Crystal Plasticity Mechanism of Temperature-Dependent Crack Propagation in a Single Crystal Nickel-Based Superalloy

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Temperature-dependent fatigue crack propagation in a Ni-based single crystal superalloy was experimentally and numerically investigated in a single crystal Ni-based superalloy. Fatigue crack propagation tests at room temperature 300, 450, and 700 °C were conducted using four types of compact specimens with different combinations of crystal orientations in loading and crack propagation directions. It was revealed in the experiments that the crack propagated along slip planes in crystallographic cracking manner at room temperature, while the cracking mode transitioned from the Mode I to crystallographic cracking at 300, 450, and 700°C. Mode I stress intensity factor range ΔKI values at the transitions depended on the testing temperature as well as crystal orientation. To interpret these temperature-dependent crack propagation, a crystal plasticity finite element analysis was conducted by taking into account the 3D inclined crack plane and the activity of slip planes in front of the crack. Slip plane activity, proposed as a damage parameter, could rationalize the fatigue crack propagation rates both during the crystallographic and Mode I cracking. It has been found that crack propagation resistance for crystallographic cracking is more or less the same at low temperature, while that for Mode I cracking decreases with the increase of the temperature. This damage parameter also provided an explanation of the critical condition that induces the transition from Mode I to crystallographic cracking.