晶体相场法研究应力状态及晶体取向对微裂纹尖端扩展行为的影响

采用晶体相场模拟研究了单向拉伸作用下初始应力状态、晶体取向角度对单晶材料内部微裂纹尖端扩展行为的影响, 以(111)晶面上的预制中心裂纹为研究对象探讨了微裂纹尖端扩展行为的纳观机理, 结果表明: 微裂纹的扩展行为主要发生在<011>(111)滑移系上, 扩展行为与扩展方向与材料所处的初始应力状态及晶体取向紧密相关. 预拉伸应力状态将首先诱发微裂纹尖端生成滑移位错, 进而导致晶面解理而实现微裂纹尖端沿011]晶向扩展, 扩展到一定程度后由于位错塞积, 应力集中, 使裂纹扩展方向沿另一滑移方向101], 并形成锯齿形边缘; 预剪切应力状态下, 微裂纹尖端首先在101]晶向解理扩展, 并诱发位错产生, 形成空洞聚集型长大的二次裂纹, 形成了明显的剪切带; 预偏变形状态下微裂纹尖端则直接以晶面解理形式101]在上进行扩展, 直至断裂失效; 微裂纹尖端扩展行为随晶体取向不同而不同, 较小的取向角度会在裂纹尖端形成滑移位错, 诱发空位而形成二次裂纹, 而较大的取向角下的裂纹尖端则以直接解理扩展为主, 扩展方向与拉伸方向几近垂直.

Study on the effect of stress state and crystal orientation on micro-crack tip propagation behavior in phase field crystal method

A nanometer scale mechanism for micro crack propagation under uniaxial tension in single crystals is investigated using phase field crystal (PFC) simulation. The uniaxial tensile loading is strain controlled. And three initial typical stresses of pre-existing center crack in (111) crystal plane of face centered cubic structure are chosen to study the effects of initial stress state on micro-crack propagation. Moreover, the influences of different crystal orientations, when the crystal suffers from uniaxial tension, are also investigated. Due to the influence of time scale and length scale in the PFC method, the motion of dislocations, vacancies, shear band and twinning structure should be observed and described during the propagation process of micro cracks. In addition, the free energy curves of different processes are drawn and discussed in order to explain the different behaviors of the crystal in the propagation of cracks. Simulation results show that the propagation behavior of micro cracks can be closely associated with the initial stress state. It is found that the propagation behavior mainly occurs in the <011>(111) slip system. Besides, the crystal orientation has a significant effect on the mechanism of activation and evolution. In the pre-stretching system, slip dislocation is induced near the micro-crack tip, and then its slide in 011] direction will cause the cleavage of a certain crystal plane, and promote the micro cracks to extend. However, to a certain level, the propagating direction of the micro-crack tip will turn to another slip direction 101]. As a result, zigzag edge appears. By contrast, in the pre-shear system, the tip of the micro crack propagates in a cleavage mode, and results in the appearance of slip dislocation 101] near the micro-crack tip. Afterwards, the motion of slip dislocation promotes the production of vacancies. And owing to the aggregation and combination of vacancies, secondary cracks form and propagate in the process that follows. At the same time, in a pre-deviatoric system, the micro crack propagates forward with direct cleavage of 101] slip direction near the micro-crack tip until the single crystal sample fractures. Furthermore, no slip dislocation appears during the whole process. The mechanism of micro-crack tip propagating behavior varies with crystal orientation. When the crystal orientation angle is lower, the micro-crack tip prefers to produce slip dislocation around it, and the following dislocation slide will induce vacancies, then a secondary crack also forms because of the aggregation and combination of vacancies. On the other hand, when the aggregation degree is higher, the micro-crack tip is inclined to directly propagate in a cleavage mode, and its propagating direction is nearly perpendicular to the stretching direction.