纳米多晶金属样本构建的分子动力学模拟研究

Construction of metallic nanocrystalline samples by molecular dynamics simulation

研究了分子动力学模拟中纳米多晶金属样本的构建过程.首先采用Voronoi几何方法生成初始的纳米多晶铝和铜样本,然后用快速冷凝(或共轭梯度)法得到样本的局域最低能态,最后在恒温零应力周围环境下(常温常压NPT系综)退火得到最低能态样本.使用样本的残余内应力来衡量纳米多晶样本是否与实验制备的一致.通过监测这两步弛豫过程中晶界结构的变化形态、体系平均内应力和能量下降过程及具体的局域分布和不同弛豫条件下最终样本的弹性常数,发现样本的能量和残余内应力都接近实验制备的纳米多晶金属.对Voronoi几何法生成的晶界而言,5—10ps的快速冷凝(或共轭梯度法)能量最小化,40—100ps的升温(温度控制在室温到0.65倍熔点)退火较合适,弛豫时间和退火温度选择在一定范围内时对弛豫样品终态的力学性质影响很小.

The construction of metallic nanocrystalline（NC） samples by molecular dynamics simulation is investigated.Firstly,the initial NC aluminum and copper samples are assembled by Voronoi geometrical construction method,then the local minimized energy states of the samples are obtained by quenching（or conjugate gradient method）.Finally,the simulated annealing method in normal pressure and temperature condition ensembles at zero pressure is used to approximate the global minimized energy states of the samples.The residual internal stress is employed to signify the difference between the simulated and the experimentally synthesized samples for the first time.The structure of grain boundaries,the descending process and the local distribution of the average internal stress and the energy of the samples,as well as the elastic constants of the final samples are observed during these two relaxation procedures.It is found that the energy and the residual internal stress of the samples are close to the experimental data after relaxation.It is enough to obtain the global minimum energy states through Voronoi geometrical construction to investigate the static and dynamic mechanical properties of NC metals with a 5—10 ps local energy minimization and a 40—100 ps of simulated annealing with annealing temperature between the room temperature and 65% of melting point.The annealing time and temperature are of little importantce to the mechanical properties within the parameter windows properly selected.