Orientation-Dependent Deformation Mechanisms of Uranium Under Shock Compression

Abstract

Large-scale non-equilibrium molecular dynamics (NEMD) simulations were employed to investigate the dynamic deformations of alpha-uranium (α-U) single crystals subjected to varying shock strengths along low-index crystallographic orientations. The pronounced anisotropy of α-U gives rise to a complex microstructural evolution under shock loading. In-depth microstructural analysis of post-shock specimens revealed the identification of multiple dynamic deformation mechanisms. Notably, when the shock loading direction aligned with the a-axis, α-U exhibited a high Hugoniot elastic limit (HEL), dynamic deformation of the α-U single crystals is primarily dominated by lattice instability, which attributed to a crystalline-to-amorphous transition serving as the dominant shear stress relaxation pathway. On the other hand, shock loading along the b-axis resulted in an abundance of deformation twins, with twinning planes identified as (130) and (1-30). During the twinning event, the α-U matrix underwent a transition to a metastable intermediate phase, subsequently decomposing into a composite structure comprising α-U twins and matrix. This unconventional twinning mechanism significantly deviated from classical theories. Furthermore, loading along the c-axis led to a reduced HEL in α-U, accompanied by more plentiful deformation mechanisms. The dynamic deformation of these specimens revealed the presence of α-U twins and the emergence of the body-centered tetragonal phase of uranium (bct-U). This study, from a kinetic perspective, provides unprecedented insights into the necessary conditions and evolution path for the α-U → bct-U phase transition under extreme pressures, contributing to an enhanced comprehension of the phase behavior of uranium under extreme conditions.

Keywords: Dynamic loading, Uranium, Phase transition, Deformatioin twin