Hardening by annealing in the medium-entropy alloy CrCoNi after equal-channel angular pressing

After equal-channel angular pressing (ECAP) and a subsequent heat treatment at 600 °C for 1 h, the MEA CrCoNi is characterized by a yield strength of about 1.4 GPa but a drastically low uniform elongation of 1.2 %. High-energy X-ray diffraction analysis, transmission electron microscopy, and high-resolution high-angle annular dark-field scanning transmission electron microscopy investigations were performed, but the cause of the annealing-induced hardening could not be conclusively determined, although differential scanning calorimetry measurements reveal an exothermic peak and thus microstructural changes in the temperature range between 450 °C and 650 °C. Despite the complex nature of the MEA CrCoNi, the dominant deformation mechanisms after ECAP processing and a subsequent heat treatment during straining appear to be similar to those of conventional alloys. It is assumed that the rearrangement of dislocations in low-energy configurations and, to some extent, grain and twin boundary relaxation lead to the anneal-induced hardening effect at 600 °C. Grain boundary dislocations are hindered from bowing out or activating new dislocation sources, resulting in the strong increase in yield and ultimate tensile strength.