Multi-component High Entropy Alloys Research Articles

Nano-Structure Evolution and Mechanical Properties of AlxCoCrFeNi2.1 (x = 0, 0.3, 0.7, 1.0, 1.3) High-Entropy Alloy Prepared by Mechanical Alloying and Spark Plasma Sintering.

The AlxCoCrFeNi2.1 (x = 0, 0.3, 0.7, 1.0, 1.3) multi-component high-entropy alloy (HEA) was synthesized by mechanical alloying (MA) and Spark Plasma Sintering (SPS), The impact of the percentage of Al on crystal structure transition, microstructure evolution and mechanical properties were studied. Crystal structure was investigated by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The results show that with the increasing of Al content, the crystal structure of the alloys gradually transformed from a nanocrystalline phase of FCC to a mix of FCC and BCC nanocrystalline. The hardness was found to increase steadily from 433 HV to 565 HV due to the increase in fraction of BCC nanocrystalline phase. Thus, the compressive fracture strength increased from 1702 MPa to 2333 MPa; in contrast, the fracture strain decreased from 39.8% to 15.6%.

First-principles calculation of lattice distortion, electronic structure, and bonding properties of GeTe-based and PbSe-based high-entropy chalcogenides

The massive amount of wasted heat energy from industry has pushed the development of thermoelectric (TE) materials that directly convert heat into electricity to a new level of concern. Recently, multicomponent alloys such as GeTe-based and PbSe-based high-entropy (HE) chalcogenides have attracted a great deal of attention due to their potential application as TE materials. The nature of the interatomic bonding, lattice distortion (LD), and the electronic structure in this class of materials is not fully understood. Herein, we report a comprehensive computational investigation of nine GeTe-based HE alloys with eight metallic elements (Ag, Pb, Sb, Bi, Cu, Cd, Mn, and Sn) with large supercells of 1080 atoms each; seven PbSe-based HE solid solutions: Pb0.99−ySb0.012SnySe1−2xTexSx (x = 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, with y = 0) with supercells of 1000 atoms each; and five Pb0.99−ySb0.012SnySe1−2xTexSx (y = 0.05, 0.1, 0.15, 0.2, 0.25 with x = 0.25) solid solutions with supercells of 1000 atoms each. All these HE models are theoretically investigated for the first time. The electronic structure, interatomic bonding, charge transfer, and lattice distortion (LD) are investigated by first-principles calculations based on density functional theory. Multicomponent HE alloys can cause a significant LD, which affects their mechanical, thermal, and TE properties. The calculations for the GeTe-based HE chalcogenides showed that they are semiconductors with a narrow bandgap, except for m8, which has a semi-metallic characteristic, and this makes them good candidates for TE applications. For most of these models, the Fermi level shifts upward and locates deeply in the conduction bands, resulting in the enhancement of the electrical conductivity (σ). The bonding properties showed that most bonds in m5 are more dispersed, indicating highest LD and lower lattice thermal conductivity. For PbSe-based HE solid solutions, the LD calculations showed that the models Pb0.99Sb0.012Se0.5Te0.25S0.25 and Pb0.89Sb0.012Sn0.1Se0.5Te0.25S0.25 have the higher LD, and thus a lower lattice thermal conductivity. Such investigations are in high demand since it enables us to design new HE chalcogenides for TE applications. We use the novel concept of total bond order density as a single quantum mechanical metric to characterize the internal cohesion of these HE alloys and correlate with calculated properties, especially the mechanical properties. This work provides a solid database for HE chalcogenides and a road map for many potential applications. Moreover, the computational procedure we developed can be used to design new HE chalcogenides for specific TE applications.

