单晶铝中微喷射的分子动力学研究

Molecular dynamics study on micro jet in single crystal aluminum

采用非平衡分子动力学模拟方法,研究了方形冲击波在含有圆锥和正弦缺陷的单晶铝中一维和二维的微射流演化.利用正向和反向粒子追踪技术获得了微射流的粒子源,发现微射流不同部位的粒子源结构与缺陷类型密切相关,微射流的形成是从缺陷表层到内层的分层聚集过程.锥形情况下的射流头部速度最初急剧增加,然后快速下降至恒定速度;锥形和正弦情况下的喷射体积先经历非线性增加,后保持不变.通过进一步对喷射粒子数量、密度、速度和温度之间的关系分析,发现在圆锥和正弦情况下,当冲击速度为6 km/s时喷射物速度在冲击方向上基本呈线性变化,且射流最大速度位于射流头部;但在正弦情况下,当冲击速度为2 km/s时喷射物速度在冲击方向上波动变化,射流头部速度低于为头部后侧的速度.更重要的是,本文揭示了两种机制,即一维射流的颈缩与二维射流的孔洞增长和贯穿,它们主导了射流的破碎.实际上,速度、温度和压力的扰动都有助于射流破碎,其中速度扰动对其破碎具有最重要影响.此外,速度梯度会引起孔洞边界变厚,从而导致射流头部密度增加.

Non-equilibrium molecular dynamics simulations are performed to explore the evolution of one-and two-dimensional micro jets induced by square shock wave in single crystal Al with conical and sinusoidal defects.The particle source of micro jet is obtained using the forward and backward particle tracing techniques,and it is discovered that the structure of particle source in different parts of micro jet closely depends on the defect type,and the formation of micro jet is a layered aggregation process from the defect’s surficial layer to its inner layer.The jet head velocity in the conical cases initially experiences a sharp increase,then a rapid decline to be constant,and jet volume in both conical and sinusoidal cases undergoes a nonlinear increase before remaining unaltered.A further analysis on the relationship among particle count,density,velocity,and temperature is performed,where the ejecta velocity along the shock direction fundamentally shows a linear variation and the maximum velocity is in the jet head in the conical cases and sinusoidal case of shock velocity u_(p)=6 km/s,but the jet velocity presents a fluctuant variation and the head velocity lowers than that in the post-head in the sinusoidal case of u_(p)=2 km/s.More importantly,it is revealed that two mechanisms,the necking for the one-dimensional jet and void growth and coalescence for the two-dimensional jet,dominate the jet breakup.In fact,velocity,temperature and pressure perturbations all contribute the jet breakup,with the velocity perturbation having the most important influence.Furthermore,the thickening of void border is due to the velocity gradient,which produces an increase in density in the jet head.