Modeling thermal exposure effects on fatigue of gamma titanium aluminides

Aircraft engine applications of gamma titanium aluminide alloys require these materials to undergo extended periods of high temperature exposure in air. In order to determine whether gamma alloys will function satisfactorily in such applications, the effects of long-time air exposure need to be understood, and quantified. To date, investigations in this area have been conducted on cast Ti–48Al–2Nb–2Cr and wrought Alloy 395. Both alloys showed reductions in fatigue capability that tended to peak in the intermediate temperature (∼540 °C) range, with smaller or no reductions at lower or higher temperatures. This paper develops a model for fatigue of thermally exposed gamma alloys based on propagation of surface cracks initiating from an embrittled surface layer. The model successfully predicted the observed maximum effect at intermediate temperature and provides quantitative agreement with the observed post-exposure fatigue capability.