Structure and Properties of Melt-Quenched Al<sub>4</sub>CoCrCuFeNi High-Entropy Alloy

The structure and mechanical properties of a multicomponent high-entropy Al4CoCrCuFeNi alloy in the as-cast and melt-quenched states were investigated. The alloy composition was analyzed based on the literature criteria for predicting the phase formation in high-entropy alloys, which considered the entropy and enthalpy of mixing, valence electron concentration as well as the atomic size difference of the components. The alloy films were synthesized by quenching from the melt using a splat-quenching technique. The cooling rate of the films was estimated to be ~ 106 K/s based on the film thickness. The X-ray diffraction analysis revealed that both as-cast and melt-quenched Al4CoCrCuFeNi alloy samples had an ordered B2 phase in their structure. The microhardness of the as-cast alloy was 6500 MPa, while the microhardness of the melt-quenched film was significantly higher and reached 9400 MPa.

Thermophysical Properties of Cu-Based Subsystems of High-Entropy Alloys

According to the well-known concept of multicomponent high-entropy alloys, high entropy of mixing can stabilize the formation of solid solutions (simple bcc or fcc crystal structure) during solidification. Stabilization of the solid solution and prevention of the formation of intermetallic phases during solidification is provided by the high entropy of mixing in the solid and liquid states. High-entropy alloys have increased strength, high hardness, thermal stability in combination with good resistance to oxidation and corrosion. These properties allow to significantly expand the scope of these alloys. In this work, the electrical resistivity, thermoelectric power and surface tension of binary Cu–Sn, Cu–Ga and Cu–Bi alloys, which are the sub-system components of model low-temperature high-entropy Bi–Cu–Ga–Pb–Sn alloys, have been studied in a wide temperature range including solid and liquid states. The lack of the surface tension data of the above-mentioned alloys is compensated by the model predicted values.

The influence of cooling rate on the structure and mechanical properties of Al4CoCrCuFeNi multicomponent alloy

The present study focused on examining the composition, structure, and mechanical properties of an Al4CoCrCuFeNi multicomponent high-entropy alloy. Two states of the alloy, namely as-cast and melt-quenched, were investigated. The composition of the alloy was determined based on established criteria found in literature, which consider factors such as entropy, enthalpy of mixing, valence electron concentration, and atomic size difference of the components. To synthesize the alloy films, a splat-quenching technique was employed, where the films were rapidly cooled from the molten state. The estimated cooling rate of the films was approximately 106 K/s, calculated based on the film thickness. X-ray diffraction analysis was conducted on both the as-cast and melt-quenched Al4CoCrCuFeNi alloy samples, revealing that they exhibited an ordered B2 phase in their structure. The microhardness measurementswere carried out to evaluate the mechanical properties of the alloy. The as-cast alloy displayed a microhardness of 6500 MPa, while the melt-quenched film exhibited a significantly higher microhardness, reaching 9400 MPa.

Design of self-stable nanocrystalline high-entropy alloy

Nanocrystalline (NC) materials are prone to grain-coarsening at low temperatures—requiring extra solute-element addition for stability. While this approach is established mainly in simple-binary-alloys, it is adjudged “complex” for multicomponent-alloys due to complex-interactions among constituent-elements. We report for the first time that nanograins in multicomponent-high entropy alloy (HEA) stabilize themselves without requiring additional solute if constituent-HEA-elements with highest mixing enthalpy and melting point preferentially segregate to grain boundaries (GBs); a process we term self-stabilization effect in HEAs. Using in-situ X-ray diffraction, scanning/transmission electron microscopy, and atom-probe-tomography (APT), we show that Cr and Fe in NC-AlCoCrFe-HEA (9 nm grain-size) segregate at GBs by site-competition to stabilize it at 0.5Tm (Tm–melting temperature). At 0.6Tm, GB-desegregation is established to be precursor to phase decomposition, and it competes with nanograin stability; this culminates in the onset of grain coarsening at this temperature. Compared with the literature (e.g., NC-AlCoCrFeNi), NC-AlCoCrFe HEA shows exceptional nanograin-stability at high homologous-temperatures; this suggests possible breakdown of the cocktail and sluggish-effects in HEAs, since more elements do not necessarily improve nanograin stability. To the authors’ knowledge, this is the finest stable-NC-HEA produced—it paves a new way of engineering NC-HEAs without coarsening in scalable solid-state processes that require substantially-high temperatures.

