热机械疲劳是单晶高温合金在实际服役过程中的一种重要损伤模式，澄清合金的热机械疲劳行为及损伤机制对于提升单晶高温合金的服役可靠性具有重要意义。本工作以先进航空发动机高压涡轮叶片用第四代单晶高温合金DD91为对象，采用SEM、EBSD、TEM等手段研究了合金在600~1000℃下的同相位热机械疲劳断裂特征及损伤机理。结果表明，随应变幅的升高，合金的疲劳寿命大幅下降，迟滞曲线明显张开，且应力响应行为由高温半周循环软化和低温半周循环硬化转变为以循环稳定为主导的特征。在不同应变幅下疲劳断裂后，合金断口呈韧性断裂特征，随应变幅的升高，韧窝区面积分数不断下降。在低应变幅下，合金主要承受氧化损伤，并伴随一定程度的蠕变损伤，合金的主要变形机制为位错在γ基体中滑移，并以Orowan机制绕过γ'强化相；而在高应变幅下，合金承受严重的塑性变形损伤而氧化损伤程度减轻，此时界面位错能够以生成层错或反相畴界的形式切割γ'相。另外，合金在不同应变幅下循环至断裂后，均未出现再结晶晶粒和变形孪晶。

During the service, the turbine blades of aero-engines are subjected to a complex and ever-changing combination of temperature and stress, resulting in severe cyclic temperature/strain damages and thermal-mechanical fatigue (TMF) failures of the alloy. In this work, in-phase (IP) TMF tests under 600-1000^oC were conducted on a newly developed fourth-generation single-crystal superalloy. The alloy's fracture characteristics and comprehensive damage mechanisms were examined via SEM, EBSD, and TEM. The results showed that when the strain range increased, the fatigue life of the experimental alloy noticeably decreased, and the hysteresis loop clearly opened. Stress response behaviors shifted from cyclic softening at high temperatures and cyclic hardening at low temperatures into a dominant characteristic of cyclic stabilizing. The fracture surfaces of alloys displayed ductile features after fatigue fracture under various circumstances, and the area fraction of dimples reduced with increasing strain amplitude. When the strain amplitude was low, the alloy was mainly subjected to oxidation damage, accompanied with a certain degree of creep damage. In contrast, the dominant deformation mechanism of the alloy was dislocation slipping in γ matrix and Orowan by-passing through γ' particles. As the strain amplitude increased to higher levels, the alloy was subjected to severe plastic deformation damage, while the degree of oxidation damage had been alleviated. Under this condition, the interfacial dislocations could shear into the γ' phase with the generated stacking fault or anti-phase boundary. Notably, no recrystallization grains or deformation twins were formed in the DD91 alloy during the IP-TMF experiments at different mechanical strain amplitudes.