ACTA Vol. 198, 1 Oct. 2020, P1-9

The aim of the present study is to extend the knowledge about the formation and thermal stability of vacancy-type defects in tungsten under neutron irradiation, thereby mimicking the temperature and neutron flux expected in the ITER divertor. Neutron irradiation of single crystal tungsten, W(100), in the temperature range 600-1200 °C is performed up to 0.12 dpa. Positron annihilation spectroscopy is employed to detect the presence of open volume defects, while hardness tests are applied to relate the irradiation-induced defects with the modification of mechanical properties. Rationalization of the experimental results is enhanced by the application of a kinetic Monte Carlo simulation tool, applied to model the microstructural evolution under the neutron irradiation process. The relation between radiation microstructure and hardness is explained via a dispersed barrier model.

ACTA Vol. 198, 1 Oct. 2020, P10-24

Recent experimental studies reported that the bamboo-grained oligocrystalline shape memory alloy (SMA) microwire showed excellent two-way shape memory effect (TWSME) and elastocaloric effect (eCE) due to the reduced constraints from grain boundary. In this paper, a theoretical model is established to predict the TWSME and eCE of such a kind of SMAs. At single crystal scale, an anisotropic thermo-mechanically coupled constitutive model is proposed to describe the thermo-elastic martensitic transformation based on the crystal plasticity theory. The driving force of martensite transformation, the internal heat release/absorption, and the evolutions of internal variables and temperature are deduced within the framework of irreversible thermodynamics. To calculate the temperature, stress and strain fields in the bamboo-grained oligocrystalline SMA microwire and obtain the overall thermo-mechanical response of the microwire, the proposed constitutive model is implemented into the finite element program ABAQUS/Explicit by writing a user-defined material subroutine. Meanwhile, to reduce the computational cost faced in the fully coupled thermo-mechanical analysis by using the finite element method, a scale transition rule from single crystal scale to macroscopic oligocrystalline one is constructed based on a new proposed sub-region integral method by considering the special geometric characteristics of the microwire. The capability of proposed model to describe the TWSME and eCE of bamboo-grained oligocrystalline SMA microwire is validated by comparing the predictions with the experimental data. In addition, the effect of grain orientation is discussed. The proposed model can provide a theoretical guidance for the optimal design of grain microstructures to obtain the SMAs with excellent TWSME and eCE.

ACTA Vol. 198, 1 Oct. 2020, P35-46

Magnesium (Mg) and its alloys usually show relatively low strength and poor ductility at room temperature due to their anisotropic hexagonal close-packed (HCP) crystal structure that provides a limited number of independent slip systems. Here we report that unique combinations of strength and ductility can be realized in bulk polycrystalline pure Mg by tuning the predominant deformation mode. We succeeded in obtaining the fully recrystallized specimens of pure Mg having a wide range of average grain sizes, of which minimum grain size was 650 nm, and clarified mechanical properties and deformation mechanisms at room temperature systematically as a function of the grain size. Deformation twinning and basal slip governed plastic deformation in the conventional coarse-grained region, but twinning was suppressed when the grain size was refined down to several micro-meters. Eventually, grain boundary mediated plasticity, i.e., grain boundary sliding became dominant in the ultrafine-grained (UFG) specimen having a mean grain size smaller than 1 μm. The transition of the deformation modes led to a significant increase of tensile elongation and breakdown of Hall-Petch relationship. It was quantitatively confirmed by detailed microstructural observation and theoretical calculation that the change in strength and ductility arose from the distinct grain size dependence of the critical shear stress for activating different deformation modes.

ACTA Vol. 198, 1 Oct. 2020, P47-60

Ferritic-martensitic steels are attractive candidates for structural materials in next generation nuclear reactor systems due to their resistance to radiation induced swelling. Cavity and dislocation loop evolution was characterized in dual ion irradiated T91 steel in three separate irradiation campaigns examining single parameter dependencies of temperature, helium co-injection rate, and damage rate. Irradiations resulted in bimodal cavity size distributions across nearly all ranges of experimental parameters. It was determined that irradiation temperature and helium co-injection rate are stronger influences on bubble stability and the transition from bubbles to voids than is the irradiation damage rate. At low helium injection rates all helium is in vacancy clusters that evolve into bubbles or voids. At high helium injection rates, bubbles become saturated with helium resulting in accumulation of helium at other traps such as dislocation loops. At intermediate levels of He that should aid in the nucleation of bubbles and enhance swelling, the high density of sinks in the F-M microstructure suppresses bubble nucleation and therefore, the onset of swelling. At high enough temperatures, helium is only in bubbles as other strong helium traps, such as dislocation loops, did not form. The mechanism of bubble to void transition was found to shift from being driven by the accumulation of helium to the critical bubble at low damage rates to being driven by spontaneous formation by stochastic vacancy fluctuation at high damage rates.

