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Phase stability is an important topic for high entropy alloys (HEAs), but the understanding to it is very limited. The capability to predict phase stability from fundamental properties of constituent elements would benefit the alloy design greatly. The relationship between phase stability and physicochemical/thermodynamic properties of alloying components in HEAs was studied systematically. The mixing enthalpy is found to be the key factor controlling the formation of solid solutions or compounds. The stability of fcc and bcc solid solutions is well delineated by the valance electron concentration (VEC). The revealing of the effect of the VEC on the phase stability is vitally important for alloy design and for controlling the mechanical behavior of HEAs. [ABSTRACT FROM AUTHOR]

Highlights: [•] A series of fcc structured equiatomic solid solutions were designed. [•] The solid solution limit in multi-component equiatomic alloys was explored. [•] Topological instability sets the solid solution limit. [•] Decomposition occurs when the lattice strain reaches a critical value of 0.018. [ABSTRACT FROM AUTHOR]

In the search for electronic phenomena in high-entropy alloys (HEAs) that go beyond the independent-electron description, we have synthesized a series of hexagonal rare earth (RE)-based HEAs: CexLaLuScY (x = 0.05–1.0). The measurements of electrical resistivity, magnetic susceptibility and specific heat have shown that the CexLaLuScY HEAs exhibit the Kondo effect, which is of a single impurity type in the entire range of employed Ce concentrations despite the alloys being classified as dense (concentrated) Kondo systems. A comparison to other known dense Kondo systems has revealed that the Kondo effect in the CexLaLuScY HEAs behaves quite differently from the chemically ordered Kondo lattices but quite similar to the RE-containing magnetic metallic glasses and randomly chemically disordered Kondo lattices of the chemical formula RE1xRE21−xM (with RE1 being magnetic and RE2 being nonmagnetic). The main reason for the similarity between HEAs and the metallic glasses and chemically disordered Kondo lattices appears to be the absence of a periodic 4f sublattice in these systems, which prevents the formation of a coherent state between the 4f-scattering sites in the T → 0 limit. The crystal-glass duality of HEAs does not bring conceptually new features to the Kondo effect that would not be already present in other disordered dense Kondo systems. This study broadens the classification of HEAs to correlated electron systems. [ABSTRACT FROM AUTHOR]

The effect of minor additions of substitutional elements such as Al, Cu, Mn, and Fe on the room-temperature (RT) and elevated-temperature hardness of a single bcc phase Hf20Nb31Ta31Ti18 refractory high entropy alloy is studied here. Interestingly, 2.5 at. % nominal addition of Fe hardened the base Hf20Nb31Ta31Ti18 alloy in the temperature range from RT to 800 °C, while the same nominal content of addition of Al, Cu, and Mn softened the base alloy from RT to 1000 °C. Regardless of solid solution hardening or solid solution softening, the hardness variation with temperature essentially showed the same three-stage pattern for all studied alloys here: a temperature-dependent decrease in hardness below 300 °C/400 °C, followed by a temperature-independent hardness plateau between 300/400 and 800 °C, and finally a temperature-dependent decrease in hardness at temperatures higher than 800 °C. The mechanism for solid solution hardening or softening in bcc-structured refractory high entropy alloys is discussed, together with their temperature dependence. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) are characterized by a simultaneous presence of a crystal lattice and an amorphous-type chemical (substitutional) disorder. In order to unravel the effect of crystal-glass duality on the electronic transport properties of HEAs, we performed a comparative study of the electronic transport coefficients of a 6-component alloy Al0.5TiZrPdCuNi that can be prepared either as a HEA or as a metallic glass (MG) at the same chemical composition. The HEA and the MG states of the Al0.5TiZrPdCuNi alloy both show large, negative-temperature-coefficient resistivity, positive thermopower, positive Hall coefficient and small thermal conductivity. The transport coefficients were reproduced analytically by the spectral conductivity model, using the Kubo-Greenwood formalism. For both modifications of the material (HEA and MG), contribution of phonons to the transport coefficients was found small, so that their temperature dependence originates predominantly from the temperature dependence of the Fermi–Dirac function and the variation of the spectral conductivity and the related electronic density of states with energy within the Fermi-level region. The very similar electronic transport coefficients of the HEA and the MG states point towards essential role of the immense chemical disorder. [ABSTRACT FROM AUTHOR]

