Body-centered Cubic Solid Solution Research Articles

Achieving room temperature hydrogen storage reversibility in Nb-rich alloys of the Nb-Cr-Mn system

Recent developments in the design of body-centered cubic (BCC) multicomponent alloys via computational tools have demonstrated the possibility of obtaining alloys with excellent hydrogen storage behavior. In this work, we employ the CALPHAD (Calculation of Phase Diagrams) method to design Nb-rich alloys of the Nb-Cr-Mn system that present hydrogen storage reversibility at room temperature under moderate pressure conditions. We employ the valence electron concentration (VEC) factor as a compositional guide to select compositions with suitable thermodynamic properties. Using electric arc melting, we synthesize two alloys, namely Nb85Cr10Mn5 and Nb70Cr20Mn10, both forming predominant BCC solid solutions with VEC ∼ 5.2 and minor amounts of a eutectic microconstituent composed of BCC and Laves C14 phases. Both alloys are easily hydrogenated at room temperature without the need for an activation treatment. The Nb85Cr10Mn5 alloy reaches a storage capacity of 2.1 wt% of H (298 K; Peq∼ 20 bar) whereas the Nb70Cr20Mn10 alloy reaches a capacity of 1.4 wt% (298 K; Peq∼ 21 bar). Benefits in the storage kinetic performance are correlated with the BCC + C14 microstructure. Pressure-Composition-Temperature (PCT) diagrams show moderate values of equilibrium pressure for hydrogen storage reversibility at room temperature. Room temperature absorption/desorption cycling measurements demonstrated a reversible capacity of 1.2 wt% of H (Peq∼ 29 bar) for the Nb85Cr10Mn5 alloy and 0.8 wt% of H (Peq∼ 31 bar) for the Nb70Cr20Mn10 alloy after twenty cycles.

Construction of hard BCC phases FeCrVMnTix wear-resistant high-entropy alloy coatings enhanced by solid solution of Ti elements

To address the complex working conditions encountered by 1Cr13 ferritic/martensitic steel during operation, it is imperative to develop high-entropy alloys (HEAs) protective coatings with enhanced friction resistance. In this work, guided by the valence electron concentration criterion, we constructed a single-phase body-centered cubic (BCC) structure with high hardness. This was achieved through the introduction of titanium (Ti), which has a significantly different atomic radius compared to other elements. FeCrVMnTix (x = 0, 0.5, 1.0, 1.5, and 2.0) coatings were fabricated on 1Cr13 steel surfaces using laser cladding techniques, and the microstructure, mechanical properties, and wear-resistance properties of Ti-doped HEA coatings were investigated. The results reveal that the FeCrVMnTix coatings are comprised of a BCC solid solution. Increasing Ti content refined the dendritic structure of the coatings, enhancing the Vickers hardness from 4.35 GPa to 8.27 GPa and the Young's modulus from 232.6 GPa to 273.2 GPa. Additionally, the wear rate decreased from 3.07 × 10−5 mm3·N−1·m−1 to 4.94 × 10−6 mm3·N−1·m−1. Notably, the FeCrVMnTi2.0 coating demonstrated excellent wear resistance that can be attributed to the solid solution strengthening effect of Ti. These findings underscore the pivotal role of Ti in enhancing the wear resistance of the coatings.

Oxygen-induced decomposition of the body-centered cubic HfNbTaTiZr high-entropy alloy

The factors affecting the stability of an initially single-phase body-centered cubic (BCC) HfNbTaTiZr solid solution are investigated by heat treatments at 900 °C up to 1000 h in different atmospheres. The BCC solid solution is stable but oxygen (O) contamination originating from the aging atmosphere leads to the co-precipitation of an O-enriched hexagonal close-packed (HCP) Hf-Zr-rich phase and a second Nb-Ta-rich BCC phase. As O from the atmosphere diffuses from the surface to the core of the sample at a much faster rate along grain boundaries than through the grain interior, this results in an inhomogeneous distribution of precipitates, which formed at grain boundaries by a monotectoid reaction. In contrast, coincidence-site-lattice boundaries are not affected by this transformation owing to their lower diffusivities and lower capacity for heterogeneous nucleation. Similar phases were observed in a HfNbTaTiZr enriched with 3 at.% O, thus confirming the role of oxygen in phase stability.In both alloys, the orientation relationship (OR) between the HCP and BCC phases is predominantly of Burgers type but our analyses further revealed the presence of two minor ORs. More interestingly, both alloys exhibit rod-shaped HCP precipitates in contrast to the plate-shaped precipitates observed in conventional Ti and Zr-based alloys. This result can be rationalized by calculations of elastic strain energy density, which suggest that the rods should grow along 〈100〉BCC to minimize the elastic strain energy. Deviations of up to 20° from this theoretical direction were observed and are likely related to relaxation by misfit dislocations or by plastic deformation.

