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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 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.