• CoCrFeMoNi alloys were prepared by vacuum arc-melting from pure elements. • The validity of new PHACOMP method for high-entropy alloys are verified. • New PHACOMP method can be used to evaluate thermal stability of high-entropy alloy. • The designed CoCrFeMoNi high-entropy alloys show competitive corrosion resistance with commercial alloy. [Display omitted] Various methods can be used to predict the phase composition of conventional alloys. However, it is relatively difficult to predict high-entropy alloys (HEAs) owing to their complexity. There remain many challenges in designing HEAs with desirable properties. In this study, two strategies were applied to design corrosion-resistant non-equal-molar CoCrFeMoNi HEAs with a single phase. The two strategies were prediction for valence electron concentration dependent structural behavior to evaluate the matrix of HEAs, and average d -orbital energy level ( M ̅ d) that was initially used to predict precipitation in superalloys under different temperatures. The validity of these two approaches was determined by thermal stability. The corrosion behaviors of the designed HEAs were investigated by polarization tests. In conclusion, the phase compositions of CoCrFeMoNi HEAs were predicted using a combination of the two approaches. The corrosion resistance was also competitive compared to that of Hastelloy C276. This research can provide a reference for the future development of corrosion-resistant HEAs with a single phase. [ABSTRACT FROM AUTHOR]

Abstract: An Al–10Ni–8Y (at.%) alloy was atomized by Ar gas and the morphology, microstructure, thermal stability, phase composition and microhardness of the as-atomized powder were investigated. Most of the powders are spherical in shape, but the surface morphology was different for powder of different size. The cross-section microstructure of powder with size below 15μm in diameter showed no detailed feature, indicating existence of amorphous phase or nanocrystalline structure. The as-atomized powder showed four distinct exothermic peaks when heated at 296, 340, 366 and 456°C. The glass transition temperature T g, crystallization temperature T x , and the temperature interval of the supercooled liquid region ΔT x (=T x −T g) were detected to be about 266, 288 and 22°C. The Al82Ni10Y8 alloy powder exhibits a high Vickers hardness of 230.6, and shows great potential for structural application. [Copyright &y& Elsevier]

Eutectic high-entropy alloys (EHEAs), as a sub-group of high-entropy alloys (HEAs), are becoming a new research hotspot in the metallic materials community because of their excellent castability, fine and uniform microstructures even in the as-cast state, high strength, and good ductility. Some of the EHEAs have shown promising potentials for industrial applications. Here, the history, interesting solidification microstructure and mechanical properties, and the design strategy of EHEAs are reviewed, and their future prospects are outlined. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract The Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr refractory high-entropy alloy with excellent corrosion resistance in the 3.5 wt% NaCl solution is identified in this work. This refractory high-entropy alloy exhibits much better general corrosion resistance than that of the 316L stainless steel, due to its corrosion current density being about one fifth of that in the latter. Meanwhile, the pitting potential of Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr reaches an unusually high value of +8.36 V, much higher than that of reported high-entropy alloys. The superior passivity of Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr is accredited to the formation of a single-phase solid solution containing high amount of homogenously distributed passivity-promoting elements, and also the existence of metallic Ta and OH− species in the passive film, which contribute to the high immunity to passive film breakdown. Highlights • Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr contains finely distributed passivity-promoting elements. • Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr exhibits better general corrosion resistance than 316L SS. • E pit of Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr reaches +8.36 V, beating all other reported RHEAs. [ABSTRACT FROM AUTHOR]

