The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 °C by in situ observation with a scanning electron microscope (SEM) for the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.

中文翻译：

---

镍基单晶高温合金在550°C时局部重结晶的疲劳断裂机理

首次在550°C下通过扫描电子显微镜（SEM）进行了现场观察，研究了具有重结晶晶粒的镍基单晶（NBSC）高温合金的疲劳断裂机理。观察到与再结晶的晶界和碳化物有关的多个裂纹萌生。通过分析滑移轨迹和裂纹扩展平面，可以确定单晶衬底和重结晶晶粒的滑移系统均为八面体。明显的晶体学裂缝占主导，伴随着重结晶的晶界伴随的裂纹萌生。这不同于广泛报道的高温下的晶间开裂。疲劳裂纹扩展速率曲线显示出明显的波动，由于疲劳裂纹与局部微观结构的相互作用以及裂纹的聚结机理。再结晶晶粒和不同滑移系统之间的竞争都是造成疲劳和组织微细裂纹行为加速和减速的原因。