Regional high temperature fatigue crack growth behavior of a microstructure-gradient nickel-based superalloy

In this work, the regional fatigue crack growth (FCG) behavior of the FGH9X nickel-based superalloy with gradient microstructure (grain size, volume fraction of primary gamma ' phase and size of secondary gamma ' phase) at high temperatures (650 degrees C and 700 degrees C) were investigated. The grain size and the volume fraction of gamma ' phase were confirmed to be critical factors thus to influence the FCG behavior at high temperature. Sample with coarse-grain has the highest FCG resistance with less primary gamma ' phase. As the test temperature increased from 650 degrees C to 700 degrees C, the resistance of FCG consequently decreased, which is due to more severe high-temperature oxidation of the specimens at higher test temperature, accompanied by a decrease in static properties. The effects of both microstructure and temperature on the rate of fatigue crack growth weaken with an increase in Delta K. The apparent activation energy (Eapp) calculations concentrated in the static property change range(0-60 KJ/mol) and showed a clear decreasing trend with the increasing of Delta K, which indicated that the main factor affecting the fatigue crack growth rate in this study is the change of yield strength a and the elastic modulus E of the superalloy.