A comparative study on the evolution of microstructure and hardness during monotonic and cyclic high pressure torsion of CoCuFeMnNi high entropy alloy

Discs of CoCuFeMnNi face centered cubic high entropy alloy were subjected to monotonic and cyclic high pressure torsion (HPT) in a single step and multiple steps of 5 degrees forward and reverse cycle for 100 degrees and 360 degrees twist, respectively, under 5 GPa pressure at room temperature. It was observed that the 100 degrees cyclic HPT sample shows the highest hardness at the periphery comparable to 360 degrees monotonic HPT sample, while the cyclic 360 degrees HPT sample shows the lowest hardness throughout the sample. High hardness of 100 degrees cyclic HPT sample can be attributed to finer grain size and unstable dislocation substructure by continuous change in strain path from initial compression to forward-reverse torsion, while stable dislocation structure corresponding to shear contributes to increase in hardness from 100 degrees to 360 degrees for monotonic HPT sample. The unstable dislocation substructure promotes grain boundary migration-enabled grain growth leading to low hardness throughout the 360 degrees cyclic HPT sample.