Cyclic HCP<->FCC Phase Transformation Crystallography in Pure Cobalt

The phase transformations between HCP and FCC structures are among the most important transformations in metallic materials. The memory effect during cyclic transformation around the transus temperature in pure cobalt was investigated using the in situ electron backscatter diffraction (EBSD) technique. The crystallographic variants of orientation were systematically derived and compared with the observations. Texture memory effect was observed at both room and high temperatures, and a notable variant selection was observed, with the microstructure being preserved after cyclic heat treatment. Based on EBSD observations, the transformation mechanism is explained based on nucleation from a crystallographic perspective. Restricted nucleation of the transformation variants by grain boundaries is proposed to explain the observed phenomena, and these proposals could be extended to similar transformation systems.