Fatigue crack propagation in single-crystal CMSX- 2 at elevated temperature
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The fatigue crack propagation (FCP) behavior of the nickel-base superalloy CMSX-2 in single-crystal form was investigated. Tests were conducted for two temperatures (25 and 700 °C), two orientations ( and ), and in two environments (laboratory air and ultra-high vacuum 10-7 torr). Following FCP testing, the fracture surfaces were examined using scanning electron microscopy (SEM). The FCP rates were found to be relatively independent of the temperature, environment, and orientation when correlated with the conventional mode I stress-intensity factor. Examination of the fracture sur-faces revealed two distinct types of fracture. One type was characterized by 111 fracture surfaces, which were inclined relative to both the loading and crack propagation directions. These features, al-though clearly a result of the fatigue process, resembled cleavage fractures along 111 planes. Such fea-tures were observed at 25 and 700 °C; they were the only features observed for the 25 °C tests. The second type had a macroscopically dull loading appearance, was microscopically rough, and grew normal to the loading axis. These features were observed on the specimens tested at 700 °C (in both air and vacuum) and appeared similar to conventional fatigue fractures. Although in this region the crack plane was mac-roscopically normal to the loading direction, it deviated microscopically to avoid shearing the y ’ precipi-tates. In view of the complex crack growth mechanisms, mixed fracture modes, 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 higher temperature turbine components based on conventional fracture mechanics are significant and should be investigated further.
Keywordsfatigue crack propagation nickel-base alloy single crystal
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