不同加载速率下氧化铝纳米线弯曲力学行为的分子动力学研究

Molecular dynamics analysis on bending mechanical behavior of alumina nanowires at different loading rates

采用LAMMPS软件建立弯曲加载下α-Al_(2)O_(3)纳米线的分子动力学(MD)模型,计算并分析不同加载速率下α-Al_(2)O_(3)纳米线的原子应力和应变,揭示加载速率对其弯曲力学行为的影响规律。研究结果表明:α-Al_(2)O_(3)纳米线在不同加载速率下的最大表面应力−转角曲线均可分为弹性变形、塑性变形和破坏3个阶段,弹性极限点可通过加、卸载循环下的曲线对称性来确定;加载速率对α-Al_(2)O_(3)纳米线的塑性变形影响很大,但对弹性模量影响较小;当加载速率增加时,塑性变形阶段缩短,材料更易发生脆性断裂,且弹性极限和强度极限(由直接和间接MD法确定)更加接近。MD模拟结果与改进的弯曲环路试验结果吻合良好,从而验证分子动力学建模和计算方法的有效性。直接MD法是确定在任意加载速率下(无论发生脆性还是韧性断裂)纳米晶须弹性极限和强度极限的有效方法。

The molecular dynamics(MD)model ofα-Al_(2)O_(3) nanowires in bending is established by using LAMMPS to calculate the atomic stress and strain at different loading rates in order to study the effect of loading rate on the bending mechanical behaviors of theα-Al_(2)O_(3) nanowires.Research results show that the maximum surface stress−rotation angle curves ofα-Al_(2)O_(3) nanowires at different loading rates are all divided into three stages of elastic deformation,plastic deformation and failure,where the elastic limit point can be determined by the curve symmetry during loading and unloading cycle.The loading rate has great influence on the plastic deformation but little on the elastic modulus ofα-Al_(2)O_(3) nanowires.When the loading rate is increased,the plastic deformation stage is shortened and the material is easier to fail in brittle fracture.Therefore,the elastic limit and the strength limit(determined by the direct and indirect MD simulation methods)are closer to each other.The MD simulation result ofα-Al_(2)O_(3) nanowires is verified to be valid by the good agreement with the improved loop test results.The direct MD method becomes an effective way to determine the elastic limit and the strength limit of nanoscale whiskers failed in brittle or ductile fracture at arbitrary loading rate.