FATIGUE-CRACK GROWTH-BEHAVIOR OF A NEW SINGLE-CRYSTAL NICKEL-BASED SUPERALLOY (CMSX-4) AT 650-DEGREES-C

The fatigue crack growth behavior of a new single crystal nickel-based superalloy containing rhenium (CMSX-4) has been studied at 650 degree s C. The investigation also examined the influence of gamma' precipita tes (size and distribution) on the near-threshold fatigue crack growth rate and the fatigue threshold. The influence of load ratio on the fa tigue crack growth rate and the fatigue threshold was also examined. D etailed fractographic studies were carried out to determine the crack growth mechanism in fatigue in the threshold region. Compact tension s pecimens were prepared from the single crystal nickel-based superalloy CMSX-4 with [001] orientation as the tensile loading axis direction. These specimens were given three different heat treatments to produce three different gamma' precipitate sizes and distributions. Fatigue cr ack growth behavior of these specimens was studied at 650 degrees C in air. The results of the present investigation indicate that the near- threshold fatigue crack growth rate decreases and that the fatigue thr eshold increases with an increase in the gamma' precipitate size at 65 0 degrees C. The fatigue threshold decreased linearly with an increase in load ratio. Fractographs at 650 degrees C show a stage II type of crack growth along {100} type of crystal planes in the threshold regio n, and along {111} type of crystal planes in the high Delta K region.