Bcc Alloys Research Articles

Deformation Mechanisms of (100) and (110) Single-Crystal BCC Gum Metal Studied by Nanoindentation and Micropillar Compression

In this paper, small-scale testing techniques—nanoindentation and micropillar compression—were used to investigate the deformation mechanisms, size effects, and strain rate sensitivity of (100) and (110) single-crystal Gum Metal at the micro/nanoscale. It was observed that the (100) orientation exhibits a significant size effect, resulting in hardness values ranging from 1 to 5 GPa. Conversely, for the (110) orientation, this effect was weaker. Furthermore, the yield strength obtained from the micropillar compression tests was approximately 740 MPa for the (100) orientation and 650 MPa for the (110) orientation. The observed deformations were consistent with the established features of the deformation behavior of body-centered cubic (bcc) alloys: significant strain rate sensitivity with no depth dependence, pile-up patterns comparable to those reported in the literature, and shear along the {112}<111> slip directions. However, the investigated material also exhibited Gum Metal-like high ductility, a relatively low modulus of elasticity, and high yield strength, which distinguishes it from classic bcc alloys.

A single-phase Nb25Ti35V5Zr35 refractory high-entropy alloy with excellent strength-ductility synergy

Refractory high entropy alloys (RHEAs) are promising candidates for high-temperature structural materials due to their excellent high-temperature performance. However, the room-temperature brittleness of most alloys results in poor plastic formability, thereby limiting their engineering applications. In this study, Nb25Ti35V5Zr35 alloy with cold-rollability was developed, and its phase stability, tensile mechanical properties, elastic properties, strengthening mechanism, and deformation mechanism were investigated by combining experiments, CALPHAD, DFT and molecular statics (MS) methods. The results demonstrate that Nb25Ti35V5Zr35 alloy possesses good phase stability, with both the as-cast and annealed states maintaining a single-phase BCC structure without undergoing phase transformation. The tensile yield strength of the alloy reaches its peak at 1152 MPa in the cold-rolled condition, while its annealed state exhibits an optimal balance of strength and plasticity, with a yield strength of 836 MPa and an elongation of 22.45 %, respectively. Solid solution strengthening is the primary source of its high strength, with V and Zr playing key roles in this strengthening mechanism. Dislocation slip plays a dominant role in plastic deformation. Additionally, compared to traditional BCC alloys, the significance of edge dislocations in the deformation process is notably enhanced, with the {112} plane serving as the preferred slip plane for dislocations. DFT results indicate that the alloy exhibits significant anisotropy in both Young's modulus and shear modulus. This work contributes to understanding the material properties of the Nb25Ti35V5Zr35 alloy and provides a reference for the design of new ductile RHEAs.

Plasticity and strength of an equiatomic and a non-equiatomic HfNbTaTiZr high entropy alloy during uniaxial loading : a molecular dynamics simulation study

Abstract Understanding plasticity and strength of high entropy alloys of HfNbTaTiZr is extremely significant in building nuclear reactors, gas turbines, aerospace devices etc. Here we study an equiatomic (Hf0.20-Nb0.20-Ta0.20-Ti0.2-Zr0.20) and a non-equiatomic (Hf0.35-Nb0.20-Ta0.15-Ti0.15-Zr0.15) mixture of two alloys under uniaxial tensile loading from molecular dynamics simulations. Modified Embedded atom potential is used to model both these bcc alloys and all simulations are performed at 300 K with three different tensile strain rates–0.0002, 0.0005 and 0.001 ps−1. Radial distribution functions, bond-orientational parameters and OVITO are used to analyse the MD trajectories. At 0.001 ps−1 strain, both these alloys deform similarly, but differences are observed at 0.0005 and 0.0002 ps−1 strains. At these rates, both alloys deform elastically till 3%, thereafter they deform plastically till 15%–20% strain. Yield strengths are comparable in the elastic limit but in the plastic limit non-equiatomic alloy have higher strength. In equiatomic alloy, bcc phase transforms to fcc whereas in non-equiatomic alloy bcc phase transforms to both fcc and hcp. Formation of hcp atoms (50%) decrease the plasticity of the non-equiatomic alloy but increases its strength. We also observe that in both these alloys and at all strain rates, bcc atoms transform to fcc/hcp atoms through an intermediate amorphous like state. Local coordination and orientation of all atoms change similarly in equiatomic mixture. But in non-equiatomic mixture local orientation in Hf, Ti and Zr changes differently compared to Nb and Ta.

