The creep behaviour of a Nickel-based single-crystal superalloy

Creep tests have been carried out on the single-crystal superalloy SRR99 at 750°C under 800 MPa. The observed primary creep strain was extremely variable, with some crystals exhibiting values of up to 10%. Lattice rotation measurements and transmission electron microscopy studies show that primary creep takes place on a single ( 1 11) [1 1 2] system, involving the shear of γ′ precipitates by dissociated dislocations. The observed variations in creep response are rationalized in terms of the extent and direction of misorientation of the tensile axis from [001]. For a given crystal orientation close to [001], the primary creep strain is reduced by a heat treatment of 16 h at 870°C prior to testing. Atom probe microanalysis indicates that the 870°C heat treatment brings out slight chemical changes at the γ−γ′ interfaces, and it is suggested that these, together with an increase in the volume fraction of the dispersed phase, influence the creep behaviour of the material. The dislocation configurations formed during secondary creep were consistent with many models of steady state creep, and the final fracture was shown to be associated with the growth of cracks which nucleate at voids in the alloy.