ACTA Vol. 198, 1 Oct. 2020, P61-71

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

ACTA Vol. 198, 1 Oct. 2020, P72-84

The ability to process metallic samples at the sub-micrometer scale raised the strength limits of pure metals to the Giga Pascal (GPa) range. Here, we fabricated Mo nanoparticles with a giant compressive strength surpassing the previous strength records of metallic materials. Round and faceted particles were produced by manipulating the annealing atmosphere during two-stage solid-state dewetting of Mo thin films deposited on sapphire. The round particle underwent a huge elastic deformation before yielding abruptly. Using finite element analysis, we found that the resolved shear stress on a {112}〈110〉 slip system beneath the punch reaches an enormous value of 20±1 GPa at yield, regardless of particle size. The faceted nanoparticles, contrarily, followed a “smaller is stronger” rule, with uniaxial compressive strength of up to 46 GPa for the smallest nanoparticles. Molecular dynamics simulations indicated that the size effect diminishes with increasing roundness of the particle edges. This work demonstrates how shape and size of particles can be manipulated to achieve giant strength.

ACTA Vol. 198, 1 Oct. 2020, P85-99

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr₁₈Fe₂₇Mn₂₇Ni₂₈, Cr₁₅Fe₃₅Mn₁₅Ni₃₅, and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of displacements per atom (dpa) using in-situ transmission electron microscopy. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in face-centered cubic (FCC) CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect production is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr₁₈Fe₂₇Mn₂₇Ni₂₈ and 10 nm in Cr₁₅Fe₃₅Mn₁₅Ni₃₅. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.

ACTA Vol. 198, 1 Oct. 2020, P111-120

We show that molecular dynamics (MD) simulations are capable of reproducing the drag of solute segregation atmospheres by moving grain boundaries (GBs). Although lattice diffusion is frozen out on the MD timescale, the accelerated GB diffusion provides enough atomic mobility to allow the segregated atoms to follow the moving GB. This finding opens the possibility of studying the solute drag effect with atomic precision using the MD approach. We demonstrate that a moving GB activates diffusion and alters the short-range order in the lattice regions swept during its motion. It is also shown that a moving GB drags an atmosphere of non-equilibrium vacancies, which accelerate diffusion in surrounding lattice regions.

ACTA Vol. 198, 1 Oct. 2020, P121-133

The aqueous passivation of a non-equiatomic Ni₃₈Fe₂₀Cr₂₂Mn₁₀Co₁₀ - at.% (Ni₄₀Fe₂₀Cr₂₀Mn₁₀Co₁₀- wt.%) multi-principal element alloy (MPEA) was investigated in 0.1 M NaCl at pH 4 and compared to a conventional binary Ni₇₆Cr₂₄ – at.% (Ni₇₈Cr₂₂–wt.%) alloy. The electrochemical behavior and oxide film characteristics were explored utilizing in-situ electrochemical and ex-situ surface-sensitive techniques. The passive film composition, thickness, and elemental valence states, as well as, the fate of each element were studied by in-situ atomic emission spectro-electrochemistry, ex-situ X-ray photoelectron spectroscopy, and atom probe tomography. The Ni₃₈Fe₂₀Cr₂₂Mn₁₀Co₁₀ MPEA demonstrated slightly better corrosion resistance compared to the binary Ni₇₆Cr₂₄, alloy. Passive films on the MPEA contained primarily Cr, and small amounts of Ni, Fe, Co and Mn, while dissolution of Ni, Fe, Co was observed. Ni⁰ was enriched at the oxide/metal interface while Cr was depleted. Enrichment of Cr in the passive film occurred to a greater extent in the MPEA than for the Ni-Cr binary alloy. Enrichment factors were determined and the origins of enrichment are discussed.

ACTA Vol. 198, 1 Oct. 2020, P168-177

From the beginning of crystal plasticity studies, the cause for the small percentage of expended work retained as stored work has been a quandary. From forensic analyses of plastic deformation studies, it was deduced that coplanar annihilation of dislocations occur since slip takes place in small slip-patch areas of mean slip distance squared. A simple model predicts that the athermal annihilation factor A between expended and stored energy is about 20 comparable to the observed 5% for stored work. The derived A ranged from 19 at 20 K to 25 at 300 K for copper. Using the functional constitutive relation which can replicate the stress-strain diagram, analyses invoking the imposed shear strain to assess the height between coplanar slip planes indicate that structure evolution due to monopole dislocations result in the principle of similitude. However, the break-down of this Stage II structure occurs when cross-slip and the accompanying double cross-slip is incurred resulting in increasing height between slip planes. The consequences of these bypass mechanisms are the change of the random array of dislocations to that of clustered patterns with the increase in mean slip distance leading to cell structure formation. This evolution can be examined using the shear flow stress versus the inverse mean slip distance plot. Hence the bases for decoding work-hardening of face-centred cubic metals using constitutive relations have become possible.