Temperature-dependent magnetic properties of bcc and fcc FeNi alloys: A machine learning assisted Monte-Carlo approach

The estimation of exchange integrals in metals and alloys from first-principle calculations yields Curie temperature (TC), which is not in agreement with the experiments. The microscopic exchange interactions of the bcc and fcc FeNi alloys are predicted by training large datasets of random magnetization curves simulated using the atomic scale Monte Carlo method with regression models to accurately reproduce the experimental TC. The estimation of Fe-rich bcc alloys reveals predominant Heisenberg-like Ruderman-Kittel-Kasuya-Yosida (RKKY) type oscillations in Fe–Fe interactions, similar to the behavior in bcc Fe. Furthermore, frustration in Fe–Fe and Fe–Ni/Ni–Fe couplings of fcc Invar alloy results in the reduction of TC. Additionally, the temperature dependence of cubic anisotropy is calculated using constrained Monte Carlo algorithm, and the validity of Callen-Callen 10th power law is demonstrated, which shows large deviations of scaling exponent L≪10 near permalloy composition attributing to dispersion of magnetic easy axis of Fe and Ni.

Comparison of mathematical and supervised machine-learning models for ductile-to-brittle transition in bcc alloys

This study focuses on modelling Ductile-to-Brittle Transition (DBT) curves using various mathematical and supervised machine learning models. Charpy impact energy values are converted to normalized energy values to account for reductions in upper-shelf energy. The research introduces a saturation parameter in mathematical models to capture these variations and examines the influence of alloying elements, microstructure, and neutron irradiation on DBT behaviour in nuclear structural materials. Detailed analyses reveal how fitting parameters vary with these factors and demonstrate that mathematical models’ fitting parameters generally align with observed DBT curve trends. The predictive capabilities of these mathematical models are also compared with those of supervised machine learning models, highlighting the strengths and limitations of each approach in modelling DBT behaviour. An explainable approach is used for interpretation of machine learning models and it is shown that this approach can be effectively used for the influence of various independent parameters on impact energy.

Microstructural Evolution of Tantalum During Deformation and Subsequent Annealing

Microstructure-aware models are necessary to predict the behavior of material based on process knowledge or to extrapolate mechanical properties of materials to environmental conditions which are not easily reproduced in the laboratory, e.g., nuclear reactor environments. Elemental Ta provides a relatively simple BCC system in which to develop a microstructural understanding of deformation processes which can then be applied to more complicated BCC alloys. In situ neutron diffraction during compressive deformation and subsequent heat treatment have been used to monitor the evolution of microstructural features in Ta throughout simulated processing steps. Crystallographic texture and dislocation density are determined as a function of first plastic strain, then temperature. Lattice strains are determined and attributed to stresses at macroscopic, grain and dislocation length scales. The increase of the dislocation density through deformation and subsequent recovery during heat treatment is monitored through the changing diffraction line profile. Also, randomization of the texture is used as a signature of recrystallization. The recovery of dislocations through annihilation is not observed to depend on the initial dislocation density in the range studied here. In contrast, recrystallization is observed to depend strongly on the initially dislocation density.

Destabilizing high-capacity high entropy hydrides via earth abundant substitutions: From predictions to experimental validation

The vast chemical space of high entropy alloys (HEAs) makes trial-and-error experimental approaches for materials discovery intractable and often necessitates data-driven and/or first principles computational insights to successfully target materials with desired properties. In the context of materials discovery for hydrogen storage applications, a theoretical prediction-experimental validation approach can vastly accelerate the search for substitution strategies to destabilize high-capacity hydrides based on benchmark HEAs, e.g. TiVNbCr alloys. Here, machine learning predictions, corroborated by density functional theory calculations, predict substantial hydride destabilization with increasing substitution of earth-abundant Fe content in the (TiVNb)75Cr25-xFex system. The as-prepared alloys crystallize in a single-phase bcc lattice for limited Fe content x < 7, while larger Fe content favors the formation of a secondary C14 Laves phase intermetallic. Short range order for alloys with x < 7 can be well described by a random distribution of atoms within the bcc lattice without lattice distortion. Hydrogen absorption experiments performed on selected alloys validate the predicted thermodynamic destabilization of the corresponding fcc hydrides and demonstrate promising lifecycle performance through reversible absorption/desorption. This demonstrates the potential of computationally expedited hydride discovery and points to further opportunities for optimizing bcc alloy ↔ fcc hydrides for practical hydrogen storage applications.

