Understanding the Role of Microstructure on Fe’s BCC → HCP → BCC Transformation using Molecular Dynamics and Virtual Texture

Fe phase transforms from BCC → HCP when compressed above a threshold pressure of ~13 GPa, then reverse transforms to BCC when unloaded. This follows Burgers pathway for selection of HCP variants during loading and BCC variants during unloading. One fingerprint of this BCC → HCP → BCC transformation is suggested to be the distribution of twins in unloaded microstructures. However, the factors that influence transformation twin formation are not well understood. Their stability can be related to the selection of HCP variants during compression and to the Schmid/non-Schmid effects (twinnability) for the BCC phase wherein loading orientation plays a role. Polycrystalline microstructures add the complexity of neighboring grain misorientations that can influence both factors. This study uses molecular dynamics simulations to understand the role of microstructure on the stability of transformation twins in Fe after unloading. Virtual texture analysis (VirTex) enables characterization of phase variants throughout the stages of transformation and elucidates mechanisms of transformation twinning. This talk will discuss the role of phase variant selection and twinnability using various single crystal orientations and modifications due to grain misorientations using bi-crystal systems. Finally, transformation twinning is investigated in a polycrystalline microstructure of Fe.