Optimizing Al content to eliminate the brittle phase in lightweight TiZrNbTa0.1Alx refractory high-entropy alloys

Body-centered cubic (BCC) lightweight refractory high-entropy alloys (LWRHEAs) with Al contents have attracted much attention due to their low density and excellent mechanical properties. However, these typical lightweight alloys often suffer from poor room temperature plasticity. In this study, we prepared TiZrNbTa0.1Alx LWRHEAs by using a high-vacuum arc-melting technique and investigated the influence of Al content on the phase structures and mechanical properties. It was found that the TiZrNbTa0.1Al1 alloy showed a BCC solid solution matrix with some micrometer-sized Al3Zr5 precipitates and exhibited a density of 6.110 ± 0.003 g/cm3. The TiZrNbTa0.1Al1 alloy had a low mixed enthalpy of −20.831 kJ/mol, a compressive yield strength of 1037 ± 178 MPa, and a fracture plasticity of ∼6%. As a result of reducing the Al content, the TiZrNbTa0.1Al0.2 alloy showed a simple BCC phase structure without any precipitates and maintained a low density of 6.743 ± 0.008 g/cm3. The TiZrNbTa0.1Al0.2 alloy had a relatively high mixed enthalpy of −4.5577 kJ/mol, a high yield strength of 1022 ± 51 MPa, and a plasticity of >70%. The TEM analysis results demonstrated that the excellent mechanical properties of this LWRHEA were mainly attributed to the reducing Al content, which could elevate the mixed enthalpy of the alloy to eliminate the brittle Al3Zr5 phase and induce the formation of dense network dislocations at the grain boundaries.

Effect of industrial grade raw materials and its associated impurities on first hydrogenation of BCC solid solution alloys

This study explores the effects of using industrial-grade raw materials on the hydrogen storage properties of body-centered cubic (BCC) solid solution alloys. Industrial scrap metals were chosen as the raw material, and the desired alloys were synthesized using a vacuum arc melting furnace. Instead of using pure vanadium (V), different inexpensive industrial substitutes such as low-grade vanadium (V), vanadium nitride (VN), vanadium oxide (VO), and ferrovanadium (FeV) were utilized. The crystal structure, morphology, first hydrogenation, thermodynamics, and cyclability of the prepared alloys were analyzed by using XRD, SEM-EDS, and XPS methods. The crystal structure and morphology analysis revealed multiphase alloys in all the prepared alloys and a Ti rich third phase in the VN sample, which was absent otherwise. The first hydrogenation was performed at room temperature under a hydrogen pressure of 2 MPa without any prior heat treatment. The VO alloy displayed negligible hydrogen absorption during first hydrogenation, while the V, VN, and FeV alloys showed maximum hydrogen storage capacities of 3.49 wt%, 2.47 wt%, and 2.18 wt%, respectively. The presence of oxygen hinders the activation of the VO alloy and requires two activation cycles to activate the sample. A short-term cyclability analysis showed a reversible hydrogen storage capacity of 2 wt%. The change in enthalpy of hydrogen absorption and desorption for V alloy were calculated to be −36.73 ± 8.6 and 57.34 ± 8.3 kJ/mol H2, respectively. One of the prominent advantages of utilizing scrap raw materials was the substantial reduction in overall raw material costs, which saw an impressive 95% decrease when compared to using pure raw materials. These results highlight the economic benefits of incorporating recovered industrial scrap into hydrogen storage alloys.