Exceptional cryogenic-to-ambient impact toughness of a low carbon micro-alloyed steel with a multi-heterogeneous structure

A low-carbon micro-alloyed (LCMA) steel with a body-centered cubic (bcc) crystal structure suitable for extremely low temperatures was developed by overcoming the intrinsic ductile-to-brittle transition in bcc alloys at cryogenic temperatures. The excellent cryogenic-to-ambient impact toughness in the LCMA rolled plate results from its heterogeneous microstructure, which gradually changes from bamboo-like ultrafine grains (∼ 1.1 μm) on the surface to relatively equiaxed coarse grains in the core (∼ 3.4 μm), accompanied by a distinct texture gradient variation. The heterostructured LCMA steel displays a cryogenic impact toughness of ∼200 J/cm2 at 77 K, which is 24 times higher than the coarse-grained LCMA steel. Such high impact toughness of heterostructured LCMA arises from the coordinated deformation mechanisms over different length-scales coupled with delamination toughening. At 77 K, the heterostructured steel plate deforms by forming cellular sub-structures at the core to the surface, which refines the microstructure and promotes hetero-deformation induced (HDI) hardening to improve intrinsic toughening. Moreover, the subsequent delamination process induces extrinsic toughening by shielding and blunting the cracks, with the local plane-stress conditions induced by delamination promoting ductile fracture of the coarse grains in the core regions. This low alloy steel with its heterogeneous microstructure exhibits extraordinary impact toughness at cryogenic temperatures highlights the possibility of materials design strategies for sustainable development.

Investigations on microstructure and properties of five TiZr-based refractory multi-principal element alloys for clinical metallic biomaterials

The development of new materials with high strength, ductility, low modulus, wear resistance, and good biocompatibility remains a requirement in the field of clinical biomaterials. Based on the concept of high-entropy alloys, five TiZr-based refractory multi-principal element alloys (RMPEAs), i.e. MoTiZr, NbTiZr, MoTiVZr, NbTiVZr, and NbMoTiZr were designed and their microstructure, mechanical properties, elastic properties, and biocompatibility were assessed in this study. Results indicate that NbTiZr and NbTiVZr RMPEAs are single-phase BCC alloys, while the other alloys exhibit multiphase structures. In terms of mechanical behaviors, NbMoTiZr RMPEA has the highest hardness (496±15 HV) among the five alloys and also combines a high yield strength and good ductility, with values of 1275 MPa and 21.66%, respectively. According to atomic calculations, the ideal shear strengths of the five alloys on the {110} plane are all lower than those on the {112} plane. Furthermore, NbMoTiZr RMPEA also exhibits a lower Young’s modulus than that of Ti6Al4V, which can reduce “stress shielding” effects. The in vitro biocompatibility evaluation revealed that NbMoTiZr RMPEA can support cell adhesion and proliferation of MC3T3-E1 cells and exhibits good biocompatibility comparable to Ti6Al4V. These results demonstrate the remarkable potential of NbMoTiZr RMPEA for orthopedic bioimplant applications.

Enhanced Hydrogen-Storage Properties of MgH2 Catalyzed via a Cerium Doped TiCrV BCC Alloy

