Surface hardening of high- and medium-entropy alloys by mechanical attrition at room and cryogenic temperatures

Surface hardening by mechanical attrition at room and cryogenic temperatures has been studied on CrMnFeCoNi high- and CrCoNi medium-entropy alloys. The hardness gradient is produced by severe plastic deformation through dislocation slip and mechanical twinning, finally leading to an ultrafine or even nanocrystalline structure. The hardness of samples after surface attrition at cryogenic temperature is slightly less than that of samples deformed at room temperature. A small amount of deformation-induced martensitic transformation is only observed in a narrow surface layer of the medium-entropy alloy severely deformed under cryogenic conditions. Reasons for these observations are discussed.