A comparison of high temperature fatigue crack propagation in various sub-solvus heat treated turbine disc alloys

The microstructure and fatigue performance of three sub-solvus heat treated nickel based disc superalloys for turbine disc applications are reported. The alloy variants studied are RR1000, N18 and Udimet 720 Low Interstitial (U720Li), with the latter tested both in a standard and large grain variant (LG). Their microstructures are examined in terms of grain and gamma prime size (γ'). Fatigue crack growth (FCG) rates for all materials at 650oC show that RR1000 provides the best performance, followed by U720Li-LG, N18 and U720Li. Some of the variations in FCG rate between the alloys are due to reduction in grain boundary oxidation processes with increased grain size, but more subtle interplays between grain boundary character, alloy composition and slip character are also important.