循环载荷下单晶高温合金变形和裂纹形核扩展的位错机理研究

单晶镍基超合金具有独特的两相微结构，其优良的高温抗疲劳性能源于增强相与位错演化的交互作用。本项目拟通过宏细观实测和多尺度计算相结合的研究方法，以"材料微结构"-"位错微结构"以及其相互关联为切入点，对该材料循环载荷下的变形和裂纹形核扩展进行研究。首先，在宏细观实验上，对三种沉淀相大小的光滑、切口和裂纹试样进行低周疲劳测试，在关注宏观循环响应、疲劳裂纹开裂和扩展的同时，观察位错微结构演化及其与材料微结构的相互作用。然后，开展分子动力学模拟，提炼位错运动和位错-微结构相互作用的本构法则，在此基础上发展具有原子信息的离散位错动力学模拟程序，用于探索高温环境中不同加载幅下循环位错微结构（花样）的形成过程和机理，揭示材料微结构与位错微结构形成的内在联系，获得位错微结构演化与循环塑性行为和裂纹形核扩展内在关联，探讨材料微结构尺寸和形状效应对疲劳裂纹形核和扩展的影响，为该材料的安全设计提供理论支持。

Since the constraint effect on dislocation evolution by its special two-phases microstructure, the Nickel-base superalloy has excellent fatigue resistance. This project aims to perform the reseach on the cyclic plasticity and fatigue damage evolution of this material by macro/micro experiments and multi-scale simulation, with the emphasis focused on the influence of material microstructure, dislocation pattern and their mutual interaction. First, in the macro/micro experiments, materials with three different precipitate size will be prepared. Then these materials will be machined into three different kinds of specimens which are the smooth round-bar specimen,the notched specimen and the cracked one. In the experiments, the overall cyclic response, the crack initiation and propogation will be observed, as well as the the dislocation distribution. Second, the MD simulation will be performed, from which the constitutive law for dislocaton motion and dislocation-material microstructure interaction will be suggested. Basing on this, the atom-informed three dimensional discrete dislocation dynamics program will be developed. Using this programe, the mechanism and process of the dislocation structures(pattern, such as persistent slip band) formation under different load and temperature range will be discussed; the relation between the dislocation pattern and material microstructure (such as the precipitate size and morphology)will be investigated; the influence of the dislocation structure evolution on the cyclic stress-strain response, fracture initiation and propogation will be probed; and the precipitate size/morphology effect on fracture initiation and propogate will also be studied carfully. All these works tend to provide the scientific support for the development of new kind superalloy and its safety assessment.