Enhanced mechanical and corrosion properties of cold-rolled CoCrFeMoNi high-entropy alloy, as a proposed material for vascular stents

Studying stent materials has been done extensively, but their significance and wide use continue to make them a topic of ongoing research in the scientific community. Multi-component high-entropy alloys are unique and distinguishable from traditional alloys because of their properties. This study describes the design and synthesis of a non-equivalent atomic ratio CoCrFeMoNi HEA. Following cold-rolling processing, the structural and mechanical behavior of this alloy was examined, and its ability for usage as vascular stent implants was explored. By forming numerous twins in the presence of Mo, the FCC solid-solution matrix created a combination of strength and ductility. Potentiodynamic polarization measurements and electrochemical impedance spectroscopy were conducted in Hank's solution at 37 °C. The findings demonstrated that all the samples exhibited superior corrosion resistance compared to the 316 L alloy, with the 60% and 70% cold-rolled samples showing the lowest corrosion rate. The SEM results of the corroded surfaces confirmed the occurrence of general corrosion. Also, in the alloy's design, elements that are widely used in the manufacturing of stents were chosen to be potentially biocompatible. All the results suggest that this alloy possesses features that make it suitable for vascular stent implants, and could enhance their capabilities.

Phonon instability of a multi-principal element alloy

It is a common wisdom that the emergence of plasticity in crystalline solids is accompanied by the collapse of the softest vibrational mode. However, the recent advance in the multi-principal element alloys (MPEAs) complicates this scenario with essential local chemical inhomogeneity rendered by formation of the chemical short-range order (CSRO) via strong enthalpic interaction between specific atom pairs. Here we develop a set of computational metrics to characterize the features of phonons and the instability pathway in a prototypical ternary CoCrNi MPEA by atomistic simulations. There exists strong anharmonicity in MPEA quantified by a large Gr\"uneisen parameter, which is modulated by the degree of CSRO. Both localized and extended vibrational modes exist at either low- or high-frequency vibrations, whereas extremely high-frequency modes prefer local vibration. Introduction of CSRO increases the phonon density of states at high frequency and reduces the degree of anharmonicity. Therefore, CSRO enhances elastic stability. Upon loading, phonon instability occurs via annihilation of the softest mode, akin to the scenario seen in conventional crystals. However, softening of several low-frequency modes occurs simultaneously in MPEA, and it is the cooperative softening of them that leads to the onset of plasticity. Amid phonon instability, essential variations in the Gr\"ueisen parameter and participation ratio are observed, signifying the correlation between vibrational and configurational space in such compositional complex alloys. The results of unusual phonon features could give insight into how the structure, elasticity, and plasticity interact in generic multi-component high-entropy alloys.

Lattice distortion in FCC HEAs and its effect on mechanical properties: Critical analysis and way forward

Lattice distortion is considered to be one of the four core effects in a multicomponent high-entropy alloy. However, their effect is least understood from experiment and theoretical standpoints. The present investigation revealed a unique way to understand this effect by combining experiments with density functional theory (DFT) calculations. A small amount of Al and Si were carefully added to the whole-solute matrix of Cantor alloys. The different-sized atomic species introduces a huge lattice distortion in the matrix, leading to a simultaneous improvement in yield strength (YS), ultimate tensile strength (UTS), and percent elongation. An extensive DFT simulation indicates that a lattice distortion is prominent in an Al-containing alloy, whereas Si does not induce a lattice distortion. However, Si leads to severe interlayer lattice distortion, caused by the displacement of Si, during twinning. This leads to the improvement of YS, UTS, and ductility. Lattice distortion and its variants play significant effects on the mechanical properties of high-entropy alloys (HEAs) in terms of local lattice distortion, providing an uneven energy landscape for the movement of line defects or interlayer distortion. The inherent nature of local lattice distortion in HEAs leads to the wavy or tortuous dislocation, unlike a straight dislocation in conventional alloys. The movement of the wavy type of dislocation through a distorted or defective lattice requires large stress, resulting in a pronounced effect on solid solution strengthening. This local lattice distortion also dictates the degree of the interlayer distance distortion in the vicinity of atoms, leading to an increase or decrease in stable stacking fault energy that decides the deformation mode via slip or twinning.