ACTA Vol. 198, 1 Oct. 2020, P178-222

For the past decade, considerable research effort has been devoted toward computationally identifying and experimentally verifying single phase, high-entropy systems. However, predicting the resultant crystal structure(s) “in silico” remains a major challenge. Previous studies have primarily used density functional theory to obtain correlated parameters and fit them to existing data, but this is impractical given the extensive regions of unexplored composition space and considerable computational cost. A rapidly developing area of materials science is the application of machine learning to accelerate materials discovery and reduce computational and experimental costs. Machine learning has inherent advantages over traditional modeling, owing to its flexibility as new data becomes available and its rapid ability to construct relationships between input data and target outputs. In this article, we propose a novel high-throughput approach, called “ML-HEA”, for coupling thermodynamic and chemical features with a random forest machine learning model for predicting the solid solution forming ability. The model can be a primary tool or integrated into existing alloy discovery workflows. The ML-HEA method is validated by comparing the results with reliable experimental data for binary, ternary, quaternary, and quinary systems. Comparison to other modeling approaches, including CALPHAD and the LTVC model, are also made to assess the performance of the machine learning model on labeled and unlabeled data. The uncertainty of the model in predicting the resultant phase of each composition is explored via the output of individual predictor trees. Importantly, the developed model can be immediately applied to explore material space in an unconstrained manner, and is readily updated to reflect the results of new experiments.

ACTA Vol. 198, 1 Oct. 2020, P223-229

We demonstrate large, crystallographic anisotropy in nonequilibrium solute capture. Under identical conditions, differently oriented grains of a Ni-30Cr (wt.%) alloy were oxidized at 600°C for 60 s. For a Ni(Cr) (100) oriented grain, a solute captured rock salt oxide formed with a cube-cube epitaxial orientation. In contrast, for a Ni(Cr) (111) oriented grain, a solute captured corundum oxide formed with the (0001) basal plane parallel to (111). There are clear indications from the morphologies that are present – islands growing outwards for rock salt and islands growing inwards for corundum – that the oxide growth is dominated by kinetics, not thermodynamic factors. Since the interfacial conditions for nonequilibrium solute capture differ for cases where the metal/oxide boundary is moving (the (111) case) and where it is not (the (100) case), these results indicate that crystallographic anisotropy will have substantial effects which cannot be ignored in oxidation or corrosion.

ACTA Vol. 198, 1 Oct. 2020, P230-241

Even though it is widely accepted that the Laves phase is a promising strengthener for chromium-containing stainless steels, there is little information available on its microstructural evolution and growth. Therefore, in this study, we analysed the behaviour of Laves (C14) precipitates in high-Cr ferritic stainless steel, Fe-20Cr-2Mo-0.5Nb (at%), annealed at 1073 K for different time periods. A high density of (Fe,Cr)₂(Mo,Nb) precipitates was formed in the α matrix during annealing. Morphological evolution in the Laves phase followed the sequence of spheroidal → faceted ellipsoidal → faceted needle-like → faceted plate-like. The Laves phase is related to the α matrix by two sets of orientation relationships, viz. OR1:  [-1-12] _α/[1-210]_L、[111]_α/[0-110]_L、and [1-10]_α/[000-1]_L，and OR-2: [1-10]_α/[0001]_L、[113]_α/[-1-120]_L、and [-3-32]_α/[1-100]_L. The habit plane of the Laves phase was found to deviate slightly from the plane. Interfacial defects, namely disconnections (bD, h) and super disconnections (mbD, mh), observed experimentally along the habit plane, were consistent with theoretical results based on OR-2 as the reference coordinate frame. The Burgers vector b^D exhibited an edge component by^D (0.1539 nm) parallel to Y/[113]_α/[-1-120]_L and a small normal component bz^D (0.0747 nm) perpendicular to the terraces; meanwhile, the overlap step height h =d[1-100]_L. The morphological evolution of Laves precipitates was described based on the lateral motion of disconnections and super disconnections on {-3-32}_α/{1-100}_L terraces in the direction. Arrays of the disconnections (bD, h) were found to be capable of accommodating the misfit strain on {-3-32}_α/{1-100}_L terraces.

