Orientation-dependent deformation mechanisms of alpha-uranium single crystals 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 reveals the identification of multiple dynamic deformation mechanisms. Notably, when the shock loading direction aligns with the a-axis, dynamic deformation of the α-U single crystals is primarily dominated by lattice instability, which attributes to a crystalline-to-amorphous transition serving as the dominant shear stress relaxation pathway. On the other hand, shock loading along the b-axis results in an abundance of deformation twins, with twinning planes identified as (130) and (13¯0). During the twinning event, the α-U matrix undergoes a transition to a metastable intermediate phase, subsequently decomposing into a composite structure comprising α-U twins and matrix. This unconventional twinning mechanism significantly deviates from classical theories. Furthermore, upon loading along the c-axis, twinning and a phase transition from α-U to body-centered tetragonal phase (bct-U) occur in α-U single crystal samples. Given that the pressure threshold of this phase transition predicted by ab initio calculations is as high as ∼270 GPa, the phase transition from α-U to bct-U might be implausible. An alternative interatomic potential of uranium with the higher pressure threshold was employed to reinvestigate the shock response of α-U single crystals along the c-axis. The phase transition of α-U to bct-U disappears, and twinning dominates the plastic deformation, with the twinning orientation conforming to the {112} twinning. The strong anisotropy of the α-U lattice triggers a wealth of orientation-dependent dynamic deformation mechanisms. The activation of the twinning system is evidently associated with the loading direction, constituting the potential cause for the discovery of multiple twinning variants during the deformation in polycrystalline uranium.