Abstract
The fatigue strength of a nickel-base superalloy was studied. Stress-controlled fatigue tests were carried out at $700^{\circ}C$ and 5 Hz using triangular wave forms. In this study, two kinds of testing procedures were adopted. One is the conventional tension-zero fatigue test(R = 0). The other was a procedure in which the maximum stress was held at 1000 MPa and the minimum stress was diverse from zero to 1000 MPa at 24 and $700^{\circ}C$. The results of the fatigue tests at $700^{\circ}C$ indicate that the fracture mechanism changed according to both the mean stress and the stress range. At a higher stress range, ${\gamma}^{\prime}$ precipitates are sheared by a/2<110> dislocation pairs coupled by APB. Therefore, in a large stress range, the deformation occurred by shearing of ${\gamma}^{\prime}$ by a/2<110> dislocations, which brought about crystallographic shear fracture. As the stress range was decreased, the fracture mode gradually changed from crystallographic shear fracture to gradual growth of fatigue cracks. At an intermediate stress range, as it became more difficult for a/2<110> dislocation pairs to shear ${\gamma}^{\prime}$ particles, cracks started to propagate in the matrix, avoiding the harder ${\gamma}^{\prime}$ particles. High mean stress induced creep deformation, that is, ${\gamma}^{\prime}$ particles were sheared by {111}<112> slip systems, which led to the formation of stacking faults in the precipitates. Thus, the change in fracture mechanism brought about the inversion of the S-N curves.