原子热运动的非线性和对纳米材料变形动力学过程的影响

Experiments show that temperature and loading strain rate can dramatically affect mechanical properties of materials. Microscopically, temperature is corresponding to atomic thermal vibrations with their intrinsic time scale of about 1 fs to 1 ps. The strain rate is related to atomic motions due to dynamic deformations. Normally, the time scale of mechanical loadings in experiments is about 1 ms to 1 s which is at least 10 orders of magnitude larger than that of atomic thermal vibrations. Because of this huge temporal gap, it is rather difficult to simulate micro/nano-materials' mechanical responses at a realistic loading strain rate comparable to that in experiments yet with temperature effects considered simultaneously using a molecular simulation tool. In this application, we are going to simulate the dynamic loading processes of nano-materials using molecular dynamics. By analyzing the effect of atomic thermal vibrations on the deformation dynamics, three problems will be studied systematically: approximation method for presenting the non-linear atomic thermal vibrations, the microscopic mechanisms of loading strain rate effects on mechanical properties of materials and the interaction between thermal vibrations and deformation dynamics. Based on these results, a new molecular modeling method is planned to be developed for simulating micro/nano-materials at finite temperature and under realistic loading strain rates. It is expected that this project will fill the huge gap between the time scales of atomic thermal motions and deformation dynamics and, provide a tool for simulating a micro/nano mechanical process similar to that of experiments.

实验表明，温度和加载应变率对材料力学性能有着显著影响。温度源于原子热运动，特征时间尺度为ps量级；而应变率则与动力学变形引起的原子运动相关，通常实验加载时间尺度为ms~s量级，与热运动特征时间相差近10个量级。这一巨大差距导致微/纳米尺度分子模拟方法难以在刻画了温度效应的同时，又做到与实验加载过程直接相比。本申请项目拟先采用分子动力学方法模拟纳米材料的动力学加载过程，分析原子热运动对变形动力学过程的影响，来系统研究三个基本问题：原子热运动的非线性近似表征方法、应变率效应的微观机制、热运动和变形运动的相互作用规律。以此为基础，探索发展新的分子模拟方法，用于纳米材料在有限温度、实际加载应变率下变形行为的计算。此项研究将有助于解决原子热运动对动力学过程模拟在时间尺度上的制约，并促进跨时间尺度分子模拟方法的发展。对微/纳米尺度材料力学性能研究，具有重要的理论价值和实际意义。

项目针对目前分子模拟方法不能有效同时处理微/纳米材料在加载过程中所出现的“温度效应”和“应变率效应”问题，从原子热运动和加载变形运动的关联入手，展开了以下研究内容：原子热运动的非线性近似表征方法、应变率效应的微观力学机制、热运动和变形运动的相互作用规律。主要研究结果有：(1) 针对原子非线性热运动的三种模式，提出Duffing振子近似表征方法，从理论上给出了频率计算表达式，为非线性振子频率的计算提供了理论基础；(2) 从原子热运动与势阱演化相耦合的角度，阐明了分子应变率效应的微观机制；(3) 提出分子动力学(MD)/分子统计热力学(MST)耦合算法模拟“低应变速率”问题，其计算结果与MD一致，计算效率比MD高两倍以上；(4) 结合实验和分子模拟结果，揭示了纳米压入硬度尺寸效应的来源和机理。项目研究加深了原子非线性热运动对纳米材料力学性能影响的理解，同时为发展有限温度下低应变率或长时间分子模拟方法提供了理论依据和参考。