| γ-TiAl alloy has a series of advantages, which make it become one of competent high-temperature structural material. At present, the laws governing γ-TiAl alloys under thermal fatigue and dead-load thermo-mechanical fatigue are still unclear. Therefore, it has a great significance for the purpose of reasonable application, design and lifetime prediction.Present work, using ray radiation heating, investigated the thermal fatigue behaviors in the temperature ranges of 200-700℃ and 200-900℃, and the dead-load thermo-mechanical fatigue behaviors of 200-700℃ with 70MPa and 140MPa dead-load tensile stress respectively, and 200-900℃ with 70MPa. The relative change of modulus DE and that of electrical resistance Dr were employed to characterize damage. SEM, TEM and XRD were used to investigate the microstructure, phase identification and fracture pattern after thermal cycling. And the damage and fracture mechanisms were also discussed. In addition, several damage evaluation methods were compared.In the shorter thermal fatigue, the damage increased intensely, and the discrepancy between DE and DR was very limited. Then, the damage increased very slowly as the number of cycles increased and approached a constant, and the discrepancy between DE and Dr also trended to a stable constant.The negative increase of DE occurred after dead-load thermo-mechanical fatigue. The differences among three testing conditions are as follow: The negative increase of DE of 200-700℃ with 70MPa and 140MPa dead-load stress occurred at the very start;The DE of 200-900℃ with 70MPa dead-load stress, at the beginning of cycling, increased intensely, and descended slightly after the 20th cycle. Thereafter, DE increased slightly and maintained at a constant. At the stable damage stage, DE hardly changed. The DE of 200-700℃ with 140MPa dead-load stress was the biggest among three testing conditions, and that of 200-900℃ with 70Mpa was larger than 200-700℃ with 70MPa. The negative increase did not occur on DR. The trend of the three curves of Dr versus the number of cycles was consistent, and all of them increased intensely in the first few cycles and followed by fluctuation. Although the two damage curves of DE and Dr did not consistent, the curves corresponded well on some inflexion.The fine and dark γ phase increased and the size of lamellar colony reducedobviously after thermal cycling, which occurred not only in thermal fatigue but also in dead-load thermo-mechanical fatigue. After thermal cycling, the damage appeared as micro-pores and micro-cracks, etc. And the density of dislocations and twins increased simultaneously.The fracture position of samples after thermal fatigue and dead-load thermo-mechanical fatigue located mostly at the transition region between direct heated region and grasp side. The miss-sticking occurred between layers in inner lamellar colonies, and the cracks emerged between layer and y phase. The characterization of river pattern occurred on y phase of fracture surface. But the microcosmic fracture after dead-load thermo-mechanical fatigue became much complicated, and tearing rib existed obviously. The fracture style of 7-T1AI under thermal fatigue, either analyzed from the microcosmic fracture mechanism or observed from macrocosmic fracture pattern, was brittle fracture. But there were some differences on fracture pattern of two temperature ranges. Although the fracture style after dead-load thermo-mechanical fatigue was brittle fracture macroscopically, there was some ductile fracture mechanism macroscopically.The five damage evaluation methods all could reflect the thermal fatigue damage course of y-TiAl. The micro-hardness damage parameter Dhv, bend strength Dbs and bend modulus Dbm coincided during the intense increase stage, and the curves corresponded well on seme inflexion during stable damage stage. De was the biggest of all, and Dbm was least.
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