Modeling the Shock-Induced Phase Transformation Behavior in Fe Microstructures at the Atomic Scales and Mesoscales

Shock compression of Fe microstructures above threshold pressures results in a BCC→HCP phase transformation. The experimental study of the mechanisms of phase transformation is largely limited to in situ diffraction methodologies as the post-shock characterization results in a reverse phase transformation from the HCP→BCC phase occurs upon unloading. This study demonstrates the capability of molecular dynamics (MD) simulations to investigate these mechanisms at the atomic scales. MD simulations are carried out to investigate the role of loading orientations and shock pressures on the thresholds of phase transformation and identify the correlations between loading orientations, and shock pressures as predicted using MD simulations, and unravel the conditions of reverse transformation induced twinning during shock release in single-crystal microstructures. To model the shock response of polycrystalline microstructures, a novel mesoscale modeling method, Quasi-Coarse-Grained Dynamics (QCGD), is used to extend the study to investigate the behavior of polycrystalline microstructures at the experimental scales with grain sizes of up to a few microns. The QCGD method retains the atomistic mechanism of dislocation slip, twinning, and phase transformation behavior as predicted using MD simulations. This capability is based on coarse-graining of atomic-scale microstructures using representative atoms (R-atoms) and using scaling relations for defining the interactions between R-atoms. The phase transformation and twinning variants in the atomic scale and mesoscale microstructures is characterized using orientation matrix and angle/axis pair from virtual texture analysis at different stages of shock evolution. MD and QCGD simulations reveal that Fe oriented along [110] direction is prone to twin with four different variants originating from the dominant hcp phase with the arrival of release wave. The talk will discuss the mechanisms of the formation of various HCP phase variants formed during shock compression and the mechanisms of reverse transformation and twinning during shock release to identify the role or loading orientations and shock pressures as predicted using MD and QCGD simulations.