Effect of Ti content and ball-milling time on the microstructural, thermal, and microhardness properties of AlCoCuNiTi high entropy alloys

In this study, AlCoCuNiTi high entropy alloy was produced by high-energy ball-milling (mechanical alloying) and sintering methods. The microstructural, thermal, and microhardness properties of the milled powders and sintered bulk alloy samples were examined by X-ray diffraction (XRD), field emission scanning electron microscopy with energy dispersive X-ray (FESEM-EDX), differential thermal analyser (DTA), optical microscope (OM), and Vickers hardness tester. XRD results revealed that the intermetallic and solid solution phases which consist of body-centered cubic (BCC) and face-centered cubic (FCC) structured phases were formed after 80–120 h of milling. It was found that the crystallite size decreased to ⁓5.67 nm, the lattice strain increased and the dislocation density increased to ⁓31.14 × 1015/m2 after the increase in Ti ratio and milling up to 120 h in the produced samples. FESEM/EDX analyses also revealed that as the mechanical milling time increased, fracture-fragments, welds, re-fractures, and agglomerations occurred, the particle size decreased to a minimum of 8 μm, and a more homogeneous structure containing spherical-shaped particles was formed. DTA curves showed an exothermic peak indicating the crystallization of the Al–Cu-based FCC solid solution phase between 550 °C and 600 °C temperatures in the samples milled up to 30 h, while it did not show any reaction peak in the samples milled up to 120 h. Additionally, it was observed that the mechanically milling time and sintering, and adding Ti led to an increase in the microhardness of the bulk alloy samples due to the deformation hardening, and the appearance of new intermetallic phases and FCC + BCC solid solution phases. Finally, after 120 h of milling and sintering at 815 °C, the maximum microhardness of the 8 % Ti-doped AlCoCuNiTi alloy was measured as 882 ± 20 HV1, which indicates that its mechanical properties are improved.

Effect of Aluminum Addition on the Microstructure, Magnetic, and Mechanical Properties of FeCrCoNiMn High-Entropy Alloy

High-entropy FeCoCrNiMn (C1) and FeCoCrNiMn10Al10 (C2) alloys (HEAs) were mechanically alloyed for 24 h and heated to 900 °C (C1_900 °C and C2_900 °C). The powders were also compacted into pellets (C1_pellet and C2_pellet) and sintered at 500 °C for 1 h. Crystal structure, microstructure, magnetic, and mechanical properties were investigated by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometry, and microindentation. During the milling process, a mixture of body-centered-cubic (BCC) and face-centered-cubic (FCC) phases with a crystallite size in the range of 9–13 nm was formed in the C1 HEA alloy. The dual FCC + BCC solid solutions remain for the C1_pellet and transform to a single FCC for the C1_900 °C powders. Al addition stabilizes the BCC structure in the FeCoCrNiMn10Al10 HEA alloy, as revealed by the structural refinement. The structure exhibits a mixture of BCC + FCC solid solutions for the C2 powders and BCC + FCC + CrCo sigma phase for the C2_pellet and C2_900 °C powders. The crystallite sizes are in the range of 6-93 nm for all the samples. The saturation magnetization (Ms), coercivity (Hc), and squareness ratio (Mr/Ms) are estimated to be 24.2 emu/g, 153.62 Oe, and 0.165, respectively, for C1 and 28.45 emu/g, 188.48 Oe, and 0.172 for C2. The C1_900 °C and C2_900 °C powders exhibit, respectively, paramagnetic and soft magnetic behaviors and an exchange bias at room temperature. The C1_pellet and C2_pellet HEAs show high hardness values of 584.85 Hv and 522.52 Hv, respectively.

Designing Nuclear Fuels with a Multi-Principal Element Alloying Approach

Previous research has shown that multi-principal element alloys (MPEAs) using chromium, molybdenum, niobium, tantalum, titanium, vanadium, and zirconium can form stable body-centered-cubic (BCC) structures across a large temperature region (25°C to 1000°C). This is the same crystal structure as γ-uranium (U), which has shown desirable thermal and irradiation behavior in previous alloy fuel research. It is hypothesized then that the MPEA alloying approach can be used to produce a stable BCC uranium-bearing alloy and to retain its stability throughout anticipated operating regimes of power-producing reactors. Candidate elements were assessed using Monte Carlo N-Particle (MCNP) analysis to determine uranium densities necessary to make the alloy an economically viable fuel compared to conventional fuel forms. Following neutronic considerations, materials property databases and empirical predictors were used to determine the compositions with a high potential to form a BCC solid solution alloy. The final four alloys were MoNbTaU2, MoNbTiU2, NbTaTiU2, and NbTaVU2, which were cast using arc melting of raw elemental foils and chunks. Characterization of the fabricated alloys included scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and transmission electron microscopy. The results showed a two-phase system with a U-rich matrix phase surrounding the refractory precipitates. The U phase was found to contain varying concentrations of the alloying elements and was a BCC γ-U phase. These results warrant further research to identify ideal compositions for use as an advanced alloy fuel.