In this work, Ce-doped Ti6Cr14V80 BCC hydrogen-storage alloys have been synthesized as catalysts to enhance the hydrogen-storage performance of MgH2 based on its room-temperature activation features and excellent durability. The Ti6Cr14V80Ce1 alloy was pre-ball milled under a hydrogen atmosphere into a Ti6Cr14V80Ce1Hx hydride. Different amounts of the Ti6Cr14V80Ce1Hx hydride were incorporated into MgH2 by ball milling to obtain the MgH2 + y wt%Ti6Cr14V80Ce1Hx (y = 0, 3, 5, 10, 15) nano-composites. With an optimization doping of 10 wt%Ti6Cr14V80Ce1Hx, the initial dehydrogenated temperature was decreased to 160 °C. Moreover, the composite can rapidly release 6.73 wt% H2 within 8 min at 230 °C. Also, it can absorb 2.0 wt% H2 within 1 h even at room temperature and uptake 4.86 wt% H2 within 10 s at 125 °C. In addition, the apparent dehydrogenated activation energy of the MgH2 + 10 wt%Ti6Cr14V80Ce1Hx composite was calculated to be 62.62 kJ mol−1 fitted by the JMAK model. The capacity retention was kept as 84% after 100 cycles at 300 °C. The ball milled Ti6Cr14V80Ce1Hx transformed from the initial FCC phase structure into a BCC phase after complete dehydrogenation and back into an FCC phase when fullly hydrogenated. A catalyst mechanism analysis revealed that the ‘autocatalytic effect’ originating in Ti6Cr14V80Ce1Hx plays a crucial role in boosting the de-/hydrogenation properties of MgH2. This work provides meaningful insights into rational designs of nano-compositing with different hydrogen-storage alloy catalyzed MgH2.

Evolution of Microstructure and Crystallographic Texture in Deformed and Annealed BCC Metals and Alloys: A Review

Dislocation slips, twinning, shear banding (SBs), strain localization, and martensite formation are a few deformation modes that are activated in BCC metals and alloys. Strain, strain rate, and deformation temperature are other parameters that determine the activation of deformation modes in BCC alloys. This review focuses on several BCC alloys, such as beta-titanium (β-Ti), tantalum (Ta), and ferritic stainless steels (FSSs), all of which exhibit differences in deformation behavior. These alloys often undergo thermo-mechanical processing (TMP) to enhance their mechanical properties. TMP leads to the evolution of deformation-induced products, such as SBs, strain-induced martensite (SIM), strain localizations, and mechanical/deformation twins (DTs) during plastic deformation, while also influencing crystallographic texture. The deformation modes in β-Ti depend upon the stability of the β-phase (i.e., β-stabilizers); low-stability alloys show the formation of SIM along with slips and twins, whereas in highly stable β-Ti alloys, only slip+twin modes are observed as the primary deformation mechanisms. In the case of Ta, slip activity predominantly occurs on {110} planes, but it can also occur on planes with the highest resolved shear stress. The breakdown of Schmid’s law or non-Schmid behavior for Ta and Ta-W alloys has been discussed in detail. The cold rolling (CR) of FSSs results in the formation of ridges, which is an undesirable phenomenon leading to very low formability. The microstructures of the rolled sheets consist of elongated ferrite grains with in-grain SBs, which are preferentially formed in the γ-fiber-oriented grains. The formation of finer grains after recrystallization improves both the mechanical properties and ridging resistance in FSS. Therefore, this review comprehensively reports on the impact of TMP on the microstructural and crystallographic texture evolution during the plastic deformation and annealing treatment of β-Ti, Ta alloys, and FSSs in BCC materials, using results obtained from electron microscopy and X-ray diffraction.

Tailoring the non-coherent interfacial precipitation to overcome the trade-off between strength-ductility and impact energy release in Ti–Zr–Hf–Nb–Ta high entropy alloy

The energetic high-entropy alloys have brought a new idea for the development of new energetic structural materials (ESMs) with high energy density and excellent mechanical properties. However, the energetic high-entropy alloys exhibit a significant trade-off between strength-ductility and impact energy release efficiency. Here, we develop a new strategy to introduce non-coherent interfacial nanoprecipitates into the Ti–Zr-Hf-Nb-Ta BCC energetic high-entropy alloy, which can not only exert the precipitation strengthening mechanism to maintain excellent mechanical properties, but also transform the local shear fracture into multiple cracks along the grain boundary to promote energy release. The nanoprecipitation-strengthened energetic high-entropy alloy achieved a four-times greater fireball area generated by impact energy release than that of the single-phase BCC alloy, while maintaining an excellent mechanical property combination of high dynamic compression strength (1787 MPa) and high fracture strain (40 %). The results provide new insights into the development of energetic high-entropy alloys with superior impact energy release and strength-ductility synergy.

Entropy-Enthalpy Compensation in Ti-V-Mn-Cr BCC Alloys Used as Hydrogen Storage Materials

In this paper, we report the effect of the Cr/Mn ratio on the thermodynamic properties of Ti30V60Mn(10−x)Crx (x = 0, 3.3, 6.6 and 10) + 4 wt.% Zr alloys. It was found that the enthalpy and entropy change with the Cr/MN ratio and that the entropy and entropy variation is coupled in an enthalpy-entropy compensation fashion. Using a compensation quality factor, it was established that the enthalpy-entropy compensation is not due to a statistical origin, with a confidence of more than 95%.

