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TiVCr-based alloys are well-explored body-centered cubic (BCC) materials for hydrogen storage applications that can potentially store higher amounts of hydrogen at moderate temperatures. The challenge remains in optimizing the alloy-hydrogen stability, and several transition elements have been found to support the reduction in the hydride stability. In this study, Ni and Nb transition elements were incorporated into the TiVCr alloy system to thoroughly understand their influence on the (de)hydrogenation kinetics and thermodynamic properties. Three different compositions, (TiVCr)95Ni5, (TiVCr)90 Ni10, and (TiVCr)95Ni5Nb5, were prepared via arc melting. The as-prepared samples showed the formation of a dual-phase BCC solid solution and secondary phase precipitates. The samples were characterized using hydrogen sorption studies. Among the studied compositions, (TiVCr)90Ni10 exhibited the highest hydrogen absorption capacity of 3 wt%, whereas both (TiVCr)95Ni5 and (TiVCr)90Ni5Nb5 absorbed up to 2.5 wt% hydrogen. The kinetics of (de)hydrogenation were modeled using the JMAK and 3D Jander diffusion models. The kinetics results showed that the presence of Ni improved hydrogen adsorption at the interface level, whereas Nb substitution enhanced diffusion and hydrogen release at room temperature. Thus, the addition of Ni and Nb to Ti-V-Cr-based high-entropy alloys significantly improved the hydrogen absorption and desorption properties at room temperature for gas-phase hydrogen storage.

For the first time, a powder of refractory body-centered cubic (bcc) HfTaTiNbZr-based high-entropy alloy (RHEA) was prepared by short-term (90 min) high-energy ball milling (HEBM) followed by spark plasma sintering (SPS) at 1300 °C for 10 min and the resultant bulk material was characterized by XRD and SEM/EDX. The material showed ultra-high Vickers hardness (10.7 GPa) and a density of 9.87 ± 0.18 g/cm³ (98.7%). Our alloy was found to consist of HfZrTiTaNb-based solid solution with bcc structure as a main phase, a hexagonal closest packed (hcp) Hf/Zr-based solid solution, and Me2Fe phases (Me = Hf, Zr) as minor admixtures. Principal elements of the HEA phase were uniformly distributed over the bulk of HfTaTiNbZr-based alloy. Similar alloys synthesized without milling or in the case of low-energy ball milling (LEBM, 10 h) consisted of a bcc HEA and a Hf/Zr-rich hcp solid solution; in this case, the Vickers hardness of such alloys was found to have a value of 6.4 GPa and 5.8 GPa, respectively.

High-entropy alloys attract the attention of researchers due to a set of new properties. We consider the factors affecting the structure of high-entropy alloys (HEAs) based on elements Ti, Zr, Hf, V, and Nb. The structures of four-component Ti 25 Zr 25 V 25 Nb 25 and five-component Ti 20 Zr 20 Hf 20 V 20 Nb 20 alloys fabricated under the same melting and cooling conditions in an arc furnace are studied. Energy dispersive chemical analysis demonstrates that the chemical compositions of the alloys correspond to the nominal ones. Based on an analysis of micrographs of the ingot surfaces, we conclude that the melting conditions lead to overheating of the four-component alloy and do not cause overheating of the five-component. The primary formation of the four-component alloy is found to occur faster than that of the five-component one; however, further remelting under overheating conditions brings about the formation of a multiphase structure. The maximum content of the bcc solid solution (98%) in the Ti 25 Zr 25 V 25 Nb 25 alloy is achieved during the first remelting, and another phase (2%) was an fcc solid solution. The maximum content of the bcc solid solution (95%) in the Ti 20 Zr 20 Hf 20 V 20 Nb 20 alloy is achieved during repeated remelting, and bcc and hcp solid solutions and a Laves phase are present in an amount of 3%. The unit cell parameters of the main bcc phases in the Ti 25 Zr 25 V 25 Nb 25 and Ti 20 Zr 20 Hf 20 V 20 Nb 20 alloys are 3.270 and 3.362 Å, respectively. The choice of temperature–time conditions of melting and solidification for each specific alloy composition is shown to be important along with the fulfillment of thermodynamic conditions for the production of refractory single-phase HEAs.

We have proposed new hydrogen absorbing alloys of the ‘Laves phase related BCC solid solution alloy’, the hydrogen capacity of which reaches almost double that of conventional rare-earth based AB5 alloys. We have reported the hydrogen absorbing properties of Ti−V−Mn, Ti−V−Cr and T−V−Mn−Cr alloys. It has been accepted that the crystal structural change of BCC hydrogen absorbing alloys is the same as that of V metal. The mono-hydride (H/M=1) of V metal has a BCT structure and the di-hydride (H/M=2) has an FCC structure. However, we recently found that the Ti−V−Mn alloy shows different behaviors in phase transformation with hydrogenation to V metal. We found three hydride phases with a BCC, a deformed FCC and an FCC structure in the Ti−V−Mn solid solution alloy-H2 system. The deformed FCC hydride phase has not yet to our knowledge been reported. The lattice constant of the deformed FCC was 0.407 nm, one axis of which is reduced by about 4%. Its single-phase region appeared at a hydrogen content between 0.8 H/M and 1.0 H/M in absorption at 298 K. The lower plateau observed due to formation of the deformed FCC hydride phase gives an increase of effective hydrogen capacity by decreasing hydrogen remaining in the alloy in the desorption process.

