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 -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 -axis results in an abundance of deformation twins, with twinning planes identified as (130) and (10). 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 -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 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 -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.

中文翻译：

---

冲击压缩下α-铀单晶的取向相关变形机制

采用大规模非平衡分子动力学（NEMD）模拟来研究α-铀（α-U）单晶在沿低折射率晶体取向受到不同冲击强度时的动态变形。 α-U 明显的各向异性导致冲击载荷下复杂的微观结构演化。对冲击后样本的深入微观结构分析揭示了多种动态变形机制的识别。值得注意的是，当冲击载荷方向与 - 轴对齐时，α-U 单晶的动态变形主要由晶格不稳定性主导，这归因于晶体到非晶态的转变作为主要的剪切应力松弛途径。另一方面，沿 - 轴的冲击载荷会导致大量变形孪晶，孪晶面被标识为 (130) 和 (10)。在孪晶过程中，α-U 基体转变为亚稳态中间相，随后分解为包含 α-U 孪晶和基体的复合结构。这种非常规的孪生机制明显偏离了经典理论。此外，在沿 - 轴加载时，α-U 单晶样品中会发生孪生和从 α-U 到体心四方相 (bct-U) 的相变。鉴于计算预测的相变压力阈值高达~270 GPa，从 α-U 到 bct-U 的相变可能是难以置信的。采用具有较高压力阈值的铀的替代原子间势来重新研究 α-U 单晶沿 - 轴的冲击响应。 α-U向bct-U的相变消失，塑性变形以孪晶为主，孪晶取向符合{112}孪晶。 α-U晶格的强各向异性引发了大量与方向相关的动态变形机制。孪晶系统的激活显然与加载方向相关，这构成了在多晶铀变形过程中发现多个孪晶变体的潜在原因。