Initiation of Fatigue Cracks in a Single-Crystal Nickel-Based Superalloy at Intermediate Temperature

Abstract

Fatigue tests under a series of stress amplitudes at 760 °C of a single-crystal nickel-based superalloy were conducted to investigate the relationships between loading conditions and initiation sites of fatigue cracks. It was found that initiation sites of the dominant fatigue crack gradually transferred from pores to MC carbides with an increase of the stress amplitude. At low stress amplitude, the fatigue crack that causes the final fracture initiated at pores and the fatigue life depended on size and shape of pores. Although the carbides on surface cracked as a result of oxidation and cyclic loading, the cracks did not penetrate the carbide/matrix interface. However, at high stress amplitude, the cracks in the carbide can penetrate the carbide/matrix interface rapidly under applied stress, and following crack propagation resulted in decisive fatigue fracture. Compared to blocky carbides on the surface, script carbides at subsurface were more likely to induce fatigue crack initiation.