Influence of severe plastic deformation on the microstructure and hardness of a CoCrFeNi high-entropy alloy: A comparison with CoCrFeNiMn

The evolution of microstructure and hardness in a CoCrFeNi high-entropy alloy (HEA) processed by severe plastic deformation (SPD) was studied. SPD-processing was carried out using a high-pressure torsion (HPT) technique up to twenty turns at room temperature that introduces a highest nominal shear strain of about 800. It was found that most of grain refinement and an increase of lattice defect (dislocations and twin faults) density occurred up to the shear strain of ~10. The saturation grain size was about 80 nm, while the maximum values of the dislocation density and the twin fault probability were ~150 × 10¹⁴ m⁻² and ~3%, respectively. In addition, a 111 texture was formed during HPT-processing. The evolution of hardness with strain followed a trend suggested by the changes in the microstructure. The saturation hardness was as high as ~5100 MPa. The microstructure and hardness obtained for the HPT-processed CoCrFeNi were compared with the values determined formerly for CoCrFeNiMn. It was found that, although the saturation grain size in the CoCrFeNi was much higher than that for the CoCrFeNiMn HEA, the hardness was similar for the two alloys due to the close values of the twin fault probability which can be explained by the similar stacking fault energies.