The Effect of Grain Size on the Dwell Fatigue Crack Growth Rate of Alloy ATI 718Plus®

Abstract

Grain size optimization is an important method to improve the fatigue crack growth resistance of superalloys. For ATI 718Plus® alloy, the effect of grain size on the fatigue crack growth rate (FCGR) is more complex, because of the complex effects of η-Ni3Al0.5Nb0.5 precipitated at the grain boundaries. In the past, the fine-grain ATI 718Plus® alloy was found to exhibit better dwell fatigue crack growth rate (DFCGR) resistance than that of the coarse-grain alloy at high temperature. An attempt to explain this phenomenon was based on the grain boundary oxygen diffusion/oxidation viewpoint; however, the mechanism remained unclear until now. Based on this, we investigated the relationship between the grain size and the DFCGR of ATI 718Plus® alloy. The DFCGR of ATI 718Plus® alloys with different grain sizes were tested in air at 704 °C. Furthermore, the microstructures and crack growth paths were analyzed, and the plastic zone near the crack tip was calculated for specimens with different grain sizes. The better effect fine grain ATI 718Plus® has on DFCGR is related to more η-Ni3Al0.5Nb0.5 phase precipitation and reduced oxygen diffusion along grain boundaries, as well as the reduction of the plastic zone near the crack tip and the increase in secondary cracking with decreasing grain size.