Exploring strengthening mechanism of FeCoNiAl high-entropy alloy by non-metallic silicon addition produced via powder metallurgy

Multi-component high-entropy alloys (HEAs) have recently attracted interest in balancing the bottleneck of high hardness and high fracture toughness. This study demonstrates the influence of Si addition to FeCoNiAl HEA fabricated by advanced powder metallurgy. The FeCoNiAlSix (x = 0, 0.2, 0.4, 0.6, and 0.8 molar ratio) HEA was fabricated by mechanical alloying for 45 h and subsequent powder compact densification by spark plasma sintering (SPS). FeCoNiAlSix HEA morphology and phase evolution were investigated to understand the influence of Si content on the microstructure, compressive strength, and fracture mechanisms of the FeCoNiAl alloy. The results indicate that FeCoNiAlSix powders HEAs have single-phase body-centered cubic (BCC) structures. However, densification of powders HEAs facilitates dual-phase face-centered cubic (FCC) and BCC formation in FeCoNiAl, whereas the FCC phase transforms completely to BCC or BCC/B2 with Si addition. The Vickers microhardness of sintered FeCoNiAlSix HEAs varies from 524 HV (for FeCoNiAl HEA) to 798 HV (for FeCoNiAlSi0.8 HEA). The maximum compressive strength of HEAs varies from 1325 MPa (for FeCoNiAl HEA) to 2031 MPa (for FeCoNiAlSi0.8 HEA), respectively. The absorption energies of FeCoNiAl and FeCoNiAlSi0.2 were 198 and 203 MJ/m3, respectively. In contrast, the absorption energies of FeCoNiAlSi0.6 and FeCoNiAlSi0.8 HEAs reduced drastically (115 and 96 MJ/m3, respectively). The failure mechanism of the FeCoNiAlSix HEAs was a mixed ductile-brittle fracture in FeCoNiAl. Brittle fracture dominates in FeCoNiAlSi0.2 and FeCoNiAlSi0.4 HEAs. A complete brittle fracture was noticed with river-like patterns in FeCoNiAlSi0.8 HEAs. The results of this investigation suggest that HEAs with different Si molar ratios and tuned phase structures can be used to design high-strength structural alloys for automotive applications.

Preparation and Microstructure of Multi-Component High Entropy Alloy Powders Fabricated by Gas Atomization Method

As an attractive high-entropy alloy, AlCrCoNiCu high-entropy alloy has excellent corrosion resistance, wear resistance, and anti-bacterial capabilities, and is considered to be a potential substitute material for marine and nuclear industry materials with great potential. One key to further optimizing the performance of high entropy alloy was to prepare high entropy alloy powder materials with uniform composition, good flow-ability, and stable performance. In this work, the AlCrCoNiCu high entropy alloy powder was prepared by the gas atomization method. The results indicated that the powder was spherical in shape, homogeneous in composition, and composed of a face-center cubic (FCC) phase. After adding Fe and Mn elements, FCC and body-center cubic (BCC) phases appeared and the particle size of the powder was mainly located at 10–50 μm. Furthermore, the larger the particle size was, the more obvious the surface roughness was. With the decreasing powder size, its shape became relatively regular, and the surface roughness decreased. This work provided an experimental and theoretical reference for preparing high-performance single-phase and multi-phase high entropy alloy spherical powders.

