体心立方金属钨Ⅱ型微观裂纹的多尺度模拟研究

The Multiscale Simulation of ModeⅡAtomistic Crack in BCC Tungsten

摘要:

本文采取多尺度准连续介质计算模拟方法(quasi-continuum method, QC),对纳米量级体心立方(body-centered-cubic, BCC)金属钨Ⅱ型裂纹尖端缺陷生成和破坏过程进行计算模拟,得到系统的载荷位移曲线,以及加载过程中裂纹尖端原子位移图像。凭借QC方法的原子精确和较高的计算效率,共观察到了裂纹尖端五次全位错形核和发射现象。模拟结果表明,宏观载荷位移曲线各突降点对应的微观本质是裂纹尖端位错的形核和发射；位错的运动具有不连续性和周期性等特点；位错数量不断增加和快速运动最终导致Ⅱ型裂纹开裂和破坏。根据模拟结果,定量化统计得到全位错位置和加载位移的关系曲线,说明了位错的运动规律：全位错会在裂纹尖端稳定位置形核,并且后续会有不连续的周期性发射；新位错的形核会导致原位错的发射,并且随着全位错数量的增加,位错整体运动速度加快。最后,根据不稳定层错能理论和微观尺度的力平衡方程,对位错形核的初始位置做了理论计算,并对位错运动规律进行机理分析,得到的理论结果与模拟结果相一致,验证了模拟结果的正确性。

Abstract:

Atomistic cracks have been the subject of material intensive research in recent years due to the fast development of nanomaterials. In order to have a better understanding of the fracture mechanisms of body-centered-cubic (BCC) metal, the multiscale quasi-continuum method (QC) is employed to analyze the nano-sized crack of BCC tungsten. The mode Ⅱ crack of Tungsten(W) in {110} planes along the [111] direction is simulated. The load-displacement curve and atom displacement images for each loading step are presented. The generation of partial dislocations, the nucleation and emission of perfect dislocations and the movement of dislocations in crack tip have been observed. Simulation results show that partial dislocations will produce before perfect dislocation nucleation; each drop point of the load-displacement curve corresponds to the nucleation and emission of a perfect dislocation; dislocation nucleation happens several times along with the dislocation launching; the increasing number and rapid movement of dislocations eventually lead to mode Ⅱ fracture. According to the simulation results, the curve of dislocation position vs. displacement is presented, and the movement characteristics of dislocations are analyzed. The results show that all the dislocations will launch after a new dislocation nucleation, indicating that a new dislocation nucleation will promote dislocation movement, and dislocation movement will speed up with the increase in the number of dislocations. In addition, the phenomenon and mechanism of dislocation in BCC metal is analyzed according to the theory of crystallology and Rice’ theory of unstable stacking fault energy. Finally, the forces on and between dislocation are discussed. By calculating the force balance equation in microscale, the initial equilibrium position of the dislocation is forecasted, and the movement mechanism of dislocations near the crack tip is explained, which coincides well with the simulation results.