Transformation- and twinning-induced plasticity in phase-separated bcc Nb-Zr alloys: an atomistic study

Several high-temperature body-centered cubic (bcc) structural materials such as Nb-, Zr- and Ti-based alloys undergo phase separation, which is a second-order phase transformation, whereby the host lattice decomposes into distinct bcc domains with different compositions. Using atomistic simulations, we studied the high-strain-rate response of bcc-forming Nb–xZr (x = 0, 25, 50 at.%) alloys. To induce phase separation in our starter alloy, we first employed hybrid Monte Carlo/Molecular Dynamics simulations in single crystals of Nb–xZr at 1000 K. Subsequently, these crystals were deformed along different crystallographic orientations (⟨001⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 001\\rangle$$\\end{document}, ⟨110⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 110\\rangle$$\\end{document} and ⟨111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 111\\rangle$$\\end{document}) at a strain rate of 10+8s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{+8} s^{-1}$$\\end{document}, to investigate orientation dependent mechanical response. The phase-separated Nb–xZr microstructures exhibited distinct bcc domains enriched in either Zr or Nb. Notably, Nb-50 at.%Zr contained coarser Zr-domains compared to Nb-25 at.%Zr. The Zr-rich domains acted as “soft” inclusions, resulting in reduced peak strengths in the following order: pure Nb (Nb-0 at.%Zr) > Nb-25 at.%Zr > Nb-50 at.%Zr. This implies that phase separation causes softening in Nb–xZr. We also discovered two deformation pathways that depended on the crystallographic orientation: (i) For deformation along ⟨110⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 110\\rangle$$\\end{document} and ⟨111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 111\\rangle$$\\end{document} directions: Elastic deformation was followed by dislocation plasticity on {110}⟨111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\{110\\}\\langle 111\\rangle$$\\end{document} slip systems; and (ii) For deformation along ⟨001⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 001\\rangle$$\\end{document} direction: Elastic deformation was followed by the formation of a volumetric fcc structure, twinning on {112}⟨111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 111\\rangle$$\\end{document} system, and the formation fcc-phase at the twin/matrix interfacial regions. This was ultimately accompanied by dislocation plasticity on {110}⟨111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\{110\\}\\langle 111\\rangle$$\\end{document} slip system. The bcc→\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rightarrow$$\\end{document}fcc displacive transformation facilitated {112}⟨111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 111\\rangle$$\\end{document} twinning when Nb–xZr was deformed along ⟨001⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle 001\\rangle$$\\end{document}. Our investigation shows that softening of bcc alloys can result from a coupling of mechanisms involving local solute segregation, displacive phase transformation and twinning occurring across multiple slip planes.

Effect of irradiation-induced strength anisotropy on the reorientation trajectories and fragmentation behavior of grains in BCC polycrystals under tensile loading

We present a combined experimental and computational study exploring the effects of strength anisotropy on the grain reorientation trajectories in neutron-irradiated BCC polycrystals subjected to uniaxial tensile loading. We observe through in situ high-energy X-ray diffraction microscopy measurements of a model BCC alloy, Fe-9wt.%Cr, that the grains in irradiated samples exhibit reorientation trajectories that deviate more substantially from classical expectations than those in the unirradiated counterpart. We hypothesize that irradiation-induced strength anisotropy is a major influence on this behavior. Utilizing crystal plasticity finite element modeling, we isolate the effects of strength anisotropy by performing a suite of simulations in which we systematically strengthen select slip systems. Reorientation trajectories are compared against a datum of Taylor model predictions, and the deviation from classical expectations is analyzed through the lens of slip activity and availability. We further describe observations regarding the propensity of samples with high degrees of strength anisotropy to exhibit grain fragmentation. Overall, computational results provide insight on and quantification of the effects of strength anisotropy on reorientation trajectories and grain fragmentation, and align well with experimental observations, suggesting strength anisotropy as a plausible contributing mechanism to the observed phenomena.

