Understanding the deformation behavior of CoCuFeMnNi high entropy alloy by investigating mechanical properties of binary ternary and quaternary alloy subsets

Combinatorial approach has been employed to understand the deformation micro-mechanisms and strength evolution of FCC equiatomic CoCuFeMnNi high entropy alloy using equimolar subset alloys, judiciously unearthed by thermodynamic modelling using CALPHAD. A series of mechanical tests indicate absence of one-to-one correlation between entropy of mixing and mechanical properties for the alloys. Optimum combination of strength and ductility was observed in ternary equiatomic FeMnNi alloy, although quinary alloy exhibits the highest strength among all the four alloys. The deformation of all the alloys is dominated by octahedral slip as well as partial slip with the absence of deformation twinning. TEM investigation on the deformed samples reveals profuse dislocation activity. Strain rate sensitivity in the range of 0.005-0.008 and activation volume of 100-150 b(3) indicate cross slip as the dominant operative mechanism for all the alloys. Nevertheless, quinary CoCuFeMnNi HEA manifests unique behavior by showing increase in strength even after long term annealing at 1273 K for 7 days. This is attributed to the presence of copper-rich nano-clusters in the alloy which undergo coarsening and partial dissolution thereby contributing to higher solid solution strengthening. Thus, the unique microstructure of the quinary CoCuFeMnNi HEA that can explore cluster strengthening and solid solution hardening manifests new vistas to design novel alloys for potential applications. (C) 2018 Elsevier Ltd. All rights reserved.