Lightweight refractory high entropy alloys with excellent specific strength and enhanced malleability by in-situ heterogeneous structure

A novel category of lightweight Tix(x=0.5–2)AlV0.5CrMo refractory high-entropy alloys (RHEAs) have been designed by tailoring the in-situ forming of heterogeneous structures during spark plasma sintering (SPS). The effect of Ti on the microstructure evolution and mechanical properties was thoroughly investigated. All the as-milled Tix(x=0.5–2)AlV0.5CrMo powders were found to present a single body-centered cubic (BCC) phase structure. At high temperature above 1200 °C, however, the disordered BCC2 and hexagonal Al2O3 phases formed from the metastable BCC solid solution. The grain sizes of both the BCC and the newly-formed BCC2 and Al2O3 phases were in the range of 0.13–0.41 μm. By referring to the empirical parameters and non-equilibrium solidification theory, the phase transformation in the SPS sintering process was discussed in detail. Mechanical tests showed that the Tix(x=0.5–2)AlV0.5CrMo RHEAs possessed ultra-high specific strength, due to the solid solution strengthening, grain size refinement strengthening, precipitation strengthening and hetero-deformation induced (HDI) hardening. In particular, the Ti1.5AlV0.5CrMo RHEA presented an excellent specific compressive yield strength as high as 385.3 MPa.cm3/g combined with an enhanced plastic strain of 10.6%. The current research turned out to be a new strategy for preparing RHEAs with heterogeneous structures to achieve the strength- malleability trade-off.

Dry sliding wear and friction behavior of powder metallurgy FeCoNiAlSi0.2 high-entropy alloys

Wear behaviors of the FeCoNiAlSi0.2 high-entropy alloy (HEA) and Si-free FeCoNiAl during the reciprocating sliding wear process were evaluated for automotive piston rings. The HEAs were prepared using on advanced powder metallurgy route to solve the inherent shrinkage porosity and segregation defects during casting, poor densification, and wear resistance of HEAs while reciprocating motion. The morphology, hardness, wear, and friction properties were investigated. FeCoNiAl had biphasic face-centered cubic (FCC) and body-centered cubic (BCC) solid solution structures, whereas FeCoNiAlSi0.2 HEAs had a single BCC solid solution structure. Si addition significantly enhanced the hardness and wear resistance of the HEAs under reciprocating conditions because of the increased solid solution strengthening and lattice distortion effects. The maximum hardness of the FeCoNiAlSi0.2 HEA reached approximately 582 HV, which is higher than that of the equiatomic FeCoNiAl HEA (491 HV). The findings depict that FeCoNiAlSi0.2 maintains better tribological behavior than FeCoNiAl HEA against the tungsten carbide (WC) ball indenter. Oxidative delamination and cracking occurred in the FeCoNiAl/WC pair, whereas a mixed adhesive–abrasive wear mechanism was prominent in the FeCoNiAlSi0.2/WC pair because of the harder BCC solid solution phase. This study explores an effective strategy for enhancing the tribology behavior of automotive piston structures under varying loads and reciprocating dry sliding motions.

Effect of atomic size mismatch and chemical complexity on the local lattice distortion of BCC solid solution alloys

The effects of atomic size mismatch and chemical complexity on the local lattice distortion of solid-solution alloys VNbTa, TiVNbMo, TiVNbMoTa and TiVNbMoTaWRe with body-centered cubic (BCC) structure are quantitatively studied with time-of-flight neutron total scattering, extended X-ray absorption fine structure (EXAFS) measurements and first principles calculations. Neutron atomic pair distribution function (PDF) measurements found that the local lattice distortion in the ternary solid-solution alloy VNbTa is 1.1 %, obviously larger than that of the most complicated solid-solution alloy (0.27% for TiVNbMoTaWRe). Our results suggested that atomic size mismatch in high entropy alloys is more critical to the local lattice distortion than chemical complexity. Both EXAFS analysis and theoretical calculations indicate that there is a maximum of ∼4 % difference between the bonding length of different atomic pairs in VNbTa.

