A homogenized primary creep model of nickel-base superalloys and its application to determining micro-mechanistic characteristics

A primary creep model of nickel-base superalloys is developed for the low-temperature-high-stress regime, which provides insight into the active micro-mechanisms, extracted from features of the experimental creep curves. A two-parameter model is established to account for the accumulated primary creep strain, which contains a “threshold stress” representing the possible shear resistance during primary creep. Consideration of this threshold stress leads to a stress exponent n in the order of 3∼4, in sharp contrast to the conventional analysis that gives an ultra high value of n>10. A general stress-dependence of dislocation velocity is then proposed based on the framework of thermally-activated deformation, which leads to another two-parameter model for the primary creep rate. Applications to two superalloys demonstrate a good agreement between the model and experimental data. In addition, two micro-mechanistic characteristics of creep, i.e., dislocation drag coefficient and activation volume, can be unambiguously determined from the model parameters. The values of these physical quantities obtained for the two superalloys considered suggest a coupled dislocation motion of glide and climb at the γ/γ' interface as the rate-controlling mechanism during primary creep in superalloys.