High thermal stability of nanocrystalline FeNi2CoMo0.2V0.5 high-entropy alloy by twin boundary and sluggish diffusion

A nanocrystalline (NC) face-centered cubic FeNi2CoMo0.2V0.5 high-entropy alloy was produced by high pressure torsion (HPT). The evolutions of microhardness and microstructure of the NC alloy during subsequent isochronal annealing were investigated systematically by electron back-scattering diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). It was found that nano-grains and deformation nano-twin lamella were obtained at outer disk edge after HPT process with a hardness plateau of 450 HV. Isochronal annealing below 600 °C induced an evident hardening without precipitation effect, due to the annihilation of mobile dislocations and sustained deformation twin barriers. Evident recrystallization and grain growth of the NC FeNi2CoMo0.2V0.5 high-entropy alloy occurred during isochronal annealing at temperatures higher than 600 °C. The activation energies of recrystallization and grain growth of the NC FeNi2CoMo0.2V0.5 high-entropy alloy were calculated to be 350 kJ/mol and 272 kJ/mol, respectively, corresponding to a slow defects recovery process and a swift GB migration process. The high thermal stability of the NC FeNi2CoMo0.2V0.5 high-entropy was mainly caused by kinetic sluggish diffusion effect and deformation twin boundaries with thermodynamic low boundary energy, which retarded the movements of dislocations and grain boundary.