On the enhanced hardening ability and plasticity mechanisms in a novel Mn-added CoCrNi medium entropy alloy during high-pressure torsion

Microstructure and texture evolution during high-pressure torsion (HPT) of a novel Mn-added CoCrNi medium entropy alloy (Co33Ni33Cr19Mn15) is investigated for the first time. The alloy exhibited a rapid rise in hardness at relatively low shear strains (γ≤20). It is attributed to an extensive dislocation activity to achieve saturation in dislocation density of ~1016 m−2, combined TWIP and TRIP effects and microstructural refinement. At higher shear strain, hardness increased at much reduced rates owing to saturation of dislocation density, twin fault probability and the TRIP effect, besides continued grain refinement for severe nano-structuring led to subsequent strengthening. The FCC phase showed remarkable stability except a small degree of initial deformation-induced HCP martensitic transformation in an early stage of HPT. The ideal shear texture components were observed at low shear strain, and these continued to evolve up to 5 turns of HPT processing. For similar HPT processing conditions, the studied alloy showed superior hardness (~650 Hv) compared to a wide spectrum of FCC materials, which is ascribed to a combination of the strengthening mechanisms of Taylor hardening, the TRIP and TWIP effects and Hall-Petch strengthening resulting from the nano-structured grains having an average size of ~35 nm.