Phase Stability and Inverse Hall-Petch Effect of Severely Deformed and Annealed CrMnFeCoNi High-Entropy Alloy

The reverse phase transition from hexagonal close-packed (hcp) to face-centered cubic (fcc) was studied using diffraction of high-energy synchrotron radiation and hardness measurements on the CrMnFeCoNi high-entropy alloy deformed by high-pressure torsion (HPT) at 77 K under 10 GPa quasi-hydrostatic pressure. Cryogenic HPT leads to a nanocrystalline structure and induces a phase transition from fcc to hcp. To determine the stability of the hcp phase, the material was annealed at temperatures between 473 K and 973 K for 2 h. Annealing initially results in the reverse transition to the fcc phase, followed by precipitation of body-centered cubic (bcc), fcc and then tetragonal particles. The volume fractions of particular phases over the entire temperature range were calculated using Rietveld refinement. During annealing, the reverse phase transformation takes place along with grain growth and reduction in dislocation density, surprisingly leading to an increase in hardness. This phenomenon indicates an inverse Hall-Petch effect.