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-gamma titanium aluminide alloy with a duplex microstructure. The test temperature ranged from 500 to 750 degreesC 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 approximate to 650 degreesC. At temperatures < 650 degreesC, initial cyclic hardening occurred, whereas > 650 degreesC 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. (C) 2001 Elsevier Science B.V. All rights reserved.