <001>fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the tetragonal distortions of the bcc crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

ACTA Vol. 196, 1 Sept. 2020, P690-703

It has been widely accepted that the viscosity of alloy melts increases monotonically with temperature decreasing after exceeding the liquidus temperature. However, a distinct viscosity-drop has been observed in several alloy melts in our previous works. To further understand this dynamic anomaly, we investigated the viscosity of CuZrAl and CuZrTiNi melts by gradually changing the compositions of alloy systems. The results suggest that when the amounts of doping elements (Al, Ti, Ni) become large enough, this abnormal viscosity-drop disappears. For CuZr binary alloys, the anomaly exists when the composition of Zr ranges between 27.3–66.7%, which nearly corresponds to the available content of Zr in CuZr bulk metallic glasses. By conducting molecular dynamics simulation, the mechanism that four icosahedron-like clusters with high fragility evolves with temperature in Cu₆₂Zr₃₈ melts has been recognized. In contrast to general aggregation or growth of clusters during cooling process, an unexpected tendency that more segregations are formed among these clusters and the clusters become further dispersed in liquids has been discovered around 1400 K; meanwhile, the viscosity-drop in our experiments begins simultaneously at this temperature. Below 1300 K, these icosahedron-like clusters aggregate abruptly again. Experimental results from differential scanning calorimeter (DSC) and high-resolution transmission electron microscope (HRTEM) further approve the influence of these fragile icosahedron-like clusters on crystallization processes. This finding uncovers the same structural origin that underlies both the formation of bulk metallic glasses and the dynamic anomaly in melts, and demonstrates the dominant role of fragile icosahedron-like clusters in the feature of liquid dynamics.

ACTA Vol. 196, 1 Sept. 2020, P710-722

Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformation after initial stress-induced forward transformation. To this end, utilizing ab-initio simulations and thermodynamic calculations we designed and produced a V₄₅Ti₃₀Ni₂₅ (at%) alloy. In this alloy, TiNi is present as nano-precipitates uniformly distributed within a ductile V-rich base-centered cubic (bcc) β matrix, as well as being present as a larger matrix phase. We characterized the microstructure of the produced alloy using various scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. The bulk mechanical properties of the alloy are demonstrated through tensile tests, and the reversible transformation in each of the TiNi morphologies were confirmed by in-situ TEM micro-pillar compression experiments, in-situ high-energy diffraction synchrotron cyclic tensile tests, indentation experiments, and differential scanning calorimetry experiments. The observed transformation pathways and variables impacting phase stability are critically discussed.

ACTA Vol. 196, 1 Sept. 2020, P733-746

Understanding the deformation mechanisms of hexagonal close-packed (HCP) polycrystals at the grain scale is crucial for developing both macro and micro scale predictive models. Slip and twinning are the two main deformation mechanisms of HCP polycrystals at room temperature. In this paper, the development of grain-level stress tensors during nucleation and growth of twins is investigated. A pure zirconium specimen with HCP crystals is deformed in-situ while the centre-of-mass, orientation, elastic strain, and stress of individual grains are measured by three-dimensional synchrotron X-ray diffraction (3D-XRD). The observed microstructure is subsequently imported into a crystal plasticity finite element (CPFE) model to simulate the deformation of the polycrystal. The evolution of stress in twin-parent pairs at the early stages of plasticity, further into plasticity zone, and unload is studied. It is shown that twins do not relax very much at the nucleation step, but the difference between the measured stress in the twin and parent increases further into plastic zone where twins relax. While at the early stages of plasticity all six twin variants are active, a slightly better estimation of active variants is obtained using the measured grain-resolved stress tensors.

ACTA Vol. 196, 1 Sept. 2020, P747-758

We present an extensive first-principles database of solute-vacancy, homoatomic, heteroatomic solute-solute, and solute-solute-vacancy binding energies of relevant alloying elements in aluminum. We particularly focus on the systems with major alloying elements in aluminum, i.e., Cu, Mg, and Si. The computed binding energies of solute-vacancy, solute-solute pairs, and solute-solute-vacancy triplets agree with available experiments and theoretical results in literature. We consider physical factors such as solute size and formation energies of intermetallic compounds to correlate with binding energies. Systematic studies of the homoatomic solute-solute-vacancy and heteroatomic (Cu, Mg, or Si)-solute-vacancy complexes reveal the overarching effect of the vacancy in stabilizing solute-solute pairs. The binding energy database presented here elucidates the interaction between solute cluster and vacancy in aluminum, and it is expected to provide insight into the design of advanced Al alloys with tailored properties.

ACTA Vol. 196, 1 Sept. 2020, P770-775

Point defects play a crucial role in crystalline materials as they do not only impact the thermodynamic properties but are also central to kinetic processes. While they are necessary in thermodynamic equilibrium spontaneous defect formation in the bulk is normally considered highly improbable except for temperatures close to the melting point. Here, we demonstrate by means of atomistic simulations that processes involving concerted atomic motion that give rise to defect formation are in fact frequent in body-centered cubic metals even down to about 50% of the melting temperature. It is shown that this behavior is intimately related to the anharmonicity of the lattice vibrations and a flat energy landscape along certain crystallographic directions, a feature that is absent in, e.g., face-centered cubic lattice structures. This insight has implications for our general understanding of these materials and furthermore provides a complementary explanation for the so-called anomalous diffusion in group 4 transition metals.

ACTA Vol. 196, 1 Sept. 2020, P776-789

Effects derived from the simultaneous application of passive, uniform saturating magnetic field and tensile stress during long-time annealing utilizing a unique and simple processing tool, the MultiDriver furnace, are reported. In particular, the evolution of the crystalline structure and the magnetic domain configuration of severely deformed and subsequently annealed equiatomic FeNi alloys were revealed by high-energy synchrotron X-Ray diffraction and Magneto-Optic Kerr Microscopy. These alloys are known to undergo a first-order magnetic phase transformation to a chemically ordered tetragonal (L1₀) structure in astronomical timeframes. MultiDriver-annealed specimens are found to retain a tetragonal crystallographic state and texture that is similar to that of the precursor deformed state, with a smaller unit cell volume and a larger c/a ratio relative to those of control samples that were annealed under the same conditions but without magnetic field and stress drivers. Magnetic domain images echo these observations: while as-cast FeNi alloys exhibit domain patterns consistent with the presence of a stress-modified cubic magnetocrystalline anisotropy, large areas of domains with uniaxial anisotropy are present in MultiDriver-annealed samples. While no chemical order was demonstrated in these samples, a strategy is proposed for achieving the ordered state by employing a passive magnetic gradient in the MultiDriver furnace to enhance diffusion.