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Abstract: A crossover behavior in the initial creep stage during nanoindentation with a constant load for a high-entropy-based alloy CoFeNi is found. The holding time and stress in the indentation tests are evaluated to reveal the intrinsic mechanism of the crossover point. It indicated that the initial creep behavior is dominated by the strain-hardening at the beginning and then transit into the dislocation migration induced viscous stage. The transition presents the crossover, where the strain hardening effect predominates in a very short initial period while the viscous stage accounts for most of the initial creep stage with a time-dependent strain rate. [Copyright &y& Elsevier]

Highlights: [•] The solid solution phase in the high-entropy alloy was confirmed to be metastable. [•] The alloy exhibited microstructural and mechanical stability against annealing. [•] Only as-cast alloys showed sufficient tensile plasticity. [•] A large variability of the measured tensile properties was recorded. [•] The alloys showing slip banding behavior did not necessarily have tensile ductility. [ABSTRACT FROM AUTHOR]

Abstract: Metastable solid solutions can form preferably over intermetallic compounds, in cast high-entropy alloys or multi-component alloys with equi- or nearly equi-atomic compositions, due to the entropy contribution at elevated temperatures. Meanwhile, the high mixing entropy also favors the amorphous phase formation. The phase selection between solid solutions and the amorphous phase upon alloying in high-entropy alloys is intriguing. A two-parameter physical scheme, utilizing the atomic size polydispersity and mixing enthalpy, is found to be capable of capturing this phase selection mechanism. [Copyright &y& Elsevier]

Abstract: A series of cost effective Co-free Al x CrCuFeNi2 high entropy alloys were developed, following the valence electron concentration rule. The solidification microstructure in these alloys showed some anomalies that were rarely seen in other high entropy alloys or multi-component alloys. Specifically, submicron rod-like microstructures existed in the eutectic alloy in spite of the high volume percentage of the rod-like phase; sunflower-like microstructures existed in the hyper-eutectic alloys, with 200nm sized spinodal decomposed seed particles densely distributed in the disk floret. [Copyright &y& Elsevier]

Abstract: Previous work on the stability of the solid solution phases in the high entropy alloys is inconclusive. We used a series of thermo-mechanical treatments to study the stability of the solid solution phases in a high-entropy Al0.5CoCrCuFeNi alloy. The solid solution phases were found to be stable, against the intermetallic compounds, at high temperatures >850 °C and at low temperatures <300 °C. At intermediate temperatures, however, the intermetallic σ-phase co-existed with the solid solution phases. The experimental observations were verified by the thermodynamic calculation results. The mechanisms for the phase stability, both at equilibrium and after quenching-equivalent annealing treatments, were discussed, and the roles of high entropy and slow diffusion kinetics were highlighted. [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 Refractory high-entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. One critical issue to solve for RHEAs is their balanced oxidation resistance and mechanical properties, mainly room-temperature ductility for the latter. Recently, it was found that existing ductile RHEAs are subject to catastrophic accelerated oxidation, also known as pesting. In this work, both alloying and surface coating, are applied to enhance the oxidation resistance of ductile RHEAs, with the focus on surface coating using the pack cementation method and more specifically, aluminizing. The oxidation resistance of two RHEAs, Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr, one recently identified ductile RHEA which pests in the temperature range of 600–1000 °C, and Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 , the newly designed ductile RHEA which does not pest but embrittles after oxidation, are studied after aluminizing at 900 °C using three different pack components. Aluminizing, if using the appropriate pack cementation parameters, can avoid pesting in Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr and alleviate the oxidation induced embrittlement in Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 , and holds the promise for further improving the RHEAs as potential ultrahigh-temperature materials. Highlights • Pack cementation using different pack compositions were experimented on two RHEAs. • Proper aluminizing can avoid pesting in the Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr RHEA. • Proper aluminizing can improve the embrittlement in Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5. • Aluminizing is promising to further improve RHEAs for high-temperature applications. [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]

The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi 0.5 high entropy alloy (HEA) with a composite structure reinforced by TiC nanoparticles. The initial alloy was prepared by mechanical alloying (MA) in a planetary ball mill, compacted by spark plasma sintering (SPS) and heat treated at different temperatures. Mechano-chemical reactions during the MA process as well as the microstructure and hardness of the SPS-ed compacts prior to and after the heat treatment were investigated. During MA, Cr-based supersaturated solid solution with the BCC structure was formed. After SPS at 1100 °C, the BCC solid solution decomposed into nano-grained microstructure consisting of FCC and ordered BCC solid solutions, σ phase, and in-situ formed TiC nanoparticles. The high hardness of the alloy (762 HV) was retained after the subsequent heat treatment at 1100 °C (603 HV). It was shown that the fabrication of TiC reinforced nanocomposites from elemental powders without the use of expensive nanograined powders can be achieved. [ABSTRACT FROM AUTHOR]

The formation of core–shell structure is an interesting phenomenon occurring during the solidification process, due to the liquid phase separation. The formation of core–shell structure in high-entropy alloys, a new class of advanced metallic materials, has not been reported previously, and thus constitutes an intriguing scientific question. Here, we firstly report the formation of core–shell structure in one laser cladded high-entropy alloy, where we show the nanosized-Y 2 O 3 powder addition, serves as the catalyst for the liquid phase separation. [ABSTRACT FROM AUTHOR]