Recovery of metastable solid solution from a severe plastically deformed Cu alloyed FeMnCoCr high entropy alloy

We report on the recovery of metastable Face Centred Cubic (FCC) phase from a severe plastically deformed 1 at.% Cu alloyed non-equiatomic FeMnCoCr high-entropy alloy (HEA) for the first time. The well explored Cu free variant of the non-equiatomic FeMnCoCr HEA typically exhibits Transformation Induced Plasticity (TRIP) wherein the metastable FCC solid solution transforms to Hexagonal Close Packed (ε, HCP) phase upon straining. Both Cu alloyed and Cu free HEA variants investigated in this work exhibited TRIP upon subjected to severe plastic deformation by High Pressure Torsion (HPT). A single rotation of HPT at 5 GPa of applied load resulted in the formation of ~84% ε-martensite phase in the Cu alloyed HEA. Subsequent increase in the number of rotations led to further enhancement in the ε-martensite fraction, with a maximum of 89.6% achieved after 20 rotations. While the initial single rotation substantially promotes martensitic transformation, the rate of phase transformation decreases with subsequent rotations. Corresponding to the presence of ε-martensite across various regions in the HPT samples, the hardness profiles exhibited a monotonous increase from the centre to the periphery. Annealing treatment performed at 900 °C of the HPT-processed samples led to a substantial recovery of the parent FCC solid solution highlighting the adaptable nature of these metastable HEAs facilitating composite strengthening.