Nanoparticles (NPs) have become popular photothermal therapy (PTT) photosensitizers because they can be targeted to cancer tissues and deliver a chemotherapeutic medication. Pulsed laser ablation in liquid media (PLAL) can produce stable colloidal NPs without any stabilizing agents. The stability, size distribution, and morphology of the produced colloids are influenced by the nature of NPs, laser parameters such as laser power, laser frequency repetition, ablation rate, and the specific ionic effects from the solvent. In this work, HfNbTaTiZr refractory high entropy alloy (RHEA) NPs were prepared by PLAL method in different solvents, including distilled water, ethanol, and n-hexane. The experiments were performed by irradiating a HfNbTaTiZr target with a femtosecond laser outputting 120-fs pulse at varying levels of ablation fluence (0.1, 0.16, 0.23 mJ/cm2). The elemental and structural characteristics of the prepared HfNbTaTiZr NPs were elucidated by using several advanced analytical techniques. The XRD pattern of NPs revealed that the liquid medium significantly affects the type of crystallized phases and the surface composition. The surface composition was studied by XPS, indicating that the fabricated NPs were oxidized. SEM and TEM analysis have evidenced the generation of NPs with an average diameter of less than 50 nm. The results revealed that the higher ablation fluence resulted in the formation of NPs with a larger average diameter. The highest intrinsic photothermal conversion efficiency and 12 °C time-dependent solvent heating produced in ethanol at 640 nm irradiation after 20 min were demonstrated. [Display omitted] • HfNbTaTiZr RHEA NPs prepared by Puls laser ablation in different media and ablation fluence. • The surface chemical composition of NPs after Puls laser ablation is related with ablation media. • By decreasing the ablation power, size and particle size distribution of produced NPs get smaller and narrower. • 12 °C temperature increment of NPs produced in Ethanol irradiated by 640 nm laser after 20 min was measured. [ABSTRACT FROM AUTHOR]

This work, a new composition of high-entropy alloys (HEAs) was designed. The composition was carefully tailored with the aim to obtain a solid solution with a BCC crystalline structure to be an alternative binder in cermets. Thus, the composition of the HEA has been designed taking into account various criteria which has fulfilled a metallic binder of a Ti(C,N) cermet:(1) high hardness and oxidation resistance and (2) good wetting behaviour with Ti(C,N) particles because the processing of cermets is by LPS. The design of the alloy has been performed using theoretical calculations of physicochemical properties of the elements involved and the calculation of phase diagram by Thermocalc. The designed alloy has been processed by casting and powder metallurgy (PM) to study the influence of the processing route on the phases formed and on the resulting properties. The powders were produced by gas atomisation and then consolidated by hot pressing. Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) are promising candidates for new-generation high temperature materials, but they generally suffer from room temperature brittleness and unsatisfactory high-temperature oxidation resistance. There currently lack efforts to address to these two critical issues for RHEAs at the same time. In this work, the high temperature oxidation resistance of a previously identified ductile Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr RHEA is studied. An accelerated oxidation or more specifically, pesting, in the temperature range of 600–1000 °C is observed for the target RHEA, where the oxidation leads the material to catastrophically disintegrate into powders. The pesting mechanism is studied here, and is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation. The simultaneous removal of zirconium and hafnium can eliminate the pesting phenomenon in the alloy. It is believed that pesting can also occur to other equiatomic and non-equiatomic quinary Hf-Nb-Ta-Ti-Zr or quaternary Hf-Nb-Ti-Zr and Hf-Ta-Ti-Zr RHEAs, where all currently available ductile RHEAs are identified. Therefore, the results from this work will provide crucial perspectives to the further development of RHEAs as novel high-temperature materials, with balanced room-temperature ductility and high-temperature oxidation resistance. [ABSTRACT FROM AUTHOR]

The effect of low temperature on tensile properties of a AlCoCrFeNi 2.1 eutectic high entropy alloy (EHEA) was investigated in the present work. Transmission electron microscopy (TEM) showed that the initial as-cast microstructure consisted of B2 (ordered body centered cubic structure) and L1 2 (ordered face centered cubic structure) phases. Upon tensile testing at temperatures ranging from room temperature (RT) to −196 °C (77 K), the L1 2 phase became disordered, changing to a simple FCC (face centered cubic) crystal structure whereas the B2 phase maintained an ordered structure. An increase over 300 MPa was observed in the ultimate tensile strength ( σ UTS ) of the −196 °C tensile tested sample compared to the room temperature tensile tested sample, while keeping almost similar amount of total elongation. TEM results indicated an increase in the dislocation activity in the FCC phase as well as B2 phase in the −196 °C tensile tested sample compared to the sample deformed at room temperature. [ABSTRACT FROM AUTHOR]

