A homogeneous HfNbTaTiZr high-entropy alloy was successfully processed via powder metallurgy route. For the initial powder feedstock material fabrication, the electrode induction-melting gas atomization procedure was used, resulting in a spherical powder morphology and dual bcc phase composition distinguishable within the individual particles. Spark plasma sintering was then used for the powder compaction at sintering temperatures ranging from 800 to 1600 °C. By the characterization of the compact microstructures, lattice defects (microscopic porosity and vacancy-like misfit defects), and mechanical properties (hardness and three-point bending strength), the sintering conditions were optimized to obtain a fully dense, homogeneous, single-phase bcc material. It was found that such properties are achieved when sintering at 80 MPa pressure for 2 min at temperatures above 1200 °C, where the single bcc phase structure exhibited ductile behavior with considerable flexural strength and ductility at ambient temperature. Positron annihilation spectroscopy was used to characterize the evolution of atomic and mesoscale defects during optimization of the sintering process.

A series of CoCrFeNiMox (x = 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2) high-entropy alloys were designed to develop a eutectic high-entropy alloy system and to acquire a superfine eutectic structure. The results show that for the CoCrFeNiMox alloys, with the increase of Mo content from 0.2 to 1.2, the microstructures shift from a typical dendrite structure to a hypoeutectic microstructure (x = 0.6), and then to a fully eutectic microstructure (x = 0.8) with a lamellar spacing only 110 nm, and finally culminate in the hypereutectic structure (x = 1.0, x = 1.2). The XRD results show that CoCrFeNiMox alloys have a single FCC phase when x is 0.2 or 0.4. When Mo content is over 0.6, it begins to separate Cr9Mo21Ni20 intermetallic compounds. The hardness of the CoCrFeNiMox alloys is increasing significantly from 172.8 to 763.7 HV with the increase of Mo content. Meanwhile, the fracture strength increased but the ductility decreases. Among these alloys, the CoCrFeNiMo0.6 alloy shows excellent integrated mechanical properties of compressive fracture strength and strain, which are 2051 Mpa and 23%, respectively.

Coherent cuboidal B2 nanoprecipitation in body-centered cubic (BCC)-based high-entropy alloys (HEAs) is important for the improvement of mechanical strength. The present work primarily investigated the effect of Ti substitution for Al on the cuboidal B2 nanoprecipitates in BCC Al0.7NiCoFeCr2 HEAs. A series of (Al,Ti)0.7NiCoFeCr2 HEAs with different Al/Ti ratios were prepared by suction-cast processing, and their microstructures and mechanical properties were then characterized comprehensively. It was found that the substitution of Ti for Al can change the phase structures of ordered precipitation, from the B2-AlNi to a highly ordered L21-Ni2AlTi phase. Especially, a small amount addition of Ti (≤4.2 at.%, Al/Ti ratio ≥2/1) renders the HEAs with cuboidal L21 nanoparticles coherently precipitated into the BCC matrix, which is attributed to the moderate lattice misfit (ε = 0.5–0.6%) between BCC and L21 phases. HEAs with such coherent microstructures exhibit high compressive yield strength of about 1700–1800 MPa. When the Ti content reaches up to 6.25 at.%, the matrix of the alloy will be turned into the σ phase, rather than BCC, leading to a heavy brittleness.

A hypoeutectic CoCrFeNiNbχ system was synthesized to investigate the effect of Nb content on the thermal stability, mechanical properties, and corrosion behaviors. The hypoeutectic CoCrFeNiNbχ alloy, which contained the Laves phase, possessed two-phase eutectic structures. The elevated temperature may have an impact on the stability of the Laves phase. Nanoindentation measurements showed that the Laves phase is much harder than the FCC phase, which could be confirmed by the shallower maximum penetration depth in the typical P–h curve. Furthermore, the plasticity of the Laves phase was characterized by nanoindentation measurements. Compared with the FCC phase, the activation energy of dislocation nucleation in the Laves phase is much higher due to the large atomic size difference and the phase difference. Corrosion and passivation behaviors of CoCrFeNiNbχ were investigated in 3.5% NaCl solution. All the alloys exhibited spontaneous passivity and low current densities in 3.5% NaCl solution. Furthermore, the corrosion potential increased with the increasing Nb content, which indicated that the corrosion resistance enhanced with a higher Nb content.

This contribution presents a comprehensive analysis of the low temperature deformation behavior of CoCrFeMnNi on the basis of quasistatic tensile tests at temperatures ranging from room temperature down to 4.2 K. Different deformation phenomena occur in the high-entropy alloy in this temperature range. These include (i) serrated plastic flow at certain cryogenic temperatures (4.2 K/8 K), (ii) deformation twinning (4.2 K/8 and 77 K), and (iii) dislocation slip (active from 4.2 K up to room temperature). The importance of deformation twinning for a stable work-hardening rate over an extended stress range as well as strain range has been addressed through the use of comprehensive orientation imaging microscopy studies. The proposed appearance of ε-martensite as well as a previously uninvestigated route of analysis, essentially a quantitative time-dependent, strain-dependent, and stress-dependent evaluation of the serrated plastic flow in CoCrFeMnNi is provided.

