Low cycle fatigue of a single crystal CoNi-base superalloy

The low cycle fatigue response of a single crystal γ′-containing CoNi-base superalloy has been investigated in air and in vacuum at 950 °C. The fatigue performance is demonstrated to be similar to the 1st-generation single crystal Ni-base superalloy AM1. Fracture surface imaging and cross-sections on both alloys reveal that oxidation-assisted surface cracking is responsible for fatigue crack initiation in air. The fatigue performance improves under vacuum for both alloys as crack initiation transitions to internal casting pores retained from the solidification process. Recrystallized regions on the fracture surfaces of the vacuum-tested samples of both alloys are identified by electron backscattered diffraction, which has not been previously reported for single crystal Ni-base alloys at these test conditions. The role of the base element (Ni- vs. CoNi- vs. Co-base) on the fatigue performance is discussed.