柱面冲击载荷下铁的塑性变形与相变机理原子模拟

The study of mechanical response behavior under high-pressure dynamic loading has important significance on weapons design, materials synthesis, energy development and other related areas, and it is also an important scientific research interest of compression science. In comparison with planar shock loading, the materials under cylindrical shock loading suffer different strain constraint, different stress state and shearing or local deformation arising from the relative motion of internal material segments, and they show different response behaviors of plastic deformation and phase transition. Affect by the ultra-high temperature and pressure, and ultra-fast response process and complex geometry configuration in cylindrical shock loading, it is difficult to understand the mechanisms of plastic deformation and phase transition completely from experimental studies. In the present proposal, we intend to develop atomistic simulation technique of cylindrical shock loading, and adopt large-scale non-equilibrium molecular dynamics simulations and microstructure analysis technique suitable to dynamical processes to study the plastic deformation and phase transition of metallic iron under dynamic cylindrical shock-loading. It is expected to discover the mechanisms of plasticity and phase transition of iron on atomic scale. With simulating the process of interaction between the microscopic feature plasticity and the crystal structure changes for iron under a cylindrical dynamic loading, the lattice-level coupling mechanism is analyzed to give an appropriate physical image and build a theoretical model. It can provide the physical basis for establishing high pressure constitutive relations and enhancing our confidence in dynamic material models under the entire range of stress and strain states at which they are utilized.

高压冲击载荷下材料的力学响应研究对武器设计、材料合成和能源开发等相关领域有着重要指导意义，也是压缩科学的重要研究方向。同平面冲击相比，材料在柱面冲击载荷下受到的应变约束和所处的应力状态不同，内部单元之间的相对运动会引起显著的局域变形，所以冲击塑性和相变规律表现出明显差异。由于复杂的几何结构和超快的响应过程，且体系处于高压、高温极端状态，仅采用实验方法难以揭示柱面冲击过程中材料塑性变形和相变的微观机理。本项目拟开发柱面冲击加载原子模拟技术，采用大规模并行非平衡分子动力学方法和适用于动态过程的微结构分析技术，模拟高压动态柱面载荷下金属铁的塑性变形和相变过程，通过跟踪局域结构、应力和应变分布，揭示应力状态对塑性变形和相变的影响规律和微观机理，分析柱面动态载荷下塑性变形与相变间的相互影响过程，探讨其晶格层次的耦合机制，为认识整个应力与应变状态下的材料本构关系和提高动态材料模型的置信度提供物理依据。

高压冲击载荷下材料的力学响应是压缩科学的重要研究方向。材料在柱面冲击载荷下受到的应变约束和所处的应力比较复杂，导致塑性和相变规律与平面冲击相比表现出明显的差异。由于复杂的几何结构和超快的响应过程，仅采用实验方法难以揭示柱面冲击过程中材料塑性变形和相变的微观机理。本课题采用大规模并行非平衡分子动力学方法结合柱面冲击加载原子模拟技术和适用于动态过程的微结构分析技术，模拟高压动态柱面载荷下金属铁和金属铜塑性变形和相变规律，并揭示其微观机理。模拟结果表明金属在柱面外爆和柱面内爆两种不同加载方式下塑性和相变的微观过程有显著的差异。柱面外爆加载下单晶铁样品中观察到了伴随着晶格重定向的α→ε→α→ε 相变过程，并形成局部相变孪晶。柱面内爆加载样品中观察到大量的相变孪晶，这是两种变体均匀形核，在生长过程中相互接触而形成的。另外，柱面冲击加载下单晶铁塑性与相变的耦合方式主要是通过塑性滑移引起原子在相变滑移面面上集体运动和重排来激活相变形核，柱面内爆加载下这一过程更复杂。这两种柱面冲击加载方式对单晶铜熔化和层裂特性的影响也尤为明显。内爆冲击加载下，冲击波前端通过塑性变形和冲击熔化释放剪切应力。而在外爆加载的作用下，冲击波由里往外传播，能量逐渐耗散，这就导致外爆加载发生冲击熔化所需要的冲击速度高于内爆加载。对于层裂而言，两种加载方式引起的层裂缺陷成核位点不同，内爆加载下层裂在预熔区域成核，而外爆加载下层裂缺陷在<100>方向上的孪晶界成核。柱面内爆加载下单晶铜的位错形核的位置与冲击波速度有关。在冲击速度较低时，位错环在冲击波前沿的热点区域成核和运动，当激波速度增加到0.55 km/s时，位错在自由表明成核。另外不同柱面轴的样品启动的滑移系不同，其中柱面轴沿[001]时位错成核的因素由分切应力和滑移面法向应力共同决定的。柱面轴沿 [110]与[111]时位错环运动与平面单轴拉伸中位错环运动相似，并在微观尺度观察到了位错的分解与合成反应。项目从原子角度探究了柱面动态载荷下金属材料的塑性变形与相变及其相互影响过程，揭示了其晶格层次的耦合机制，为认识复杂应力应变状态下材料本构关系和提高动态材料模型的置信度提供物理依据。