High entropy multi-component alloys - Concepts, microstructures and mechanical properties

In metallurgy, the conventional alloy development strategy consists in selecting a majority element to meet the main constraint of the specifications, then adding minority elements, sometimes in significant numbers, to adjust the secondary properties. The main categories of metal alloys used in industry today are steels, i.e. alloys whose main element is iron, superalloys, whose main element is nickel, and also aluminum-based, titanium-based or copper-based alloys. Over the last few decades, composition adjustment and microstructure control have enabled a spectacular evolution in the diversity and performance of these materials. However, this approach seems to be reaching its limits. There are still areas in the property maps that are not occupied by any material, particularly for certain combinations of properties, such as mechanical strength and ductility. To achieve significant gains in properties, and thus fill the gaps in the property space, it would appear necessary to develop new material concepts. The research carried out on high-entropy alloys since 2004 is part of this approach. The aim of this article is to summarize the main knowledge already established on high entropy alloys (HEAs), but also to highlight the questions that remain open. To this end, the article is divided into six sections: (1) the discovery of high-entropy alloys and the resulting definitions, (2) the thermodynamic tools used to describe the stability of HEAs, (3) the main chemical compositions of HEAs and the specificities in terms of structure and microstructure, (4) the production methods already implemented, (5) the mechanical properties and the mechanisms that help to understand these properties, and (6) the outlook in terms of applications and research.

At the end of the article, readers will find a glossary of terms and a table of symbols and acronyms.