金属铍和锆高压熔化的分子动力学研究

Equation of states (EOS) of metals has wide applications in national important projects such as inertial confinement fusions and designs of nuclear weapons. Multi-phase EOS of metals is extremely important to improve these projects and designs. Solid-liquid phase transition is one of the most important contents in the EOS research. So recently the high-pressure melting of metals has become an open issue internationally. In this project, the physical mechanism of high-pressure melting of metals Be and Zr, which are widely used in nuclear industries, will be uncovered from molecular dynamics (MD) and ab initio MD simulations combined with a high-precision melting method. The contents in detail are as follows: to investigate the melting curve of Be under high pressure with the modified Z method and ab initio MD; to calculate the melting curve of Zr under high pressure with the modified Z method and ab initio MD; to gain the melting mechanism of nanocrystalline Be under high pressure; to seek the melting mechanism of nanocrystalline Be under shock loading from the MD simulations; to achieve the melting mechanism of nanocrystalline Be under release loading from the MD simulations. Some high-precision data and physical mechanism of high-pressure melting of metals, which will be produced in this project, can solve the lack of high-pressure melting data of these two metals and help us to understand the unconceivable discrepancies of the melting data from the DAC and the shock-loading experiments, which can play an important role on constructing the multi-phase EOS of metals so as to improve the confidence level of designs in these national important projects.

金属的物态方程广泛应用于惯性约束聚变和核武器等国家重大工程设计中，建立高精度的金属材料多相物态方程对提高这些工程的设计水平及其重要，而固液相界是构建多相物态方程的重要内容，因此确定金属材料的固液相界成为当前国际上物态方程研究的热点问题。本项目以核工业中的重要金属材料铍和锆为研究对象，通过经典分子动力学（MD）和第一原理MD模拟，结合先进的熔化计算方法，研究金属的高压熔点和熔化机理。具体的研究内容包括：基于改进Z方法的金属铍高压熔化的第一原理MD研究；基于改进Z方法的金属锆高压熔化的第一原理MD研究；纳米晶金属铍静高压熔化的MD研究；纳米晶金属铍冲击熔化的MD研究；纳米晶金属铍卸载熔化的MD研究。本项目的研究成果预期可解决这两种金属高压熔化数据的匮乏，增加人们对动静高压熔化实验数据的认识，有助于建立它们的多相物态方程，从而提高国家重大工程设计的置信度。

本项目聚焦于金属材料高压熔化问题，获得高精度的固液相变数据，认识动态冲击加卸载熔化机理。将熔化模拟的改进Z方法推广应用于第一原理分子动力学领域，针对铍的BCC相和HCP相、锆的BCC相和钒的BCC相，开展了高达数亿大气压的熔化模拟研究，获取了金属铍BCC相和HCP相500GPa以内的多个高精度熔点数据，揭示了HCP相是毗邻液相的固相结构，计算的熔化曲线和DAC实验结果符合较好。获取了金属锆在120GPa范围内的多个高精度熔点数据，其中常压熔化温度和实验符合非常好。获取了金属钒在470GPa范围内多个高精度数据，预测结果得到了静高压和动高压实验的验证。.利用分子动力学方法，通过构建高达数千万原子的多晶铜体系，开展了达1微米长样品的冲击加卸载熔化模拟，设计了并行的数据处理程序，提取了表征熔化的原子序参量和径向分布函数等，分析了冲击波作用下物理量演化过程，发现冲击熔化温度和相同压力下的静态熔化温度一致，这不同于单晶的过热或过冷熔化现象。在冲击压力较高至接近冲击熔化时，冲击波后温度需要长达~100ps才趋于稳定值。这是由于波后过冷液态中的晶核在逐渐生长。冲击卸载熔化的起始压力较低，完全卸载后的温度比静态熔化温度低约7%，表明卸载熔化属于过冷熔化。分析发现卸载过程中有先结晶再熔化的结构转变过程。