Variant Selections under Shock-induced Phase Transformation and deformation Twinning in BCC Metal Microstructures.*

The deformation behavior of BCC metal microstructures at high pressures has contributions from dislocation slip, deformation twinning, and phase transformation. While deformation twinning is a common mode of deformation in BCC metals at high pressures, Fe-based microstructures also demonstrate a BCC  →  HCP phase transformation when deformed above a threshold pressure of ~13 GPa. Recent studies have demonstrated the BCC  → HCP  → BCC phase transformation can result in a distribution of twins in the bcc microstructure. This distribution of twins is attributed to the selection of HCP phase variants during compression and their stability and reverse transformations during unloading. Understanding and predicting the role of microstructure and stress-states on these variant selections during deformation and their stability and reverse transformation behavior during unloading is important. The current efforts to investigate variant selections in BCC metals are largely limited to real-time in situ x-ray diffraction (XRD) and for single-crystal (sc) systems. In addition, the interpretations of the plasticity contributions from diffractograms are non-trivial, especially when multiple modes of deformation may be operating.  Molecular dynamics (MD) simulations can successfully capture various deformation modes in metals and complement experiments using simulated diffractograms at various stages of evolution. This talk will discuss the use of virtual XRD and a newly developed virtual texture analysis (VirTex) algorithm to characterize the variant selections and their plasticity contributions during plastic deformation. The simulations investigate the role of BCC microstructure and loading stress-states on variant selections during phase transformation and twinning in BCC Fe microstructures as predicted using MD simulations. In addition, the simulations investigate the stability and the reverse transformation behavior of the variants during unloading. The simulations are able to unravel the role of variant selection and transformation that renders a distribution of twin boundaries in unloaded microstructures.