再结晶缺陷对涡轮叶片高温合金低周疲劳性能影响机理的定量研究

Nickel-based single crystal (SC) superalloys with excellent high temperature fatigue properties have been used as the key material for turbine blades of aircraft engine and gas turbine. Recrystallization preferentially occurs at the root of turbine blade; hence it can be potential crack initiator and lead to rupture of blade. The secondary orientation, which is not controlled during blade design, together with recrystallization, have been found to have significant influence on the fatigue property of single crystal superalloy. However, the influence mechanism is not clear yet. Hence, in this study, in-situ low-cycle fatigue tests of single crystal superalloy with different secondary orientations and typical recrystallization will be conducted, in association with DIC technique and slip trace analysis. The effects of recrystallization and secondary orientations on deformation mechanism, activated slip systems, and microstructure-dependent fatigue crack initiation and propagation behavior will be disclosed. The results will be used to disclose the mechanism for fatigue life degradation. Based on the experimental microstructures of recrystallized grains and single crystal matrix, crystal plasticity finite element model will be established to study cyclic plasticity behavior and slip system evolution. Further, by incorporating a fatigue indication parameter based on fatigue failure mechanism, a total fatigue life prediction approach based on crack initiation and propagation will be established for recrystallized single crystal superalloy. It provides a theoretical tool for quantitative evaluation of the effect of recrystallization and secondary orientation on fatigue deformation, failure mechanism and fatigue life.

镍基单晶高温合金具有优异的疲劳性能，被作为航空发动机和燃气轮机涡轮叶片的核心材料。涡轮叶片根部区域容易出现再结晶缺陷，会成为疲劳裂纹源而引发叶片断裂。由于叶片设计时对第二取向通常不予控制，再结晶及第二取向耦合作用对单晶合金的高温低周疲劳性能具有重要影响，但其影响规律和机理尚不清楚。因此，本项目针对含再结晶缺陷的单晶合金开展低周疲劳原位实验研究，结合数字图像相关(DIC)技术和滑移迹线法分析再结晶晶粒对于不同第二取向的单晶合金的滑移变形机制和滑移系激活规律的影响，进而研究含再结晶试样的疲劳裂纹萌生扩展机制。与实验相对照，建立基于再结晶晶粒-单晶合金微结构的有限元晶体塑性模型，计算再结晶区域的循环塑性变形累积和各滑移系相互作用，采用晶体学失效相关的疲劳指示参量，建立基于疲劳裂纹萌生与扩展的再结晶-单晶合金的疲劳寿命模型，为定量评估再结晶缺陷对叶片单晶合金疲劳变形失效机制和疲劳寿命提供理论方法。

本项目研究以航空发动机/燃气轮机涡轮叶片的单晶高温合金再结晶问题为工程背景，采用不同第二取向单晶合金为研究对象，在疲劳试样表面制备了典型再结晶缺陷，开展了系统的原位疲劳性能测试，考查了第二取向、再结晶缺陷对单晶合金低周疲劳性能的影响，获得了关键的疲劳性能数据，阐明了单晶合金的疲劳失效机理和影响因素，得到如下结论：.1）获得了典型第二取向（0°和45°）单晶合金的疲劳寿命曲线、含典型再结晶缺陷的不同第二取向单晶合金的疲劳寿命曲线，结果发现45°单晶合金样品相比0°单晶合金样品具有更优异的疲劳性能，含再结晶的45°单晶样品相比含再结晶0°样品具有更优异疲劳性能，再结晶样品相比原始单晶样品疲劳寿命降低约2个数量级，为评估再结晶对叶片单晶合金疲劳性能影响提供了实验依据。 .2）阐明了典型第二取向（0°和45°）和再结晶晶粒对单晶合金的循环塑性变形机制的影响，揭示了不同第二取向下激活的滑移系，通过原位观测揭示了单晶合金试样和含再结晶试样的疲劳裂纹萌生与扩展机制，获得了典型疲劳裂纹扩展规律和扩展速率曲线，结果用于解释疲劳寿命的差异性。 .3）建立了基于再结晶晶粒-单晶合金的晶体塑性有限元模型，定量评估了不同第二取向、再结晶晶粒等因素对总体循环塑性变形的影响，在此基础上提出了合适的疲劳指示因子FIP，建立了能够描述第二取向和再结晶影响的单晶合金低周疲劳寿命预测模型。