HARDENING OF LIGHTWEIGHT MULTI-PRINCIPAL ELEMENT TITANIUM-BASED ALLOY UNDER AR ION IRRADIATION

Among new prospective materials multi-principal element alloys (MPEA) have attracted considerable attention in recent years due to their excellent corrosion and irradiation resistance as well as their good mechanical properties over a wide temperature range. The new lightweight multi-principal element titanium-based alloy 61Ti-10Cr-7Al11V-11Nb (at. %) with high ductility at room and elevated temperatures is studied. This single-phase bcc alloy was irradiated with 1.4 MeV Ar ions at room temperature and mid-range doses from 1 to 10 displacements per atom. The effect of irradiation is studied by examining the hardening. A comparison was performed with irradiation-induced hardening behaviour of MPEA, 316 austenitic stainless steel irradiated under an identical condition. It was shown that hardness increases with irradiation dose for all the materials studied, but this increase is lower in multi-principal element alloys both face-centered cubic (FCC) and body-centered cubic (BCC) structures than in stainless conventional steel.

Origin of Deformation Twinning in bcc Tungsten and Molybdenum.

Twinning is profuse in bcc transition metals (TMs) except bulk W and Mo. However, W and Mo nanocrystals surprisingly exhibit twinning during room temperature compression, which is completely unexpected as established nucleation mechanisms are not viable in them. Here, we reveal the physical origin of deformation twinning in W and Mo. We employ density functional theory (DFT) and a reduced-constraint slip method to compute the stress-dependent generalized stacking fault enthalpy (GSFH), the thermodynamic quantity to be minimized under constant loading. The simple slipped structures and GSFH lines show that compressive stresses stabilize a two-layer twin embryo, which can grow rapidly via twinning disconnections with negligible energy barriers. Direct atomistic simulations unveil the explicit twinning path in agreement with the DFT GSFH lines. Twinning is thus the preferred deformation mechanism in W and Mo when shear stresses are coupled with high compressive stresses. Furthermore, twinnability can be related to the elastic constants of a stacking fault phase (SFP). The hcp phase may serve as a candidate SFP for the {112}⟨1[over ¯]1[over ¯]1⟩ twinning system in bcc TMs and alloys, which is coincident with the {111}⟨112[over ¯]⟩ twinning in fcc structures.

Ti-Zr-Nb based BCC alloy containing Mo prepared by laser directed energy deposition—ω phase and cellular structure

This study focuses on the fabrication of Ti-Zr-Nb β-Ti alloys via laser direct energy deposition (DED) technique using pre-alloyed powder with and without having Mo traces. Microstructural analysis revealed the presence of cellular structures within the grains, resembling dendritic solidification, although no discernible dendritic segregation was observed. Nano-scale solute element clustering, particularly the formation of Nb depletion pockets, led to the development of the ω phase within the β matrix. Strengthening effects resulting from the presence of the ω phase were confirmed through TEM analysis. The DED-fabricated β-Ti alloy exhibited enhanced strength (∼1 GPa), significant elongation (∼15%), and a low elastic modulus (∼65 GPa), positioning it as a promising candidate for load-bearing biomedical implants.

Chromium-based bcc-superalloys strengthened by iron supplements

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials.

Composition stability of single fcc phase in Cr–Fe–Mn–Ni alloys: First-principles prediction and experimental validation

The relative phase stability of fcc and bcc Cr–Fe–Mn–Ni alloys has been investigated using a combination of density functional theory, cluster expansion (CE), and Monte Carlo (MC) simulations. The MC simulations for fcc and bcc Cr–Fe–Mn–Ni alloys performed using the CE models enabled computing the Gibbs free energies of formation of 1767 fcc and bcc alloys in the whole range of compositions of each element and analyzed as a function of temperature. In order to determine regions of stability of single fcc/bcc structures and atomic fractions of these phases in two-phase regions, we constructed the thermodynamic databases (TDBs) derived from MC simulations with their application in the OpenCalphad calculations. The results obtained using this approach are in qualitative agreement with the available experimental data from the literature and the experiments performed within this work for the samples of [CrFeMn]100−xNix (x = 20, 25 and 35 at.%) alloys synthesized using arc-melting and annealed at 1273 K for 48 h. The analysis of fcc fractions of the near-equiatomic alloy compositions has enabled the identification of the alloys that are predicted to be single fcc phase for a wide range of temperatures.