In situ EBSD study on the abnormal grain growth phenomenon in high-purity cobalt

The abnormal grain growth of high-purity cobalt during high-temperature annealing was analyzed in this work. The microstructure and orientation evolutions were characterized using the in situ electron backscatter diffraction technique. It was found that the HCP ↔ FCC phase transformation follows the orientation relationships of {0001}_HCP//{111}_FCC, < 11–20 > _HCP// < 1–10 > _FCC during both the heating and cooling process. At around 500 °C, the {100} oriented FCC grains exhibited abnormal growth, with the size exceeding 300 μm. The origin of this phenomenon was analyzed from the perspectives of grain orientation distribution and surface energy. The growth of {111} oriented grains is inhibited by the {111} texture, leading to the abnormal growth of {100} oriented grains, which have the second lowest surface energy. Additionally, the transformation microstructure of the abnormally large grains after cooling is analyzed. This work provides insights into the microstructure stability of high-purity cobalt sputtering targets during their service.