Revealing the Impact of Initial Textures on Orientation Memory Effect During HCP → FCC → HCP Transformation in High-Purity Cobalt

The orientation memory effect after the phase transformation (HCP → FCC → HCP) cycle was observed in high-purity cobalt with different initial textures using the in situ electron backscatter diffraction technique. Profuse <111>/60° twins are formed during the HCP to FCC phase transformation upon heating and subsequently transform back to original HCP grains after cooling. Unlike previously reported mechanisms in other alloy systems, such as phase transformation strain or residual parent phase, we found that the HCP variant grains maintain a Shoji-Nishiyama orientation relationship with neighboring FCC grains during the FCC to HCP transformation. This unique relationship lowers the nucleation barrier, leading to a remarkable orientation memory effect.