Influence of processing route on microstructure and properties of Al13.45FeCrNiCo high entropy alloys

The Al13.45FeCrNiCo high entropy alloys (HEAs) were obtained using two different processing routes, namely (i) mechanical alloying (MA) and spark plasma sintering (SPS), and (ii) vacuum induction melting – copper cast molding. A final homogenizing treatment step was applied to the as synthesized materials. Based on thermodynamic and empirical criteria for phase formation, the alloy was designed to develop a mixture of the face-centered-cubic (FCC) and body-centered-cubic (BCC) solid solution. The contribution of aluminum content on the alloy microstructure in terms of hardening of solid solution and precipitation hardening was highlighted. The influence of the processing route on the microstructure evolution and the relationship between the microstructure and properties of HEA materials were established by performing X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and high-resolution (HR) TEM analyses, as well as microindentation and compression tests. The powder metallurgy processes produced a two-phase structure consisting of at least two types of crystalline grains based on FCC and BCC solid solutions. Increasing the sintering temperature from 950 °C to 1050 °C contributed to structural changes in the materials. Lower sintering temperatures (950 °C and 1000 °C) generated a single FCC phase structure, while at 1050 °C two FCC and BCC phases were obtained without any evidence of the liquid phase sintering. The as cast route generated a hypoeutectic type structure with a very fine and preferentially oriented dendritic structure, in which the interdendritic space is occupied by a second phase arranged in a network where the microstructure appears to be the result of a eutectic transformation with precipitation of secondary phases rich in Ni and Al. All the synthesized alloys revealed high hardness and a good mechanical behavior in compression. Sintered HEA materials exhibited higher Vickers hardness than as cast HEA, even after the heat treatment process, while the as cast HEA material has better ultimate strength and deformation behavior than sintered HEAs. The processing route did not affect the nature of the FCC and BCC solid solution phases formed in the Al13.45FeCrNiCo HEA materials but was responsible for the variation of phase proportion in the HEA’s microstructure, which in turn influenced the mechanical properties.

Phase equilibria of VCrMnFeCo high entropy alloys

The almost infinite compositional space is one of the most attractive aspects of High Entropy Alloys (HEAs), but finding promising compositions is usually a great challenge. The most studied families of HEAs include the 3d transition metals, in which promising face-centered cubic alloys have been found, but few single-phase body-centered cubic (BCC) alloys were designed and produced with these elements. In the present work, a systematic exploratory study was performed within the VCrMnFeCo system, a vast compositional space currently underexplored and prone to form BCC solid solutions. It is shown that the brittle and usually undesirable sigma phase is often observed in this HEA family and, therefore, predicting its formation is essential in alloy design. The phase equilibria were studied in a wide range of compositions by the CALPHAD method and the combination of two empirical methods proposed by Tsai et al. (2013; 2016) for predicting the sigma phase. Six compositions were produced and characterized in the as-cast and annealed condition (1150 ºC, 4 h), namely V42Cr41Mn17, VCrFe, VCrMnFe, V37Mn25Fe38, VCrMnCo, and VCrMnFeCo. The first three alloys presented a single-phase BCC microstructure, while the others were sigma dominant. The CALPHAD and Tsai criteria disagreed on some predictions, and both were partially accurate when compared to the experimental characterization. Considerations on alloy design of 3d transition metal HEAs were discussed, as well as the advantages and limitations of these predictive methods when applied in the proposed system. The simultaneous application of CALPHAD and Tsai criteria is recommended for the sigma-prone 3d transition metal HEAs. This work collaborates to a better understanding of the dimension of compositional fields in 3d transition metal HEAs.

