Effect of change in stress amplitude on the fatigue damage of Ni-base superalloy at room temperature and 500°C

In order to study the fatigue damage of a Ni-base superalloy Inconel 718 at an elevated temperature, the plain specimens were fatigued at room temperature and 500°C. The crack initiation and crack growth behavior was monitored by the plastic replication method. Experimental results showed that the resistance for slip and crack initiation decreases due to the softening of matrix at elevated temperature. With regard to the crack growth at 500°C, the growth rate of large crack was accelerated, however the growth of microcracks less than 50 μm was suppressed. In addition to the fatigue tests under constant stress amplitudes, the fatigue tests under the two step loading was also performed to clarify the effect of change in stress amplitude on the fatigue damage. There was no effect of the change in stress amplitude on the crack growth behavior at room temperature. For low-to-high block loading at 500°C, the crack growth behavior was hardly affected by the change in stress amplitude. For high-to-low block loading at 500°C, however, the growth behavior of microcracks under a two stress amplitude was influenced by the high stress repetitions. The cumulative cycle ratio Σ(N/Nf) was calculated. The values of Σ(N/Nf) exhibit the distinct tendency determined by the load pattern and temperature. Above results were discussed from the viewpoints of the matrix softening and the oxide films formed at the elevated temperature.