High-entropy alloys: a critical assessment of their founding principles and future prospects

Abstract

High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years. They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements. Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect. By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed. The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs suggests their diffusion kinetics are not necessarily anomalously slow in comparison to conventional alloys. The meaning and influence of the cocktail effect is also a matter for debate. Nevertheless, it is clear that HEAs represent a stimulating opportunity for the metallurgical research community. The complex nature of their compositions means that the discovery of alloys with unusual and attractive properties is inevitable. It is suggested that future activity regarding these alloys seeks to establish the nature of their physical metallurgy, and develop them for practical applications. Their use as structural materials is one of the most promising and exciting opportunities. To realise this ambition, methods to rapidly predict phase equilibria and select suitable HEA compositions are needed, and this constitutes a significant challenge. However, while this obstacle might be considerable, the rewards associated with its conquest are even more substantial. Similarly, the challenges associated with comprehending the behaviour of alloys with complex compositions are great, but the potential to enhance our fundamental metallurgical understanding is more remarkable. Consequently, HEAs represent one of the most stimulating and promising research fields in materials science at present.

Face Centred Cubic Multi-Component Equiatomic Solid Solutions in the Au-Cu-Ni-Pd-Pt System

“…An origin for the highly segregated microstructure observed in AuCuNiPd and AuCuNiPt might be connected to sluggish diffusion [20]. According to Table 1 gold atoms have the largest radius among the elements under investigation, while copper and nickel have the smallest radii.…”

Science and technology in high-entropy alloys

“…However, if such data are not available unfortunately, it becomes necessary to design and carry out experiments. In this regard, the thermodynamic data obtained from the DFT calculations will be useful to fill up experimental data [31,116]. Thus, the reliable thermodynamic databases, containing a series of functions related to the composition and temperature, are assessed by the CALPHAD method, based on the reliable experimental data and accurate DFT results [40,50,[113][114][115][116][117][119][120][121][122].…”

“…Since then, the new alloy-design strategy opens a huge, unexplored field of multi-component alloys. The strategy of alloy design has obtained unimaginable successes and great efforts have been devoted to the development and application of many HEAs [30,31] in various fields due to their excellent performance, such as the unique wear resistance [32], excellent strength and thermal stability at elevated temperatures [33,34], superior high elongation [35,36], great fatigue and fracture resistance [37][38][39], etc.…”

Atomic displacement in the CrMnFeCoNi high-entropy alloy – A scaling factor to predict solid solution strengthening

“…16], have been receiving tremendous attention in the literature [e.g., Refs. [17][18][19][20][21][22]. At least part of this interest stems from the fact that some of them exhibit intriguing and exceptional mechanical properties that are challenging to interpret.…”