This article describes the process of grain refinement behavior in fcc high-entropy alloys (HEAs) by severe plastic deformation using high-pressure torsion (HPT) method. Disk samples of CrMnFeCoNi and Al0.3CrFeCoNi were deformed by HPT. It was revealed that the two HEAs exhibit formation of equiaxed nanograins even after relatively low degree of deformation. Formation of fine lamellar structures by nanotwins or shear bands may be an important factor to drastically increase dislocation density, which leads to the formation of equiaxed nanograins by continuous dynamic recrystallization.
Since the discovery of superconductivity in a high-entropy alloy (HEA) Ti-Zr-Hf-Nb-Ta, we have developed HEA-type superconductors based on various electronic materials. Since compounds basically possess two or more crystallographic sites and characteristic chemical bonds, the modulation of local crystal structure and electronic states and the improvement of functionality are expected by the introduction of HEA-type site. In this paper, we introduce the variation of HEA-type superconductors and their unique superconducting properties are reviewed. Furthermore, the results of pressure effects on the crystal structure and the superconducting properties in HEA-type metal tellurides are reviewed.
The high-entropy alloys (HEAs) receive much attention due to their superior mechanical properties, high corrosion resistance, enhanced thermoelectric properties, and so on. Superconducting or magnetic HEAs also show interesting properties. In superconducting HEAs, the robustness of superconductivity against extremely high-pressure or magnetism is reported. For magnetic HEAs, tailoring magnetic properties by changing the microstructure recently attracts much attention. In this article, we introduce materials research on superconducting or magnetic HEAs. The perspectives relating to high-pressure physics are also mentioned.
High-entropy alloys consisting of equal amounts of multiple elements are attracting much attention because their properties are far exceeding expectations. These have spread to compounds, and various metal oxides have been reported to be stabilized by the high-entropy effect. In this article, we overview the structures of simple metal-chalcogenides, MX and MX2 (X = S, Se), then introduce our recent study on the high-pressure synthesis of the high-entropy metal chalcogenides together with related studies. The notable point is that most of the compounds stabilized in simple structures such as NiAs-type in MX and pyrite-type in MX2 by increasing numbers of elements even though the original structure of MX/MX2 is different from NiAs-type/pyrite-type.
Shock compression research of solids started at USA and former Soviet Union before 1960 age using explosive method, and had developed mainly at USA using also propellant guns. However, research of ceramics had been almost left untouched. We have established various kinds of measurement methods, and have led the shock compression study of ceramics. The elasto-plastic transition, high-pressure phase transition, equation of state of Al2O3, ZrO2, TiO2, Si3N4, AlN, NiO, ZnS, B4C, TiB2, Gd3Ga5O12, MgO, ZnO, TeO2, PbF2, etc. had been investigated by shock compression. And, the synthesis of new metastable substances and the shock metamorphism, such as B1-type TaN, orthorhombic HfO2, CaCl2-type RuO2, Sm2Fe17Nx bulk magnet, defective ZnMgS and some solid-solution bulk alloy of immiscible systems such as Fe-Cu, Co-Cu system, and the shock metamorphism have been studied as well as some unique physical properties of these materials. Here, our developments of shock-wave measurement systems and the results are reviewed mainly on ceramics.