利用SEM原位观察技术研究了近片层Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金在750 ℃疲劳蠕变交互作用下的裂纹萌生及扩展行为, 循环实验采用在最大拉应力保载的梯形波. 结果表明, 裂纹主要在片层团界面萌生, 裂纹萌生方式包括蠕变空洞和疲劳微裂纹. 片层团界面处的微裂纹先通过吞并蠕变空洞或在裂纹尖端应力集中作用下沿片层团界面进行扩展, 然后相互连接长大; 当裂纹扩展受到不同取向的片层团界面阻碍时, 受阻的裂纹开始沿试样厚度方向扩展, 且附近伴随出现垂直于载荷方向的微裂纹; 最终受阻的裂纹相互连接直至合金断裂. 将实验结果与该合金在相同条件下疲劳变形和蠕变变形的原位观察结果进行了比较. 结合实验结果建立了高铌TiAl合金在疲劳蠕变交互作用下裂纹萌生及扩展示意模型.

TiAl-based alloys appear as potential competitors to steels and superalloys applied in aerospace and automotive industries due to their low density, high specific strength and stiffness and good oxidation resistance at elevated temperatures. As a new generation of TiAl-based alloys, high Nb-containing TiAl alloys have become a promising high temperature structural material due to their better high temperature strength and oxidation resistance than ordinary TiAl alloys. TiAl-based alloy components such as low pressure turbine blade and compressor impeller often serve in near steady conditions for a duration of time once peak operating conditions are achieved at high temperature. The components suffer not only from rapidly induced damage from start-up and shutdown cycles, but also from creep damage under sustained loading periods. Moreover, the possible interaction damage between fatigue and creep must be considered. Thus, the study of fatigue-creep interaction for TiAl-based alloys is of great practical importance. Large numbers of researches were focused on the fatigue or creep properties of TiAl-based alloys, however, the fatigue-creep interaction behavior was rarely reported. Therefore, the crack initiation and propagation behavior of a nearly lamellar Ti-45Al-8Nb-0.2W-0.2B-0.1Y alloy in fatigue-creep interaction was observed at 750 ℃. The cyclic loading tests were carried out using a mini servo-hydraulic fatigue machine in a SEM chamber. The entire process of crack initiation and propagation was observed. The load cycling was trapezoidal by applying a dwell time at the maximum tension stress. The results indicated that micro-cracks mainly occurred at internal grain boundaries in the form of creep void or fatigue micro-crack. The micro-cracks firstly extended along the grain boundary by absorbing the creep voids or the stress concentration around crack tips, then connected with each other forming a longer crack. As the crack was frustrated by grain boundaries of other orientations, the crack began to grow in the thickness direction. Meanwhile, the micro-cracks perpendicular to loading direction emerged. Eventually, the frustrated cracks interconnected resulting in fracture. Compared to the in situ SEM observations in fatigue deformation, the dwell time resulted in the increase of probability of grain boundary crack initiation and the changes of crack propagation path. Thus, the fracture mode transform from transcrystalline to intercrystalline and the fatigue lifetime significantly decreased. The model of the crack initiation and propagation behaviors of high Nb-containing TiAl alloys in fatigue-creep interaction was presented in this work.