Orientation and grain-boundary dependence of shock-induced plasticity and transformation in nanocrystalline Ti

Abstract

The plasticity and $\ensuremath{\alpha}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}\ensuremath{\omega}$ transformation are important for the toughness and ductility of hcp titanium under shock loading. However, three questions remain outstanding: (i) what mechanisms govern the plasticity and transformation, (ii) how does the microstructure, i.e., grain boundaries (GBs) and crystallographic orientations, affect them, and (iii) does the transformation take place dependent on the plasticity, such as dislocation slips? We conduct large-scale nonequilibrium molecular dynamics simulations to study shock-induced plasticity and phase transformation in hexagonal columnar nanocrystalline Ti. Significant anisotropy and strong dependence on crystallographic orientation are presented during shock-induced plasticity and phase transformation. The shock first prompts ``heterogeneous'' dislocation slips and ${90}^{\ensuremath{\circ}}$ lattice reorientation, via coupling deformation twinning and slips. Then, the on-going plastic deformation induces a ``heterogeneous'' $\ensuremath{\alpha}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}\ensuremath{\omega}$ phase transformation at lower impact velocities or a ``homogeneous'' solid-state disordering at higher impact velocities. The phase transformation mostly obeys the TAO-1 pathways originated from GBs, while a few of them are governed by Silcock mechanisms within the grains. The TAO-1 and Silcock-governed transformations stem from the emission and propagation of basal-prismatic and prismatic stacking faults, respectively. At the release/tension stage, a $\ensuremath{\omega}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}\ensuremath{\alpha}$ transformation occurs, acting as the reversed process of the $\ensuremath{\alpha}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}\ensuremath{\omega}$ transformation at the compression stage. Meanwhile, structural recovery and spallation initiate in the extending tension area induced by the release fans. Serving as the nucleation of the plasticity, phase transformation, and spallation, GBs play the key role during the loading.