Effect of Initial Grain Size on Deformation Mechanism during High-Pressure Torsion in V10Cr15Mn5Fe35Co10Ni25 High-Entropy Alloy

The transition of the deformation mechanism from the dislocation slip-mediated mechanism to the twin-mediated mechanism with increasing grain size is a well-observed phenomenon in materials with low stacking fault energy during compression/tensile tests. To understand this effect further at large strains, a V10Cr15Mn5Fe35Co10Ni25 (at%) high-entropy alloy with two initial average grain sizes is processed by high-pressure torsion (HPT) at different numbers of turns. The results indicate that initial grain size plays a significant role in the deformation mechanism during the HPT process. The fine-grained (FG) sample exhibits only a tangled dislocation structure, whereas mechanical twins are observed along with the formation of dislocations in the coarse-grained (CG) sample after the one-fourth turn. High dislocation density is observed in the CG sample after the one-fourth and first turn, which leads to a higher rate of hardness increment as compared with the FG sample. However, a similar microstructure and mechanical properties are observed after five turns of HPT processing in both FG and CG samples. After five turns, the microstructure consists of nanograins (average grain size approximate to 30 nm) with nanotwins, and the samples exhibit a very high ultimate tensile strength of approximate to 2 GPa with a reasonable elongation to failure of approximate to 6%.