High-temperature fatigue behavior of a near-γ titanium aluminide alloy under isothermal and thermo-mechanical conditions

Fatigue tests were performed on smooth cylindrical specimens of a near-γ titanium aluminide alloy with a duplex microstructure. The test temperature ranged from 500 to 750 °C in the isothermal tests and these temperatures were also used as minimum and maximum temperature of in-phase and out-of-phase thermo-mechanical fatigue (TMF) tests. Environmental degradation was studied running the experiments in air and high-vacuum, respectively, and the microstructural changes were characterized by transmission electron microscopy. Fatigue behavior was found to be strongly affected by temperature as a consequence of a change in the cyclic stress–strain response at ≈650 °C. At temperatures <650 °C, initial cyclic hardening occurred, whereas >650 °C a cyclic saturation state was observed from the beginning. With increasing temperature, fatigue life decreased in vacuum, but increased in air indicating an abnormal (inverse) environmental effect. The stress–strain behavior under TMF conditions resembles that of the corresponding isothermal experiments. Out-of-phase thermo-mechanical fatigue was found to be more detrimental than in-phase, because of the positive (tensile) mean stress arising in out-of-phase testing in combination with the pronounced mean stress sensitivity of the relatively brittle alloy.