Austenitic HEA/MPEA : from the Cantor alloy to precipitation hardened Co-free alloys with optimised mechanical resistance

For nearly twenty years now, the academic – and increasingly industrial – world has been interested in new classes of metallic alloys. HEA (high entropy alloys), which are concentrated, single-phase alloys based on several metallic elements and whose Cantor alloy (Co20Cr20Fe20Mn20Ni20) is the archetype, were quickly supplemented by multiphase alloys, CCA (complex concentrated alloys) or MPEA (multi principal elements alloys) in which secondary phases bring improvements in properties.

Austenitic HEA have several major advantages: excellent mechanical behaviour at cryogenic temperatures; impact resistance higher than that of similar conventional materials (austenitic steels) and good stability of the single phase structure. Their mechanical strength is generally low.

In this presentation, a non-exhaustive overview of recent developments in austenitic HEA/MPEA will be presented. We will mention the various elementary mechanisms leading to the specificities of these alloys, such as high level of solid solution hardening, the importance of the stacking fault energy (SFE) and the existence of plastic deformation by twinning (TWIP effect).

The various ways of designing and developing alloys will be discussed and illustrated by recent results obtained in our laboratory:

• searching for austenitic cobalt-free alloys; cobalt being one of elements with many defects and disadvantages;
• the effects of deviations from equiatomic compositions, however assumed to stabilize the solid solution by maximizing configuration entropy;
• the possibilities of hardening this type of material by secondary phases.

The choice of presented results will be focused on prospective applications of these materials in nuclear industry. The presented results come from the work of several PhD students or post-doctoral researchers (Michal MROZ, Julia OLSZEWSKA, Ebert ALVAREZ, Dinesh RAM, Mathieu TRAVERSIER, Jingjun GAO). The projects needed the help of our technical team (Claude VARILLON, Mickaël HAERING, Anne-Cécile BACH) and academic collaborators (Franck TANCRET, IMN; Emmanuel RIGAL, CEA; Gilles ADJANOR, EDF; Xavier BOULNAT, Mateis; Estelle MESLIN, CEA; Brigitte DECAMPS, IJCLab).

Funding the projects by public (ANR, the French grant agency, AURA region) and private companies (EDF, CEA, Aperam, FRAMATOME, Aubert&Duval) is acknowledged.