Molecular dynamics simulation of shock-induced isostructural phase transition in single crystal Ce

Abstract

Cerium (Ce), a rare earth metal, undergoes a significant (14%−17%) and discontinuous volume shrinkage when subjected to ~0.7 GPa compression at ambient temperature: there happens a first-order isostructural phase transition from <i>γ</i>-Ce phase to <i>α</i>-Ce phase (these two phases are both face-centered-cubic (fcc) phase). Because of the <i>α</i>→ <i>γ</i> transition in Ce under shock compression, the shock front in cerium exhibits a 3-wave configuration: elastic precursor, plastic shock wave in <i>γ</i>-Ce, and phase transition wave corresponding to the <i>γ </i>→ <i>α</i> transition according to the experimental observation. In this paper, a recently developed embedded-atom-method (EAM) potential for fcc Ce is employed in the large-scale molecular dynamics simulations of shock loading onto single crystal Ce to study its dynamic behavior, especially the shock-induced <i>α</i>→ <i>γ</i> phase transition, and the orientation dependence with [001], [011] and [111] shock loading. The simulation results show single-wave or multi-wave configuration for shock wave profiles. Under the shock loading along the [001] or [011] crystallographic orientation, the shock wave possesses a 2-wave structure: an elastic precursor and a phase transition wave, while under shock loading along the [111] crystallographic orientation, the obtained shock wave shows a 3-wave profile as observed experimentally. Thus the shock wave structure is obviously dependent on loading orientation. The Hugoniot data obtained in MD simulation show good agreement with the experimental results. The shock loading MD simulation shows lower phase transition pressure than hydrostatic loading, indicating an accelerant role of the deviatoric stress played in the shock induced <i>γ </i>→ <i>α</i> phase transition in Ce. The local lattice structure before and after shocked are recognized with polyhedral template matching and confirmed with radial distribution functions. Under the [011] and [111] loading, the lattice structure maintains the fcc before and after the shocks, and experiences a collapse during the last shock (the second shock for the [011] loading and the third shock for the [111] loading). The lattice structure also maintains fcc before and after the first shock for the [001] loading, while after the second shock the structure type is considered to be body-centered-tetragonal (bct) which is a meta-stable structure resulting from the used EAM potential for Ce. The fcc lattice rotation after shock is observed in the [011] and [111] loading after the phase transition, while no re-orientation occurs in the [001] loading.