Effect of nitrogen content on microstructure and fatigue crack growth rate of high titanium-alloyed superalloy

The effect of nitrogen content on microstructure and fatigue crack growth (FCG) rate of high titanium-alloyed Ni-based superalloy FGH96 was investigated. The results show that with nitrogen content decreasing from 0.0066 wt% to 0.0012 wt%, the microstructure (gamma' phase and carbide) has little change, but the FCG rate decreases significantly. This can be explained by the sharp and rectangular morphology of TiN nitride, which could cause severe stress concentration around them. TiN nitrides were observed along the main and secondary crack paths on the transversal section of alloys with high nitrogen content. The characteristic of TiN nitrides was analyzed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS). The number density, area fraction, and maximum and average sizes of TiN nitrides decrease with decreasing of nitrogen content. Clusters of TiN with large size exist in the case of 66 ppm of nitrogen.