Tertiary creep in nickel-base superalloys: analysis of experimental data and theoretical synthesis

It has recently been established that the extended period of tertiary creep of nickelbase superalloys with good ductility is not primarily a consequence of the thermal instability of the particulate microstructure: rather it is believed to be caused by a strain-induced instability of the dislocation substructure. The instability appears to be intrinsic to this class of alloy and the resulting isotropicallyaccelerating creep rate can be represented by a scalar parameter in constitutive laws. Evidence is presented to suggest that a similar strain-induced instability operates in certain low alloy ferritic steels and precipitation-hardened aluminium alloys. The micromechanism responsible for this intrinsic instability is still a matter for speculation, but recently it was suggested that, for NiCr alloys hardened with aluminium and titanium, the acceleration in strain accumulation may be influenced by the volume fraction of γ′ particles and by the magnitude of their lattice-parameter mismatch with the matrix. In this paper, a more substantial range of data for alloys of this type is presented and it is concluded that ductility in creep is the most significant parameter correlating differences in rates of strain-accumulation in the tertiary stage: thus, the lower the ductility the greater the rate of strain-accumulation. The various shapes of uniaxial creep curve found in nickel-base superalloys have been synthesised quantitatively by developing a model with two internal state-variables: one represents damage caused by the intrinsic instability while the other represents damage caused by cavitation. The model accounts for the engineering classification of alloys into so-called “von-Mises type” and “maximum principal stress type” and also rationalises the previously unexplained fact that there is no unique correlation between this classification and uniaxial ductility in creep.