Anomalous Hardening of CrMnFeCoNi High-Entropy Alloy Deformed by High-Pressure Torsion

The CrMnFeCoNi high-entropy alloy (HEA) was severely deformed by high-pressure torsion (HPT) at temperatures ranging from 77 K to 673 K. Phase stability, microstructure and texture were investigated by diffraction of synchrotron radiation. At low HPT temperatures, the low stacking fault energy HEA transforms from the face-centered cubic to the hexagonal close-packed structure, whereas at the highest HPT temperature, the alloy decomposes into body-centered cubic and tetragonal phases. Between room temperature and 573 K the typical structural refinement behavior of HPT deformed single-phase metals to a nanocrystalline and ultrafine-grained structure is observed. The microhardness measured at room temperature increases with increasing HPT temperature. This hardness anomaly is attributed to extreme nanostructuring, where grain boundary and phase boundary sliding become the predominant deformation mechanism leading to inverse Hall–Petch behavior, i.e., increasing hardness with increasing grain size. Furthermore, short-term anneal hardening—occurring after the actual HPT experiment—could also contribute to the observed increase in hardness. Microhardness, measured at room temperature, of CrMnFeCoNi high-entropy alloy and stainless steel deformed by high-pressure torsion at various temperatures. The nanocrystalline materials exhibit inverse Hall–Petch behavior, indicating grain/phase boundary sliding as the dominant deformation mechanism.