Effect of microstructure on high-cycle fatigue properties of Alloy 718 plates

Abstract

Effect of microstructure on high-cycle fatigue properties of Alloy 718 were investigated at 77 K by using samples with three different microstructures; fine-grained (FG), coarse-grained (CG) and bimodal-grained (BG) ones. The BG sample consisted of FG and CG microstructural regions and grain sizes of those regions were close to those of the FG and the CG samples, respectively. High-cycle fatigue strength of the FG sample was higher than that of the CG sample. High-cycle fatigue strength of the BG sample was clearly lower than that of the FG sample and almost the same as that of the CG one. Flat area (facet) was found at fatigue crack initiation site in all specimens. Facet size was similar to the grain size and found to be almost same in the CG and the BG samples. Observations of the microstructure beneath the fatigue crack initiation site of the BG sample revealed that the facet corresponds to transgranular cracking in the course grain, meaning that fatigue crack initiated at the coarse grain in the BG sample. It is deduced that the high-cycle fatigue strength of Alloy 718 with the BG microstructure is strongly affected by that of the CG region in that material.