FATIGUE-CRACK PROPAGATION IN SINGLE-CRYSTAL CMSX-2 AT ELEVATED-TEMPERATURE

The fatigue crack propagation (FCP) behavior of the nickel-base supera lloy CMSX-2 in single-crystal form was investigated. Tests were conduc ted for two temperatures (25 and 700-degrees-C), two orientations ([00 1][110] and [001][010]), and in two environments (laboratory air and u ltra-high vacuum 10(-7) torr). Following FCP testing, the fracture sur faces were examined using scanning electron microscopy (SEM). The FCP rates were found to be relatively independent of the temperature, envi ronment, and orientation when correlated with the conventional mode I stress-intensity factor. Examination of the fracture surfaces revealed two distinct types of fracture. One type was characterized by {111} f racture surfaces, which were inclined relative to both the loading and crack propagation directions. These features, although clearly a resu lt of the fatigue process, resembled cleavage fractures along {111} pl anes. Such features were observed at 25 and 700-degrees-C; they were t he only features observed for the 25-degrees-C tests. The second type had a macroscopically dull loading appearance, was microscopically rou gh, and grew normal to the loading axis. These features were observed on the specimens tested at 700-degrees-C (in both air and vacuum) and appeared similar to conventional fatigue fractures. Although in this r egion the crack plane was macroscopically normal to the loading direct ion, it deviated microscopically to avoid shearing the gamma' precipit ates. In view of the complex crack growth mechanisms, mixed fracture m odes, and lack of any difference in FCP rates, it is hypothesized that the correlation between FCP rates and the stress-intensity parameter is probably coincidental. The implications for life prediction of high er temperature turbine components based on conventional fracture mecha nics are significant and should be investigated further.