Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

The mechanical properties of ex-situ and in-situ metallic glass matrix composites (MGMCs) have proven to be both scientifically unique and of potentially important for practical applications. However, the underlying deformation mechanisms remain to be studied. In this article, we review the development, fabrication, microstructures, and properties of MGMCs, including the room-temperature, cryogenic-temperature, and high-temperature mechanical properties upon quasi-static and dynamic loadings. In parallel, the deformation mechanisms are experimentally and theoretically explored. Moreover, the fatigue, corrosion, and wear behaviors of MGMCs are discussed. Finally, the potential applications and important unresolved issues are identified and discussed.

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Metallic glass matrix composites

The sliding wear behaviour of CoCrFeMnNi and AlxCoCrFeNi high entropy alloys at elevated temperatures

Additive manufacturing of magnetic materials

Single-phase high-entropy alloys – A critical update

The tribological properties of Al0.6CoCrFeNi high-entropy alloy with the σ phase precipitation at elevated temperature

Surface strengthening in Al0.25CoCrFeNi high-entropy alloy by boronizing

The formation of octonary DyErGdHoLuScTbY and senary DyGdHoLaTbY and ErGdHoLaTbY high-entropy alloys (HEAs) with the hexagonal close-packed (HCP) structure was reported in this study. Experiments using scanning electron microscopy and x-ray diffraction confirmed the single HCP solid solution in the as-cast state for these three HEAs if the presence of minor rare-earth oxides due to contamination from processing is ignored. The measured compressive yield stress values for these HEAs at room temperature are 245, 205, and 360 MPa for the ErGdHoLaTbY, DyGdHoLaTbY, and DyErGdHoLuScTbY HEAs, respectively. The corresponding solid solution strengthening contributions for these HEAs were estimated using a simple elastic model, and the resulting contributions were 28 MPa, 27 MPa, and 42 MPa for the three aforementioned HEAs.The formation of octonary DyErGdHoLuScTbY and senary DyGdHoLaTbY and ErGdHoLaTbY high-entropy alloys (HEAs) with the hexagonal close-packed (HCP) structure was reported in this study. Experiments using scanning electron microscopy and x-ray diffraction confirmed the single HCP solid solution in the as-cast state for these three HEAs if the presence of minor rare-earth oxides due to contamination from processing is ignored. The measured compressive yield stress values for these HEAs at room temperature are 245, 205, and 360 MPa for the ErGdHoLaTbY, DyGdHoLaTbY, and DyErGdHoLuScTbY HEAs, respectively. The corresponding solid solution strengthening contributions for these HEAs were estimated using a simple elastic model, and the resulting contributions were 28 MPa, 27 MPa, and 42 MPa for the three aforementioned HEAs.

Rare-earth high entropy alloys with hexagonal close-packed structure

Using thermochemical treatments, boronized layers were successfully prepared on Al0.25CoCrFeNi high-entropy alloys (HEAs). The thickness of the boronized layers ranged widely from 20 to 50 µm, depending on the heat treatment time. Boronizing remarkably improved the surface hardness from HV 188 to HV 1265 after treating at 900°C for 9 h. Moreover, boronizing enhanced the yield strength of HEAs from 195 to 265 MPa but deteriorated the tensile ductility. Multiple crackings in the boride layers significantly decreased the plasticity. The insufficient work-hardening capacity essentially facilitated the plastic instability of the boronized HEAs. With decreasing substrate thickness, the fracture modes gradually transformed from dimples to quasi-cleavage and eventually to cleavage.

Huijun Yang

Papers

The tribological properties of Al0.6CoCrFeNi high-entropy alloy with the σ phase precipitation at elevated temperature

Surface strengthening in Al0.25CoCrFeNi high-entropy alloy by boronizing

Rare-earth high entropy alloys with hexagonal close-packed structure

Deformation behavior and plastic instability of boronized Al 0.25 CoCrFeNi high-entropy alloys

Influence of lanthanum on passivity behavior of CrMnFeNi high entropy alloys