ACTA Vol. 198, 1 Oct. 2020, P242-256

Variant selection during the A1→L1₀ transformation in a polycrystalline red gold alloy close to equiatomic Au-Cu composition has been extensively studied by Electron Backscatter Diffraction (EBSD) in our previous work. The use of a mathematical description of the lattice distortion and the maximal work criterion allowed us to quantify the degree of selection. With the same approach, we investigate here an interesting shape distortion effect, discovered twenty years ago in equiatomic AuCu-Ga. The shape distortion of thin samples placed in bending condition and then heat-treated under stress is studied in details. The singular shape memory effect and the remarkable distortion amplification, which we call TADA effect, are explored by monitoring the sample radius of curvature and the advancement of the transformation. The underlying mechanisms of variant selection are revealed by EBSD analysis across the samples. The experimental crystallographic variant selection distribution is compared with the expected profile calculated with the Euler-Bernoulli beam theory. The good agreement demonstrates that variant selection during the transformation is at the origin of the macroscopic distortion of red gold alloys. The TADA effect was found to occur when external stresses are released, and strongly depends on the stress at the initial stage of the transformation. This unusual effect is assumed to result from the persistence of variant selection throughout the transformation.

ACTA Vol. 198, 1 Oct. 2020, P258-270

The formation of κ-carbides in austenite Fe-30Mn-9Al-1.2C (wt. %) lightweight steels is tuned via alloying of Si (0, 1, 2 wt. %), an element that can remarkably raise the activities of Al and C based on thermodynamic calculations. Ordered L1₂ nano-domains (with a size <1 nm), lacking elemental partition, were observed in the solution-treated steel without Si alloying, while with the increase of Si to 2 wt. %, cuboidal L′1₂ intragranular κ-carbides were well developed with an average size of 11.5 nm and a volume fraction of 25.9 %. These κ-carbides found in the solution-treated steel with 2 wt. % Si follow a different precipitation route from previous pathways that require aging. Also, particle-shaped L′1₂ intergranular κ0-carbides and DO₃ phase were formed at austenite grain boundaries in the steel with 2 wt. % Si. The precipitation of κ-carbides in grain interiors leads to an improvement of the yield strength from ~450 MPa to ~950 MPa as the Si content increases from 0 to 2 wt. %. The primary deformation mechanism is the formation of slip bands in all three steels, which involves the shear of ordered nano-domains or κ-carbides. The uniform distribution of the slip bands is essential for the high strain hardening, provided by the dynamic slip band refinement in the steel without Si. Lower strain hardening is seen in the steel with 2 wt. % Si due to the formation of localized coarse slip bands. These findings offer valuable insights into the design of high-performance lightweight steels.

ACTA Vol. 198, 1 Oct. 2020, P271-280

Low thermal expansion alloy plays a unique role in high precision instruments and devices owing to its size stability under thermal shocks. However, a low thermal expansion generally produces a poor mechanical performance, such as brittleness and low fracture resistance, which is a bottle-neck for their applications as functional materials. Here, we demonstrate a novel intermetallic compound-based dual-phase alloy of Er-Fe-V-Mo with excellent structural and functional integrity achieved by precipitating a ductile phase in the hard-intermetallic matrix with large magnetovolume effect. It is found that the compound with 12.8 ± 0.1vol% precipitate phase improves the alloy's strength and toughness by one order of magnitude, while keeping a low bulk coefficient of thermal expansion (1.87±0.02 × 10⁻⁶ K ^− 1) over a wide temperature range (100 to 493 K). The combined analyses of real-time in-situ neutron diffraction, synchrotron X-ray diffraction, and microscopy reveal that both the thermal expansion and the mechanical properties of the precipitate phase are coupled with the matrix phase via semi-coherent interfacial constraint; more importantly, the precipitate phase undergoes a pronounced strain hardening with dislocation slips, which relieves the stress localization and thus hinders the microcrack propagation in the intermetallic matrix. Moreover, the alloys are easy to fabricate and stable during thermal cycling with great application potentials. This study shed light on the development of low thermal expansion alloys as well as the implications to other high-performance intermetallic -compound-based material design.

ACTA Vol. 198, 1 Oct. 2020, P281-289

Liquid metal surfaces have gained increased interest over the last decade due to new applications in synthesis of 2D materials, catalysis, or fusion reactors. Static properties such as the reflectivity and density profile have been determined, both experimentally and computationally, for numerous liquid metals and alloys. However, the characterization of the dynamic properties has remained a challenging task and only one experimental study by Reichert et al. has evaluated the depth-dependence of different dynamic properties in the liquid indium (l-In) surface. In this paper, we present an ab inito molecular dynamics study of the collective dynamic properties of this same system at different depths, obtaining very good agreement with the experimental data. In addition, we are able to compute the properties much closer to the surface than experimentally attainable, and have discovered that at these shallower depths, the properties drastically differ from those deeper in the slab. Therefore, this study sheds light into the behavior of dynamic properties at the atomic interface and highlights the ability of ab initio molecular dynamics to study such unknown dynamic behavior of liquid metals surfaces at depths not yet attainable experimentally but of crucial importance for liquid surface physics.