In this study, we investigate the scandium-containing Sc-Hf-Nb-Ta-Ti-Zr system of refractory high-entropy alloys (HEAs). Using the arc-melting method, we synthesized nine equimolar alloys (five 4-, three 5- and one 6-component), with all of them containing Sc. The alloys were characterized by XRD, electron microscopy and EDS, while superconductivity was investigated via electrical resistivity, specific heat and the Meissner effect. The results were compared to the parent Hf-Nb-Ta-Ti-Zr refractory HEAs, forming a single-phase body-centered cubic (bcc) structure and quite homogeneous microstructure. The addition of Sc produces a two-phase structure in the Sc-Hf-Nb-Ta-Ti-Zr alloys, with one phase being bcc and the other hexagonal close-packed (hcp). The hcp phase absorbs practically all Sc, whereas the Sc-poor bcc phase is identical to the bcc phase in the Hf-Nb-Ta-Ti-Zr parent system. Upon the Sc addition, the microstructure becomes very inhomogeneous. Large bcc dendrites (10–100 µm) are homogeneous in the central parts, but become a fine dispersion of sub-micron precipitates of the bcc and hcp phases close to the edges. The interdendritic regions are also a fine dispersion of the two phases. Superconductivity of the Sc-Hf-Nb-Ta-Ti-Zr alloys originates from the bcc phase fraction, which demonstrates identical superconducting parameters as the bcc Hf-Nb-Ta-Ti-Zr parent alloys, while the Sc-containing hcp phase fraction is non-superconducting. [ABSTRACT FROM AUTHOR]

In an attempt to incorporate tin (Sn) into high-entropy alloys composed of refractory metals Hf, Nb, Ti and Zr with the addition of 3d transition metals Cu, Fe, and Ni, we synthesized a series of alloys in the system HfTiZrSnM (M = Cu, Fe, Nb, Ni). The alloys were characterized crystallographically, microstructurally, and compositionally, and their physical properties were determined, with the emphasis on superconductivity. All Sn-containing alloys are multi-phase mixtures of intermetallic compounds (in most cases four). A common feature of the alloys is a microstructure of large crystalline grains of a hexagonal (Hf, Ti, Zr)5Sn3 partially ordered phase embedded in a matrix that also contains many small inclusions. In the HfTiZrSnCu alloy, some Cu is also incorporated into the grains. Based on the electrical resistivity, specific heat, and magnetization measurements, a superconducting (SC) state was observed in the HfTiZr, HfTiZrSn, HfTiZrSnNi, and HfTiZrSnNb alloys. The HfTiZrSnFe alloy shows a partial SC transition, whereas the HfTiZrSnCu alloy is non-superconducting. All SC alloys are type II superconductors and belong to the Anderson class of "dirty" superconductors. [ABSTRACT FROM AUTHOR]

• Three Ni powders were sprayed on AZ31B by cold spray for corrosion protection. • Coating microstructures and corrosion performances were correlated with powder feature. • Powder surface protruding granules will hinder inter-particle intimate contact. • The gas-atomized smooth Ni powder can give > 3000 h corrosion protection for AZ31B. Three nickel (Ni) powders, i.e. gas-atomized spherical Ni (GA-Ni), carbonyl irregular Ni (C-Ni), and electrolytic dendritic porous Ni (E-Ni), were sprayed on magnesium alloys by an in-situ micro-forging assisted cold spray method. Although all deposited coatings present extremely low porosity, the C-Ni and E-Ni coatings reveal poorer inter-particle bonding than GA-Ni coating. Electrochemical tests reveal that the inter-particle gaps will act as fast pathways for the corrosive media to penetrate the coatings, thereby the GA-Ni coating with intimate inter-particle boundaries can isolate the corrosive media for 3000 h and the C-Ni and E-Ni coatings fail after <10 h corrosion. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications. • a two-step pack aluminizing process forms protective alumina scale in ductile RHEAs. • a TiAl 3 /TiAl/Ti 2 AlNb and substrate multi-layered gradient structure is achieved. • Crack-free TiAl 3 coating forms on the top surface of Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 • The oxidation resistance of ductile RHEAs improves tremendously after aluminizing. [ABSTRACT FROM AUTHOR]

