Four turbine disc alloys: N18, U720Li (in two grain sizes) and RR1000 are compared in terms of crack propagation behaviour at 650˚C and 725˚C in air and vacuum under 1-1-1-1 and 1-20-1-1 trapezoidal loading. Fractographic analysis, together with crack propagation analysis under the varying test conditions has been combined with a novel apparent activation energy analysis approach, to assess the multi-mechanistic processes operating. Oxidation fatigue seems to be the dominant time-dependent process while the best oxidation resistance appears to be shown by RR1000. A larger grain size confers a benefit for crack growth behaviour in time-dependent fatigue (in the absence of other factors). The grain boundary character of RR1000 and N18 is deduced to play an important role in their improved high temperature fatigue crack growth resistance over the U720Li variants.