The effect of variations in stacking-fault energy on the creep of nickel-cobalt alloys

The creep and fracture properties of a series of nickel-cobalt alloys have been determined at 500°c. The alloys covered a range of composition (0-70% Co) giving a marked change in stacking-fault energy. Cobalt was found to increase the creep resistance of nickel but for a given creep rate the fracture life was independent of cobalt content. At any one stress the steady creep rate varied by a factor of up to 5 × 102 times over the complete range of alloys. The activation energy for creep determined by temperature cycling was found to be ∼43 000 cals/g atom in all the materials tested at 500°c. However, at higher temperatures (∼600°c) the activation energy for creep became equal to that for self-diffusion (66 000 cals/g atom). The results are interpreted on the basis that the steady creep rate is controlled by diffusion along dislocations of vacancies created at non-conservatively moving jogs on screw dislocations. Variations in stacking-fault energy are then considered to affect the distance between jogs and also the number and distribution of dislocations at the onset of secondary creep. A further suggestion is that the incidence of annealing twins, which increases with decrease in stacking-fault energy, may also play a part in affecting the creep resistance.