Primary creep in single crystal superalloys: some comments on effects of composition and microstructure

Creep at low temperatures in current single crystal superalloys is characterised by a large but orientation dependent primary creep strain followed by prolonged secondary creep at a rate closely linked to the primary creep strain. The processes of incubation, nucleation and propagation are described and the linked effects of alloy composition and microstructure are discussed in relation to these processes. It is shown that the incubation period is relatively insensitive to the alloy composition but that heat treatments which vary the gamma' size can have a larger effect on the early stages of creep. The key to high primary creep appears to be the mobility of the dislocation ribbons nucleated and this is sensitive to the alloy composition. Fault energies in both the gamma and the gamma' are important in determining the form of the dislocation ribbons. Early results suggest that ruthenium may have a beneficial effect reducing the primary creep of fourth generation alloys.