Enhancing the oxidation resistance of nanocrystalline high-entropy AlCuCrFeMn alloys by the addition of tungsten

The isothermal oxidation behavior of multi-component high entropy alloys (HEAs), namely AlCuCrFeMn, AlCuCrFeMnW0.05, AlCuCrFeMnW0.1, and AlCuCrFeMnW0.5, was investigated and the behavior of the oxide layer was analyzed. All four HEAs were synthesized via mechanical alloying (MA) and consolidated by spark plasma sintering (SPS). The samples were oxidized in the air atmosphere at a temperature of 500 °C for 50 h. Based on the thermogravimetric result, the oxidation rate of the materials decreased with the increase of W content, and the values of parabolic constants were on a level similar to those observed in Ni–Al superalloys. However, higher content of W improves the continuity and internal position of the WO3 scale, which leads to increased oxidation resistance. Throughout the oxidation process, the composition of the phases of all materials changed significantly. The triple-thick oxide layer formed of Al2O3, Cr2O3, and WO3, developed in HEAs, has been carefully studied using the XPS technique.

Variation of microstructures and properties of Co0.2CrAlNi high entropy alloy doped Si

The effects of Si on the microstructures and various properties of Co0.2CrAlSixNi (in mole ratio, x = 0, 0.1, 0.3, 0.5, and 0.7) multi-component high entropy alloys were investigated. Co0.2CrAlNi was selected as the matrix, and Si was doped as the reinforcement. Microstructure analysis revealed that the Co0.2CrAlSixNi high entropy alloys consisted of two body-centered cubic (BCC) phases, the Cr-rich phase and nano-scale Ni-Al phase. With the addition of Si, the new Cr3Si phase was precipitated, and the crystallite size decreased to some extent in the alloys. Hardness test indicated that the alloys had high hardness with the maximum value of 973 at x = 0.5. The crystallite size and Cr3Si phase content were two key factors significantly affecting the hardness of Co0.2CrAlSixNi HEAs. Electrochemical test results indicated that Si affected the formation of the passivation film, the corrosion potential of Co0.2CrAlSi0.1Ni was − 0.37 V and the self-corrosion current density was 1.15 × 10−6 A/cm2, respectively. The main contribution in which Si doping affected the corrosion resistance of HEA was reduced oxidized Cr3+ and Ni2+ on the alloy surface. This study is of great significance to develop light and hard engineering materials for practical applications.

Improving mechanical properties of (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) multi-component high-entropy alloys via multi-stage strain hardening strengthening

In this study, (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) high entropy alloys were fabricated by vacuum arc melting, followed by cold rolling and aging treatment. The effects of refractory elements on the phase structure, microstructure and mechanical properties of high entropy alloys (HEAs) were systematically studied by means of XRD, SEM and TEM. The results show that the phase composition of all alloys is the face-centered-cubic (FCC) + L12 dual-phase structure, and the addition of refractory elements has not changed the phase structure of the alloys. The tensile results show that the yield strength and elongation of the matrix HEA are 750 MPa and 18 %, respectively. The strength of HEAs is improved variously by adding refractory elements. The HEAs with Ta element have the best comprehensive mechanical properties, exhibit excellent strength-ductility combination, with the yield strength and elongation of 950 MPa and 20 %, respectively, and the fracture mechanism changes from complete ductile fracture to mixed brittle-ductile fracture mode. In addition, the HEAs have multi-stage strain-hardening behavior, which has higher strength when the strain-hardening curve has larger fluctuations, which indicates that the HEAs can improve the strength-plasticity matching relationship by a multi-stage strain hardening behavior.

