Abstract: The fatigue properties of nickel-base Alloy 718 with fine- and grain-coarse grains were investigated. In the fine-grain alloy, the fatigue strength normalized by the tensile strength was 0.51 at 10^7 cycles. In contrast, the fatigue strength of the coarse-grain alloy was 0.32 at the same cycles, although the fatigue strengths in the range from 10^3 to 10^5 cycles are the same for both alloys. The fracture appearances fatigued at around 10^6 cycles showed internal fractures originating from the fiat facets of austenite grains for both alloys. The difference in fatigue strength at 10^7 cycles between the fine- and coarse-grain alloys could be explained in terms of the sizes of the facets from which the fractures originated.

Key words: aerospace material, fatigue, high temperature