斜波压缩下剪切力对铁α→ε相变的影响研究

Materials phase transition under shock compression has attracted great interest in past decades，as it has important applications in weapon physics, high pressure physics and other fields. The differences between the dynamic phase transition and the static experimental results are not only due to the loading rate effects (non-equilibrium characteristics), but also to the non-hydrostatic stress state of the dynamic loading. Despite a small amount of experiments have confirmed the conclusion predicted by numerical simulation that the shear stress has significant influence on the material phase change process , but the more adequate experimental verification is still a big challenge due to the experimental technique limits.. This application intends to use the latest developed magnetically applied press-shear loading technique to realize the control of stress state, and then to give a quantitative description of hydrostatic pressure and shear stress state, finally to obtain accurate effects of shear stress on the phase transition pressure and characteristic time. By large-scale molecular dynamics simulations of phase transition of iron under the pressure-shear stress state, we intend to obtain the influence of shear stress on the path of phase transition, phase transition barrier and phase transition pressure of iron. Based on the results of physical experiments and molecular simulations, we expect to obtain quantitative understanding of the effect of shear stress on the phase transition process of iron, and to give a direct answer to the long-standing problems how the shear stress influence the phase transition process of materials.

材料的冲击相变研究是冲击动力学界长期关注的问题，在武器物理、高压物理等领域具有重要的应用价值。材料动态相变与静态实验结果的差异，不仅源于加载的率效应（非平衡特征），也和动态加载下的非静水压状态有关。尽管已有少量的实验结果对模拟计算预测的剪应力对材料相变过程具有重要影响这一结论给予了证实，但更为充分的实验验证和分析在实验技术上仍存在很大的挑战。. 本项目拟利用最新发展的磁压剪实验技术，通过对材料内部应力状态的调控，实现相变前材料内部静水压力和剪切应力分布的定量化描述，获得切应力对铁相变压力和相变特征时间影响的准确认识。利用大规模分子动力学计算，开展压-剪复杂应力状态下铁的相变模拟，分析切应力对铁相变路径、相变势垒和相变压力的影响。基于物理实验和分子计算结果，获得切应力对铁相变过程影响的定量化认识，对切应力如何影响材料的冲击相变过程这一长期悬而未决的难题给予直接的回答。

动高压加载下材料的非平衡相变在材料动力学和武器物理研究中具有重要的价值。本项目采用分子动力学计算和磁驱动等熵压缩实验相结合的方式，开展了预制剪切应力对铁α→ε相变过程的影响研究，试图获得切向应力对铁相变过程影响的定量化认识。分子动力学模拟结果表明预剪切力的存在提升了相变成核速度，最终使得相变波传播速度增大，但是预剪切力的存在未改变单晶铁的相变机制。当冲击压力超过多晶铁的相变压力阈值时不多时，预剪力能起到促进相变的作用，导致相变速率加快和相变份额增大。当冲击压力超过铁的相变压力较多时，预剪力对铁α→ε相变过程影响减弱。等熵压缩实验结果表明多晶铁和单晶铁的相变起始压力差异并不大，主要是因为单晶铁发生弹塑性转变后其内部细观结构与多晶铁类似。多晶铁的磁压剪实验结果表明，压-剪状态下纯铁的动态相变起始于11.8GPa，高于静高压-剪状态下的相变起始压力10.8GPa，但低于准等熵压缩下的13.2GPa，这表明剪切应力有助于降低铁的α→ε相变阈值。相关研究成果对于深入了解铁相变的物理机制和高精度物理模型建立提供了基础性数据。

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