Developing metallic materials with a good combination of strength and ductility has been an unending pursuit of materials scientists. The emergence of high/medium-entropy alloys (HEA/MEA) provided a novel strategy to achieve this. Here, we further strengthened a strong-and-ductile MEA using a traditional solid solution strengthening theory. The selection of solute elements was assisted by mechanical property and microstructure predictive models. Extensive microstructural characterizations and mechanical tests were performed to verify the models and to understand the mechanical behavior and deformation mechanisms of the designated CoCrNi–3W alloy. Our results show good experiment-model agreement. The incorporation of 3 at.% W into the ternary CoCrNi matrix increased its intrinsic strength by ∼20%. External strengthening through microstructural refinement led to a yield strength nearly double that of the parent alloy, CoCrNi. The increase in strength is obtained with still good ductility when tested down to 77 K. Nanoscale twin boundaries are observed in the post-fracture microstructure under 77 K. The combination of strength and ductility after W additions deviate from the traditional strength-ductility-trade-off contour.

Environment can impact the wear behavior of metals and alloys substantially. The tribological properties of Al0.6CoCrFeNi high-entropy alloys (HEAs) were investigated in ambient air, deionized water, simulated acid rain, and simulated seawater conditions at frequencies of 2–5 Hz. The as-cast alloy was composed of simple face-centered cubic and body-centered cubic phases. The wear rate of the as-cast HEA in the ambient air condition was significantly higher than that in the liquid environment. The wear resistance in seawater was superior to that in ambient air, deionized water, and acid rain. Both the friction coefficient and wear rate in seawater were the lowest due to the formation of oxidation film, lubrication, and corrosion action in solution. The dominant wear mechanism in the ambient air condition and deionized water was abrasive wear, delamination wear, and oxidative wear. By contrast, the wear mechanism in acid rain and seawater was mainly corrosion wear, adhesive wear, abrasive wear, and oxidative wear.

High-entropy alloys (HEAs) are receiving considerable attention since last decade because of their ability to give excellent strength with reasonably good elongation during fracture. The mechanical alloying followed by sintering is one of the routes for fabrication; however, there are limited reports on sintering mechanisms of HEA powders. The present investigation studies sintering mechanisms of CoCrFeNi alloy powders in as-milled and annealed conditions using dilatometer experiments. The annealed powder shows slower densification behavior and higher activation energy of sintering, compared to the as-milled powder. Diffusion coefficients were analyzed through sintering models and compared with literature data. The as-milled powder was found to exhibit mixed response, i.e., the grain boundary diffusion seems to be dominating initially due to a large grain boundary fraction but volume diffusion (VD) also contributes significantly, due to high defect concentration and metastable phases. VD was found to be the dominating mechanism during sintering of single phase, stable annealed powder.

It has been reported that the optimal properties of materials are usually not linear to the configuration entropy of materials; in another word, the high-entropy alloys may not have the best properties among all the alloys, including medium-entropy alloys, thus all of these alloys can be universally named as entropic alloys. For entropic alloys, the design, discovery, and optimization of new materials are more complicated than conventional materials. A technique of high-throughput processing is urgently needed to improve the efficiency. In this paper, a combined method by using multitarget deposition has been proposed for parallel preparation of high-entropy to medium-entropy alloys. Films with compositional gradient were constructed in a pseudo-ternary Ti–Al–(Cr, Fe, Ni) system in this study. To facilitate the characterization of the material library, it has been divided into 144 independent units with an area of 1 cm2 and the maximum value of compositional gradient reaches ∼13 at.%/cm. The material library exhibits a high coverage of composition, and the range of element content varies from 3.3 to 89.2 at.% on average. The stability and homogeneity of the material library were analyzed from phase structure and microtopography. Preliminary screening of the phase structure and properties were performed. The phases are mainly composed of amorphous phase and body-centered cubic phase. Hardness changes nonlinearly with compositions. The material library synthesized in this study is expected to provide an effective platform for high-throughput screening of multicomponent materials.

To improve the high-temperature properties of tool steel, a microstructure-dense and crack-free Fe5Cr5Co5SiTiNbMoW high-entropy alloy (HEA) coating was successfully fabricated by laser cladding. And its microstructure and hardness evolution after various annealing temperatures of 800 °C, 850 °C, 950 °C, and 1050 °C for 4 h were carefully investigated by OM, scanning electron microscope, energy dispersion spectrum, X-ray diffraction, and microhardness tester, respectively. The experimental results show that the HEAcoating was mainly composed of body-centered cubic and (Nb, Ti)C plus few Laves phase. The high-temperature annealing processing has little influence on the phase composition. The dendrites and matrix are decomposed with the annealing temperature increasing. While annealing at 950 °C, a eutectic microstructure appeared in the coating. Moreover, the thickness of the diffusion layer of HEA coating increased with the increasing of annealing temperatures. Surprisingly, the HEA coating after annealing at 850 °C possessed ultra-high average hardness, about 1050 HV0.2, huge improvement compared with as-cladding HEA coating (∼780 HV0.2). Therefore, it might reveal that the HEA coating exhibits excellent annealing strengthening ability.

High-entropy ceramic (HEC) films refer to the carbide, boride, oxide, or nitride films of the high-entropy alloy, which have potential applications under high temperatures. In this study, we fabricated the HEC NbTiAlSiZrNx films using magnetron sputtering under various deposition atmospheres. The phase structure evolution and the mechanical properties of three HEC films under high temperatures were investigated. The HEC films demonstrated good thermal stability as well as high hardness. After annealing for 24 h at 700 °C, the films remained in an amorphous phase without obvious crystallization, and the hardness of the films declined. Nanocrystallizations occurred in films deposited at a nitrogen flow rate of 4 sccm and 8 sccm after annealing for 30 min at 900 °C and exhibited an face-centered cubic structure. HEC NbTiAlSiZrNx films have potential applications as protective coatings under high temperatures.