Microstructure Evolution During Annealing Of Cryogenically Processed Cantor High Entropy Alloy

This work focuses on the annealing behavior of a cryogenically processed Cantor high entropy alloy over a wide temperature range of 500–800°C. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscopy (TEM) were employed to analyze the microstructural evolution, with particular emphasis on twinning. Hardness and grain size were measured to correlate these microstructural changes with mechanical properties. The cryogenically processed alloy exhibited a high density of twin bundles, resulting in an initial hardness of 338 HV. Upon annealing at 500 °C, 550 °C, and 600 °C, the material showed only a limited degree of recrystallization, suggesting that the twin bundles introduced during cryogenic rolling persisted. Interestingly, hardness increased 19% after annealing at 500 °C, which may be attributed to the formation of ordered domains or nanoscale precipitates at existing defects, despite partial recovery. Nonetheless, EBSD and XRD analyses indicated the persistence of a single FCC solid-solution phase across all annealing conditions. From 550 °C and forward, the hardness progressively decreased, attributed to recovery and recrystallization processes. As the annealing temperature increased to 700 °C, 750 °C, and 800 °C, nearly complete recrystallization was achieved, reaching 153.3 HV at 800 °C, consistent with softening due to recrystallization. During this process, the twin bundles persisted in non-recrystallized regions adjacent to newly formed equiaxed grains, indicating a discontinuous recrystallization mechanism driven by grain boundary nucleation. Annealing twins remained visible even at 800 °C, suggesting that these temperatures and annealing times were insufficient to induce significant boundary migration. Simultaneously, the average grain size decreased from 72.5 µm in the cryogenically rolled condition to 27.2 µm after annealing at 800 °C, indicating grain refinement facilitated by nucleation on twinned regions and other defects. Grain coarsening was not observed within the studied temperature range, implying that higher temperatures may be required to activate substantial boundary mobility and grain growth.