Grain Growth in High-Entropy Alloys (HEAs): A Review

Grain growth is an unavoidable phenomenon occurring during processing of multicomponent high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), which significantly affects the properties and performance of these compositionally complex alloys during service. Accordingly, the present overview article is dedicated to summarize the state of the art of the grain coarsening behavior and its kinetics for HEAs and propose opportunities and scope for future research, with special attention to the Cantor-based compositions. Firstly, the primary recrystallization and grain growth of equiatomic and non-equiatomic CrMnFeCoNi alloys with face-centered cubic (FCC) crystal structure in the single-phase regime and with the presence of intermetallics such as the Cr-rich sigma (σ) phase is considered, which was accompanied by the powerful utilization of the solute drag effect, Zener pinning, and grain boundary segregation engineering via introducing carbon, nitrogen, boron, titanium, and niobium. Subsequently, the quaternary and ternary alloys based on Cr, Mn, Fe, Ni, and Co are taken into account. In these systems, the novel concept of grain boundary high-entropy effect (via the addition of molybdenum, niobium, and zirconium), the formation of pinning insoluble oxide and thermally stable carbide particles during mechanical alloying, the simultaneous solute drag and Zener pinning effects via Mo and C addition, and the formation of Cu-rich phase and the strong segregation effect of Cu atoms to grain boundaries in the FeCoNiCu HEA are critically discussed. Afterward, the thermal stability of AlxFeCoCrNi(Mn) HEAs is treated, where the formation of the Ni–Al-rich B2 phase and the corresponding Zener pinning effect, the influence of aluminum addition on the temperature range for the stability of the σ phase, and cluster drag mechanism of Al atoms are explained. What is more, the grain growth of refractory HEAs with body-centered cubic (BCC) crystal structure is also summarized, where the high grain growth activation energy is attributed to the presence of elements with very low self-diffusivity such as tantalum and niobium. Moreover, the possible effects of short-range ordering on the mobility of grain boundaries are discussed. Finally, the prospects for future research work are presented, including the development of precipitation-temperature-time (PTT) diagrams, systematic investigation of cold rolling and annealing for prior grain refinement, studying abnormal grain growth (secondary recrystallization), and assessment of grain growth behavior of HEAs manufactured by advanced methods such as additive manufacturing (3D printing).

Multicomponent Precipitation and Strengthening in Intermetallic-Strengthened Alloys

Advanced structural materials with superior mechanical properties are of technological importance for industrial applications. Multicomponent precipitation provides a potential approach for designing high-performance alloys and has been receiving increasing attention from both academia and industry. In this Perspective, we highlight the recent advances and future perspectives in multicomponent-precipitation-strengthened alloys, including multicomponent steels, superalloys, and high-entropy alloys. The emphasis is placed on the unique microstructural features and challenges of multicomponent precipitation as well as their impacts on the strengthening behavior. Finally, future research areas for this class of materials are critically discussed.

Influence of heat-treated temperature and its rate on structural, mechanical, and corrosion properties of (Ti-Mo-Zr)60CoXCrY high entropy alloys for lightweight applications

ABSTRACT Multi-component (Ti, Mo, Zr, Co, and Cr) high-entropy alloys (HEAs) are prepared are via high energy ball milling technique and followed by spark plasma sintering (SPS) technique for their compaction using the general formula (Ti-Mo-Zr)60CoxCry labeled as (TMZ)60CoxCry where x & y as at.%; equiatomic ( = and non-equiatomic ( = ,); stoichiometric ratios of Ti, Mo, Zr elements). XRD traces recorded the presence of FCC and BCC solid solution phases, at 20 h of milling schedule and stability of as precipitated phases are analyzed using DSC profiles. Mechanical properties include maximum load (19.06 kN), compressive strength (σmax: 205 MPa), Young’s Modulus (E: 57.88 GPa), and Vickers’s hardness (166HV) are achieved to be maximum for equiatomic (TMZ)60Co20Cr20 HEA pellet (rate of heat treatment: 125°C/min), compared with all other compositions under investigation. The superior corrosion properties such as corrosion current density (ic), passive current density (ip), corrosion potential (Ec), and passive potential (Ep) of obtained equiatomic (TMZ)60Co20Cr20 HEA pellet made this family of network a potential candidate for use in lightweight vehicle applications.

