SUBCRITICAL CRACK-GROWTH IN A TI3AL-BASED ALUMINIDE AT ELEVATED-TEMPERATURES

Fatigue crack growth has been studied at ambient temperature and at 700-degrees-C (in air and in vacuum) for "super alpha(2)", heat treated to give a microstructure consisting of approximately 20% volume fraction of primary alpha(2) precipitates in a transformed beta-matrix. At ambient temperature, alpha(2) precipitates fail by transgranular cleavage under cyclic loading and a steeper dependence of crack increment per cycle on the applied alternating stress intensity is observed. At temperatures of 600-degrees-C and above, the material suffers from sustained load cracking in air, particularly at test temperatures of 650 and 700-degrees-C. Static-cyclic interactions during fatigue loading are therefore possible. These have been modelled and effects of cyclic frequency have been predicted accurately at 700-degrees-C. This mechanism alone cannot account for the higher fatigue crack growth rates observed in air compared with those observed in vacuum at 700-degrees-C.