镍基单晶高温合金第二晶向对疲劳性能影响机制的实验和理论研究

Nickel-based single crystal superalloys are widely used as advanced turbine blade materials due to their excellent fatigue, creep and corrosion resistance. The secondary orientation, which is not controlled during the manufacturing process, has been found to have a significant influence on the fatigue crack growth (FCG) behavior of single crystal superalloys. Therefore, there is a great need to investigate the fatigue crack initiation and propagation performance and develop a new FCG model to take the effect of the secondary orientation into account. In-situ SEM fatigue tests will be conducted to study the FCG behavior of single crystal superalloys with different secondary orientations and clarify the mechanism of the fatigue crack initiation and the interaction between the fatigue crack and the microstructure ahead of the crack tip. Fracture modes and the dislocation structures near the fracture surface at different temperatures and secondary orientations will also be determined. Based on the damage mechanics theory and the microscopic observations, a new fatigue crack model will be proposed to describe the anisotropic FCG behavior at continuous temperatures and different secondary orientations. Moreover, this research also attempts to establish a microstructure-sensitive constitutive model for the cyclic deformation of single crystal superalloys. The analysis of the stress field and crystal slip systems near the crack tip will contribute to give the distribution of the characterization parameter of the FCG behavior. The objective of the research is to further understand the mechanism of the fatigue initiation and propagation at different secondary orientations, to provide a quantitative method to evaluate the influence of the secondary orientation on the FCG behavior, and to improve the fatigue life assessment and engineering design rules of single crystal superalloys.

镍基单晶高温合金由于具有优异的抗疲劳、蠕变和耐腐蚀性能成为目前制造先进航空发动机和重型燃气轮机涡轮叶片的主要材料。叶片设计中不加控制的第二晶向对疲劳性能具有显著的影响。因此，研究第二晶向对疲劳裂纹扩展的影响，建立各向异性疲劳裂纹扩展模型，具有重要的理论意义和工程价值。采用原位观测技术，开展固定主晶向、系统比较第二晶向对疲劳裂纹影响效应的实验研究，揭示第二晶向下裂纹萌生的物理机制和扩展过程中与微结构的相互作用机理，确定不同温度和第二晶向下的损伤断裂模式；结合损伤力学理论，探寻表征第二晶向疲劳裂纹扩展行为的各向异性表征参量，建立连续温度场下取向相关的裂纹扩展驱动力形式；发展微结构相关的循环本构模型，分析不同主晶向和第二晶向的裂尖应力场分布和开动滑移系，获得裂纹扩展控制参数的综合图谱，阐明第二晶向对疲劳裂纹扩展的影响机制，为涡轮叶片的制造工艺、结构设计和安全服役评估提供重要的技术指导和理论依据。

镍基单晶高温合金由于消除了晶界而具有优异的高温疲劳性能，已经广泛运用于发动机叶片的制造。然而，由于镍基单晶材料的各向异性和叶片设计中不加控制的第二晶向，镍基单晶的第二晶向疲劳性能研究亟待深入。本项目系统开展了600℃下镍基单晶高温合金不同第二晶向的原位扫描电镜下低周疲劳试验，结果表明不同第二晶向下裂纹萌生和扩展方向显著不同，此外第二晶向会影响小裂纹扩展速率，表现为相同的∆下，[110]晶向的裂纹扩展速率要低于[010]第二晶向。能量释放率表征的裂纹扩展速率可以很好的消除晶向带来的影响。本项目还开展了不同第二晶向下的微动疲劳实验，结果表明第二晶向对微动疲劳寿命有显著影响。借助所开发的原位SEM微动疲劳实验装置，对接触区裂纹萌生进行了实时观测，结果表明不同晶向下裂纹萌生行为有显著差异。开展了背应力相关的本构方程研究，借助晶体塑性有限元模拟，能够预测滑移面开动情况，主导滑移面的方向与裂纹萌生方向相一致，最大塑性应变的位置与裂纹萌生位置相一致，晶体塑性模拟的结果与原位SEM观测取得了良好的对照。