Iron will undergo the BCC to HCP transformation under pressure, but the reverse process during the unloading path has not been fully revealed on the atomic scale. This work investigated the unloading dynamics of HCP single crystal of iron, focusing on the microstructure evolution and related mechanical characteristics. For unloading along the normal direction of the (-12-10) and (10-10) planes, a mechanism for coupling between twinning and phase transformation was reported. The HCP to BCC transformation with the rod-like structure and twinning was revealed, which was well supported by previous experiments. For unloading along the normal direction of the (0001) plane, the HCP--FCC-BCC transition was observed. Significant shear stress was generated internally, leading to significant dislocations. There are multiple transition paths of the HCP to FCC -phase, resulting in the formation of grain boundaries and ultimately leaving the polycrystalline structures. This process was accompanied by coupled development of dislocations, grain mergers, and phase transitions. As the unloading process gradually evolves into the stretching process, detwinning and reconstruction will occur for unloading along the normal direction of the (-12-10)  and (10-10)  planes, accompanied by grain rotation.
 

Twinning,phase transition,iron