High-temperature fatigue properties of austenitic superalloys 718, A286 and 304L

High-temperature fatigue properties of austenitic superalloys 718, A286, and 304L were investigated in region between 102 and 107 cycles. The alloys 718 and A286 are precipitation-hardening type austenitic superalloys, and 304L is a solid solution-hardening type austenitic steel. The fatigue strengths of alloys 718 and A286 in high-cycle region over 104 cycles showed a grain size effect. The initiation sites of the fracture were the crystallographic facets corresponding to the austenitic grain size. Therefore the coarse-grain alloys showed the lower fatigue strength than the fine-grain alloys, because the fatigue strength is generally inversely proportionate to the initiation defect areas for the fatigue cracks. In low-cycle region under 104 cycles, on the other, the initiation sites of the fracture were the surface of the specimens. This type fracture mode is common for fatigue fracture of smoothed specimens. The ratio of the fatigue limit at 107 cycles to the tensile strength is extremely low for the coarse-grain alloys compared with the common case of 0.5. From these results fatigue fracture map is proposed.