A strategy to design eutectic high-entropy alloys based on binary eutectics

• we employed a useful strategy to design EHEAs and quickly locating eutectic compositions in multicomponent HEAs based on binary eutectic compositions. • Nb 0.62 Fe 1.98 Co 1.22 Ni 1.00 exhibits a fully eutectic microstructure confirmed by both SEM and DSC results • Our findings provide a new strategy for a starting point for quickly pinpointing EHEAs. Although eutectic high entropy alloys (EHEAs) display homogeneously fine microstructure, outstanding castability and promising industrial application potential, how to search for the eutectic compositions in multicomponent high-entropy alloys (HEAs) remains to be challenging. Herein, the strategy which was proposed for pinpointing good glass formers in the field of bulk metallic glasses was utilized for quickly locating EHEA compositions in Nb-(Fe, Co, Ni), Zr-(Fe, Co, Ni) and Hf-(Fe, Co, Ni) HEA systems based on the constituent binary eutectics. As a demonstration, the eutectic compositions in the Nb-(Fe, Co, Ni), Zr-(Fe, Co, Ni) and Hf-(Fe, Co, Ni) HEA systems were quickly determined using this new approach. The newly designed EHEAs showed ultrafine eutectic microstructures, good castability and decent compressive mechanical properties. Current findings clearly demonstrated that this proposed approach is effective for searching for eutectic compositions in HEAs, which will expedite alloy design of EHEAs with strong solid solution strengthening effects.

Multiple minor elements improve strength-ductility synergy of a high-entropy alloy

The developments of multicomponent high-entropy alloys usually put emphasis upon the roles of multiple principal elements in optimizing the various properties. Instead of tuning the principal elements, here we propose to use multiple minor alloying elements to improve performance of HEAs. To this end, the microstructure and mechanical behavior of a newly developed alloy, i.e., Fe35Ni20Co20Cr20Al2.5Ti1.0Cu1.0Si0.5 (at. %), have been investigated. The alloy decorated by the multiple minor alloying elements (i.e., Al, Ti, Cu and Si) exhibits a single face-centered cubic (FCC) γ phase in fully recrystallized state. Compared with the undecorated Fe40Ni20Co20Cr20 (at. %) counterpart with similar grain size, the yield strength of the decorated alloy sample with multiple minor elements is enhanced by ∼19% at a high elongation of ∼74%. Microstructural characterizations indicate that dislocation slip prevails at the early stages of plastic deformation while mechanical twins are observed to occur at the late deformation stage. The local stress field caused by the atomic size mismatch and elastic mismatch originated from the multiple minor elements significantly impedes the movement of dislocations thus improves the strength. Such retarding effect of the multiple minor elements on mobile dislocation further hinders the generation of stacking faults on adjacent atomic planes and thereby increases the FCC-γ phase stability, which also postpones the deformation twinning upon tension. This work demonstrates the practical significance of introducing multiple minor alloying elements with their synergistic effect into HEAs, and the design strategy enlarges the space for compositional design of advanced alloys.

High-Entropy Alloys for Solid Hydrogen Storage: Potentials and Prospects.

Hydrogen storage is one of the most significant research areas for exploiting hydrogen energy economy. To store hydrogen with a high gravimetric/volumetric density, gaseous hydrogen storage systems require a very high-pressure compressed gas cylinder which is quite unsafe and the storage in the liquid form needs cryogenic containers to be maintained at roughly 20 K under ambient pressure because hydrogen has a very low critical temperature of 33 K. However, hydrogen can be stored in solid materials with higher concentration of hydrogen compared to the gaseous and liquid hydrogen storage systems. It is therefore, worthwhile to look into the experimental and theoretical research on prospective hydrogen storage materials. The hydride-forming alloys and intermetallic compounds are found to be the most important families of hydrogen storage materials. Multicomponent alloys consisting of five or more principal elements, also known as high-entropy alloys appear to have potential for the development as hydrogen storage materials. Hydride-forming elements like Ti, Zr, V, Nb, Hf, Ta, La, Ce, Ni, and others have been shown to have hydrogen storage properties and the ability to produce single-phase high-entropy intermetallics. Here, attempts will be made to present a short review on utilization of multicomponent high-entropy alloys as solid hydrogen storage materials. Furthermore, we will also present some of our work on the synthesis, structural–microstructural characterization and hydrogen storage properties of Ti–Zr–V–Cr–Ni equi-atomic hydride-forming high-entropy alloys. From the preliminary investigation, the maximum storage capacity in this system was observed to be 1.78 wt%, which is comparable to other hydrogen storage materials. The prospects of high-entropy-based alloys for hydrogen storage will be discussed.