Single crystal Nickel-based superalloys exhibit an anomalous yield point, the yield stress increasing with temperature to a maximum at around 750 ºC. Here, we demonstrate in the alloy CMSX-4 at 750 ºC that, although there is virtually no effect of strain rate on the initial yield point, at slow strain rates a second mechanism can initiate leading to a considerable softening effect. By examining the microstructures of a series of interrupted tests, this is attributed to the initiation of stacking fault shear after the operation of a secondary slip system. Using high-resolution TEM, the dislocation structures are shown to be identical in both structure and in the segregation of Co, Cr, and W, to those observed during creep deformation of single crystal alloys, although the conformation of the dislocations and faults differs from that observed during creep. This drop in flow stress at low strain rates is not observed in the alloys TMS138A and SRR99, in the former case, the improved creep resistance of this fourth-generation alloy would require a much slower strain rate to match the creep rate achievable at this temperature.
