Temperature dependent fatigue crack propagation in a single crystal Ni-base superalloy affected by primary and secondary orientations

The effects of temperature and crystal orientations on fatigue crack propagation in a single crystal Ni-base superalloy were experimentally investigated at room temperature, 450 °C and 700 °C. Four types of C(T) specimens with different combinations of crystal orientations in loading and crack propagation directions were extracted from a partially modified CMSX-4, and fatigue crack propagation tests were conducted at room temperature, 450 °C and 700 °C. The experiments showed that the crack propagated in shearing mode at room temperature, while the cracking mode transitioned from the opening to shearing mode at 450 °C and 700 °C. ΔKI values at the transition depended on the testing temperature. These phenomena were attributed to the distinctive temperature dependence of the strength of the γ and γ' phases. The effect of crystal orientation on the cracking mode transition behavior was interpreted by the trend in the crack propagation rate of the shearing cracking at room temperature. This implied that the driving force of the shearing mode cracking has a significant effect on the cracking mode transition. Finally, crack propagation rates of the opening mode cracking at 450 °C and 700 °C were analyzed. It was revealed that difference of crack propagation rates between the two temperatures varied with the orientation in the loading direction of the specimen. This was attributed to difference in arrangement of γ/γ' microstructure respect to the loading and crack propagation direction.