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 γ′ phase and size of secondary γ′ phase) at high temperatures (650 °C and 700 °C) were investigated. The grain size and the volume fraction of γ′ 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 γ′ phase. As the test temperature increased from 650 °C to 700 °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 ΔK. The apparent activation energy (E_app) calculations concentrated in the static property change range(0-60 KJ/mol) and showed a clear decreasing trend with the increasing of ΔK, which indicated that the main factor affecting the fatigue crack growth rate in this study is the change of yield strength σ and the elastic modulus E of the superalloy.