Reversibility of the structural transition in single crystal iron driven by uniaxial and triaxial strains: Atomistic study

Atomistic simulations are used to investigate the structural transition (ST) in single crystal iron and its reverse process in isothermal (NVT) and adiabatic (NVE) ensembles. The hysteresis and microstructural reversibility of the ST are examined in detail. Under [001] uniaxial loading, we observe HCP twins oriented along the original {010} (T1) and {110} planes (T2) of BCC phase after twice HCP nucleation. Interestingly, T2 HCP twins will be annexed by T1 HCP twins with the increase of compression strain. Due to the strain rate effect, the start stress of ST and the nucleation number in NVE ensemble increase. In the unloading process, T2 HCP twins will reappear and BCC nucleation begins at the T2 twinning plane. The reverse ST in NVE ensemble begins earlier because of temperature rise, but ends at a later time. These analyses show that the [001] uniaxial strain-driven ST exhibits good microstructure reversibility with some hysteresis. Under triaxial loading, lamellar twins of HCP-FCC mixed phase along the {110} planes of BCC phase are found in NVT ensemble, where the start pressure of ST is much higher than that under [001] loading. BCC phase will nucleate at the grain boundaries in the unloading process. In NVE ensemble, plenty of nucleation takes place on the equivalent closed-packed planes and results in very small parallelepiped grains. Finally, BCC nanocrystals are formed after complete unloading, indicating an irreversibility of microstructure driven by triaxial strain at the high strain rate.