The microstructure and orientation relationships in five AlCoCrFeNi x alloys with eutectic (x = 2.1) and near-eutectic compositions (x = 1.9, 2.0, 2.2 and 2.3) have been characterized in this work. The eutectic and near-eutectic AlCoCrFeNi x alloys have microstructures consisting of face-centered cubic (FCC) and ordered body-centered cubic (BCC) B2 phases, where the volume fraction of the FCC phase is found to increase with increasing Ni content. It is found that each eutectic colony consists of several parallel lamellae in the colony center and peripheral irregular labyrinth-like structures. The frequency of the irregular structures appears to be higher in the near-eutectic alloys than in the eutectic alloy. The ratio of the average widths of FCC and BCC lamellae is observed to be sensitive to the chemical composition and increases with the increasing Ni content. The two phases in the studied alloys display relationships close to the Kurdjumov-Sachs orientation relationship. The distribution of misorientation angles between the interphase boundaries changes as a function of the Ni content. The result of this change is a small increase in the average misorientation angle across the FCC/BCC interphase boundaries with increasing Ni content. The differences in the morphological crystallographic characteristics between the alloys are briefly discussed. • Microstructure and orientation relationship are investigated in near-eutectic HEAs. • each eutectic colony consists of regular parallel lamellae and irregular structures. • Ratio of the widths of FCC and BCC lamellae increases with increasing Ni content. • Two phases in all alloys display the Kurdjumov-Sachs orientation relationship. • Misorientation angle across interphase boundaries increases at higher Ni content. [ABSTRACT FROM AUTHOR]

The state-of-the-art metallic biomaterials are 316L, CoCrMo and Ti6Al4V but they all suffer from known issues relating to biocompatibility, wear resistance and corrosion resistance. Therefore, there is always the motivation to identify novel superior metallic biomaterials to 316L, CoCrMo and Ti6Al4V. The concept of refractory high-entropy alloys (RHEAs) provides an interesting research direction towards developing novel metallic biomaterials, initially because RHEAs consist of purely biocompatible elements, but a systematic study of the performance of RHEAs targeting biomedical applications, while comparing to that of the state-of-the-art 316L, CoCrMo and Ti6Al4V, was not existing before and constitutes the theme of the current work. Two exemplary RHEAs that are studied in detail in this work, TiZrTaHfNb and Ti 1.5 ZrTa 0.5 Hf 0.5 Nb 0.5 , show highly promising characteristics as novel superior metallic biomaterials in that they possess a desirable combination of wear resistance, wettability and pitting and general corrosion resistance, outperforming 316L, CoCrMo and Ti6Al4V almost in all these important aspects. In addition, it is also shown in this work that how appropriate alloying in RHEAs can be utilized to fine-tune their performance as better metallic biomaterials, such as the correlation between lattice strain and corrosion resistance. Image 1 • As-cast TiZrTaHfNb and Ti 1.5 ZrTa 0.5 Hf 0.5 Nb 0.5 were composed of a single BBC phase. • Ti 1.5 ZrTa 0.5 Hf 0.5 Nb 0.5 showed lower lattice strain than TiZrTaHfNb. • RHEAs exhibited higher H and E ratios compared to 316L, CoCrMo and Ti6Al4V. • TiZrTaHfNb and Ti 1.5 ZrTa 0.5 Hf 0.5 Nb 0.5 revealed no pitting effect. • Ti 1.5 ZrTa 0.5 Hf 0.5 Nb 0.5 has more resistance of barrier oxide than TiZrTaHfNb. [ABSTRACT FROM AUTHOR]

Review: 69 refs. [ABSTRACT FROM AUTHOR]