Ti-Zr-Nb based BCC solid solution alloy containing trace Cu and Ag with low modulus and excellent antibacterial properties

A new series of β-type biomedical alloys Ti50Zr25Nb20Cu5−xAgx (x = 0, 1 and 2.5 at%) have been developed. Mechanical properties, antibacterial properties, and biocompatibility of the alloys were systematically investigated. These alloys are body-centered cubic (BCC) phase solid solution with dendritic structure and excellent yield strength and hardness. Meanwhile, the alloys exhibit low Young’s modulus (~61 GPa) which is only half of the Ti-6Al-4 V alloy and close to the level of human bone modulus. Due to the increases of the total content of Cu2+ and Ag+ ions with the Ag partial substitution of Cu, the antibacterial properties of the alloys improves significantly and the antibacterial rate of Ti50Zr25Nb20Cu2.5Ag2.5 alloy is above 99.6%. In vitro experiments demonstrate the biocompatibility of Ti50Zr25Nb20Cu2.5Ag2.5 alloy is comparable to Ti-6Al-4 V alloy. These β-type Ti50Zr25Nb20Cu5−xAgx (x = 0, 1 and 2.5 at%) alloys with superior material mechanical properties, excellent antibacterial properties and good biocompatibility have great potential for antibacterial biomedical implants and associated materials.

Effect of Additive Elements (x = Cr, Mn, Zn, Sn) on the Phase Evolution and Thermodynamic Complexity of AlCuSiFe-x High Entropy Alloys Fabricated via Powder Metallurgy

In this study, equimolar AlCuSiFe-x (x = Cr, Mn, Zn, Sn) HEAs were fabricated by mechanical alloying (MA) and spark plasma sintering methods (SPS). The MA was performed for 45 h followed by densification of powder compacts at 650 °C. The results revealed the formation of dual face-centered cubic (FCC) and body-centered cubic (BCC) structures in AlCuSiFe-x (x = Zn, Sn) while a single BCC solid solution was noticed in AlCuSiFe-x (x = Cr, Mn). After SPS treatment, AlCuSiFeSn alloy contained FCC with CuxSny while AlCuSiFe–Zn changed to FCC + BCC structure. Similarly, AlCuSiFeCr and AlCuSiFeMn showed the formation of BCC + FCC with additional σ- and µ-phases in the HEA matrix. The calculated thermodynamic parameters of HEAs also supported the formation of different solid-solution phases in each of the above HEAs. It was found that HEAs with the additive elements Sn and Zn tend to have major FCC phases, while those with Cr and Mn give rise to major BCC with brittle σ- and µ-phase, which further improves their mechanical strength. Graphic Abstract

Microstructure Evolution of FeNiCoCrAl1.3Mo0.5 High Entropy Alloy during Powder Preparation, Laser Powder Bed Fusion, and Microplasma Spraying.

In the present study, powder of FeCoCrNiMo0.5Al1.3 HEA was manufactured by gas atomization process, and then used for laser powder bed fusion (L-PBF) and microplasma spraying (MPS) technologies. The processes of phase composition and microstructure transformation during above mentioned processes and subsequent heat treatment were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and differential thermal analysis (DTA) methods. It was found that gas atomization leads to a formation of dendrites of body centered cubic (BCC) supersaturated solid solution with insignificant Mo-rich segregations on the peripheries of the dendrites. Annealing leads to an increase of element segregations till to decomposition of the BCC solid solution and formation of σ-phase and B2 phase. Microstructure and phase composition of L-PBF sample are very similar to those of the powder. The MPS coating has a little fraction of face centered cubic (FCC) phase because of Al oxidation during spraying and formation of regions depleted in Al, in which FCC structure becomes more stable. Maximum hardness (950 HV) is achieved in the powder and L-PBF samples after annealing at 600 °C. Elastic modulus of the L-PBF sample, determined by nanoindentation, is 165 GPa, that is 12% lower than that of the cast alloy (186 GPa).

