Corporate Effects of Temperature and Strain Range on the Low Cycle Fatigue Life of a Single-Crystal Superalloy DD10

Low cycle fatigue behavior of a nickel-based single-crystal superalloy DD10 was investigated at 760 and 980 °C under different strain ranges. Results show that the fatigue life (N _f) of DD10 alloy exhibits different temperature dependence under various strain ranges. Under low strain range, the alloy exhibits a longer N _f at 760 °C than that at 980 °C. However, under high strain range, a reverse result is obtained. This difference can be attributed to the change of dominant damage modes under various test conditions, which is manifested in different modes of crack initiation (crack nucleation and its early propagation). At 760 °C, the crack initiates at pores in subsurface due to local stress concentration. This process is mainly controlled by plastic amplitude and plastic property, but not affected by oxygen-induced damage before the crack propagates to the surface. At 980 °C, the crack initiates at surface instead of pores due to the more homogeneous plastic deformation and the disharmony between the external oxidation layer and the bulk material when the strain amplitude is high. At that temperature, the process is mainly controlled by oxidation damage and strain amplitude simultaneously. Therefore, under high strain range, the crack initiation is much easier at 760 °C due to plastic deformation and the poor plasticity, while under low strain range obvious oxidation damage at 980 °C may accelerate the crack initiation.