Modeling thermomechanical fatigue life of high-temperature titanium alloy IMI 834

A microcrack propagation model was developed to predict thermomechanical fa tigue (TMF) life of high-temperature titanium alloy IMI 834 from isothermal data. Pure fatigue damage, which is assumed to evolve independent of time, is correlated using the cyclic J integral. For test temperatures exceeding about 600 degrees C, oxygen-induced embrittlement of the material ahead of the advancing crack tip is the dominating environmental effect. To model t he contribution of this damage mechanism to fatigue crack growth, extensive use of metallographic measurements was made. Comparisons between stress-fr ee annealed samples and fatigued specimens revealed that oxygen uptake is s trongly enhanced by cyclic plastic straining. In fatigue tests with a tempe rature below about 500 degrees C, the contribution of oxidation was found t o be negligible, and the detrimental environmental effect was attributed to the reaction of water vapor with freshly exposed material at the crack tip . Both environmental degradation mechanisms contributed to damage evolution only in out-of-phase TMF tests, and thus, this loading mode is most detrim ental. Electron microscopy revealed that cyclic stress-strain response and crack initiation mechanisms are affected by the change from planar dislocat ion slip to a more wavy type as test temperature is increased. The predicti ve capabilities of the model are shown to result from the close correlation with the microstructural observations.