Effect of Iron content on the microstructure evolution, mechanical properties and wear resistance of FeXCoCrNi high-entropy alloy ‎system produced via MA-SPS

Abstract In the present study, the FexCoCrNi (x = 1, 1.3, 1.6) high entropy alloy (HEA) system was prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The effect of iron content on the microstructure, mechanical properties, thermal behavior, and wear resistance of the FexCoCrNi (x = 1, 1.3, 1.6) HEA system was separately studied. It was found that all 3 alloys showed body-centered cubic (BCC) and face-centered cubic (FCC) solid solutions after 50 h of milling and an increase in the iron content was accompanied by an increase in the fraction of the BCC solid solution. Thermal stability of the FexCoCrNi (x = 1, 1.3, 1.6) HEA system was evaluated in the temperature range of 973–1273 K. The FCC structure was retained even after thermal exposure at 973, 1123, and 1273 K for all 3 alloys. The increase in Fe content led to a decrease in the sigma phase (FeCr or CoCr) formation temperature. The results of mechanical tests indicated that increasing Fe (1–1.6 mol) resulted in enhancement of ultimate tensile strength and hardness from 480 to 560 MPa and 320–400 Vickers, respectively. Wear resistance results demonstrated that the coefficient of friction (COF) and weight loss of this system decreases with increased Fe content. Moreover, the dominant wear mechanism changed from abrasive wear to adhesive wear.

Microstructural, mechanical and electrochemical properties of AlCrFeCuNiWx high entropy alloys

AlCrFeNiCuWx high entropy alloys were fabricated on 316 L stainless steel substrate using laser metal deposition technique. The microstructural and mechanical characteristics of these alloys were studied by advanced characterization techniques. The results revealed that the alloys exhibited a dual phase structure with BCC (body centered cubic) and FCC (face centered cubic) solid solution phases. The presence of tungsten (W) particles in the HEA matrix changed the shape of grains from columnar to aquiaxed. The alloys that contained W possessed higher plasticity at maximum compressive stress. They also showed better performance when they were evaluated for wear and corrosion.

First-principles investigation of W[sbnd]V and W[sbnd]Mo alloys as potential plasma facing materials (PFMs) for nuclear application

Density-functional theory (DFT) based first-principles calculations were used to investigate the crystal structure, binding energy, phase stability and elastic properties of body-centered cubic (BCC) W-based binary solid solutions. The effect of alloying up to 50 atomic percent (at.%) concentration range is determined from the virtual-crystal approximation (VCA) approach. Resulting BCC solid solutions are assessed in comparison to the ideal Vegard's law. Solubility of the alloying elements is characterized by the negative enthalpy of mixing. The values of elastic constants computed for the ground state structures are used to assess the effect of alloying on the ductility and hardness. Based on current results, it seems key to strike a tricky balance between moderate Pugh's modulus ratio (B/G) between bulk and shear moduli (B,G) and elastic anisotropy (A) such that high hardness is not completely compromised at the expense of ductility. The predicted property trends are in agreement with existing theoretical and experimental data, which is indicative that the VCA approach is reliable in development of such alloys.

Synthesis and hydrogen storage behavior of Mg–V–Al–Cr–Ni high entropy alloys

The ternary MgVAl, MgVCr, MgVNi, quaternary MgVAlCr, MgVAlNi, MgVCrNi and quinary MgVAlCrNi alloys were produced by high energy ball milling (HEBM) under hydrogen pressure (3.0 MPa) as a strategy to find lightweight alloys for hydrogen storage applications. Most of the ternary and quaternary alloys presented multiphase structure, composed mainly of body-centered cubic (BCC) solid solutions and Mg-based hydrides. Only the quinary MgVAlCrNi high entropy alloy (HEA) formed a single-phase structure (BCC solid solution), which is a novel lightweight (ρ = 5.48 g/cm3) single-phase HEA. The hydrogen storage capacity of this alloy was found to be very low (approximately 0.3 wt% of H). Two non-equiatomic alloys with higher fraction of Mg and V (strong hydride former elements), namely Mg28V28Al19Cr19Ni6 and Mg26V31Al31Cr6Ni6, were then designed, aiming at higher storage capacity. Both alloys were produced by HEBM. The results show that the non-stoichiometric alloys also presented low hydrogen storage capacity. The low affinity of these alloys with hydrogen was discussed in terms of enthalpy of hydrogen solution and enthalpy of hydride formation of the single components. This study brought to light the importance of considering both enthalpy of hydrogen solution and enthalpy of hydride formation of the alloying elements for designing Mg-containing HEA for hydrogen storage. Once Mg has a positive enthalpy of hydrogen solution, the alloys composition must be balanced with alloying elements with higher hydrogen affinity, i.e., negative values of enthalpy of solution and hydride formation.