We have proposed new hydrogen absorbing alloys of the “Laves phase related BCC solid solution alloy”, the hydrogen capacity of which reaches almost double that of conventional rare-earth based AB5 alloys. We have reported the hydrogen absorbing properties of Ti-V-Mn, Ti-V-Cr and T-V-Mn-Cr alloys. It has been accepted that the crystal structural change of BCC hydrogen absorbing alloys is the same as that of V metal. The mono-hydride (H/M=1) of V metal has a BCT structure and the di-hydride (H/M=2) has an FCC structure. However, we recently found that the Ti-V-Mn alloy shows different behaviors in phase transformation with hydrogenation to V metal. We found three hydride phases with a BCC, a deformed FCC and an FCC structure in the Ti-V-Mn solid solution alloy-H2 system. The deformed FCC hydride phase has not yet to our knowledge been reported. The lattice constant of the deformed FCC was 0.407 nm, one axis of which is reduced by about 4%. Its single-phase region appeared at a hydrogen content between 0.8 H/M and 1.0 H/M in absorption at 298 K. The lower plateau observed due to formation of the deformed FCC hydride phase gives an increase of effective hydrogen capacity by decreasing hydrogen remaining in the alloy in the desorption process.

Atomistic simulations of dislocation mobility reveal that bodycentered cubic (BCC) high-entropy alloys (HEAs) are distinctly different from traditional BCC metals. HEAs are concentrated solutions in which composition fluctuation is almost inevitable. The resultant inhomogeneities, while locally promoting kink nucleation on screw dislocations, trap them against propagation with an appreciable energy barrier, replacing kink nucleation as the rate-limiting mechanism. Edge dislocations encounter a similar activated process of nanoscale segment detrapping, with comparable activation barrier. As a result, the mobility of edge dislocations, and hence their contribution to strength, becomes comparable to screw dislocations.

In this work, novel WNbMoTaVZr x ( x  = 0.1, 0.25, 0.5, 0.75, 1.0) refractory high entropy alloys (RHEAs) were developed, and the corresponding phase formation, microstructure and mechanical properties were investigated. As compared with the WNbMoTa and WNbMoTaV derivative alloys, the present WNbMoTaVZr x RHEAs demonstrated significantly improved strength and hardness, especially the specific yield strength. The increase of the strength was attributed to the solid solution strengthening effect, resulting from the severe lattice distortion associated with lager-atomic-sized Zr element. With the increase of Zr content, the microstructure changed from grain morphology to dendritic structures. The formation of the second phase with the increase of Zr content was also observed, and its effects on the strengthening, plastic deformation and fracture behaviors were discussed. The deformation-evolution investigations have shown that under applied loadings, microcracks initiated at interdendritic regions with relatively soft second phase. The phase thermostability analysis suggests that the phase structure of typical WNbMoTaVZr x RHEAs could be stable at elevated temperature. Graphical abstract

There has been considerable technological interest in high-entropy alloys (HEAs) since the initial publications on the topic appeared in 2004. However, only several of the alloys investigated are truly single-phase solid solution compositions. These include the FCC alloys CoCrFeNi and CoCrFeMnNi based on 3d transition metals elements and BCC alloys NbMoTaW, NbMoTaVW, and HfNbTaTiZr based on refractory metals. The search for new single-phase HEAs compositions has been hindered by a lack of an effective scientific strategy for alloy design. This report shows that the chemical interactions and atomic diffusivities predicted from ab initio molecular dynamics simulations which are closely related to primary crystallization during solidification can be used to assist in identifying single phase high-entropy solid solution compositions. Further, combining these simulations with phase diagram calculations via the CALPHAD method and inspection of existing phase diagrams is an effective strategy to accelerate the discovery of new single-phase HEAs. This methodology was used to predict new single-phase HEA compositions. These are FCC alloys comprised of CoFeMnNi, CuNiPdPt and CuNiPdPtRh, and HCP alloys of CoOsReRu.

High-entropy alloys (HEAs) prefer to form single-phase solid solutions (body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal closed-packed (HCP)) due to their high mixing entropy. In this paper, we systematically review the mechanical behaviors and properties (such as oxidation and corrosion) of BCC-structured HEAs. The mechanical properties at room temperature and high temperatures of samples prepared by different processes (including vacuum arc-melting, powder sintering and additive manufacturing) are compared, and the effect of alloying on the mechanical properties is analyzed. In addition, the effects of HEA preparation and compositional regulation on corrosion resistance, and the application of high-throughput techniques in the field of HEAs, are discussed. To conclude, alloy development for BCC-structured HEAs is summarized.

In this paper, we report an investigation of adding a non-hydride forming element in the multicomponent Ti-V-Nb-M system. By the Calculation of Phase Diagrams approach (CALPHAD), the thermodynamic phase stability of the TiVNbT (T = Cr, Mn, Fe, Co, and Ni) was investigated, and Cr was selected as the fourth alloying element due its high tendency to stabilize body-centered cubic solid solutions (BCC). The (TiVNb)100−xCrx alloys (with x = 15, 25, and 35 at.% Cr) were synthesized by arc-melting. The structural characterization reveals that the three alloys were composed of a major BCC phase, which agrees with the thermodynamic calculations. The three alloys absorb hydrogen at room temperature without any activation treatment, achieving a hydrogen uptake of about H/M = 2. The Pressure-Composition-Isotherms curves (PCI) has shown that increasing the Cr amount increases the equilibrium pressures, indicating that tunable H storage properties can be achieved by controlling the alloys’ Cr content.