Fatigue Small Crack Growth in a Single Crystal Ni-Base Superalloy at Elevated Temperature.

Abstract

Strain-controlled low-cycle fatigue tests were carried out on a single crystal Ni-base superalloy, CMSX-2, under a strain ratio of zero at 873K in air. The CMSX-2 showed higher fatigue strength, compared with some other kinds of polycrystalline Ni-base superalloys or with a directionally-solidified superalloy, CM247LC-DS. The fatigue small crack growth behavior was also investigated by using a replication technique. The small crack growth rate increased proportionally with increasing crack length on the macroscopic level. On the microscopic level, however, they were significantly affected by the microstructure. From the microscopic observations of the crack path, fracture surface and deformation around the crack tip, it was found that the small cracks propagated in the γ phase matrix on {100} cubic planes, accompanying with negligible deformation of γ' phases, when the crack length was short enough. As the crack became longer, on the other hand, the crack propagation planes were converted onto {111} slip planes. In this stage the crack rapidly grew by the slip plane decohesion mechanism, cutting the γ' phases. The above transition of crack propagation plane occurred when the stress intensity factor range reached a critical value. Based on the foregoing results, the differences in the small crack growth rate and the fatigue strength between CMSX-2 and CM247LC-DS were also discussed.