Hardening and thermal stability of a nanocrystalline CoCrFeNiMnTi0.1 high-entropy alloy processed by high- pressure torsion

A CoCrFeNiMnTi0.1 high-entropy alloy (HEA) was subjected to high-pressure torsion (HPT) processing under 6.0 GPa pressure up to 10 turns. XRD results reveal that the initial and HPT-processed microstructures consist of a single fcc phase and there is no evidence for creating a new phase and the occurrence of a phase transformation during HPT processing. It is shown that there is a gradual evolution in hardness with increasing numbers of turns but full homogeneity is not achieved even after 10 turns. Microhardness measurements reveal that the material reaches a saturation hardness value of Hv ≈ 460 which is approximately three times higher than for the homogenized alloy. The nanostructured HEA was subjected to post-deformation annealing (PDA) at 473-1173 K and it is shown that the hardness increases slightly up to Hv ≈ 550 at 773 K due to a phase decomposition and the formation of new precipitates and then decreases to the hardness of the homogenized sample (Hv ≈ 140) at 1173 K due to a combination of recrystallization, grain growth and dissolution of the precipitates. The results reveal that an addition of only 2 at.% Ti will improve the hardness and thermal stability of the nanocrystalline CoCrFeNiMn HEA.