An Interpretation of the Effects of Stress and Temperature on the Creep Properties of a Nickel-Base Superalloy

Abstract

Creep data on a directionally solidified nickel-base alloy over the range 1400-1900°F (760-1040°C) are presented; these substantiate a model of creep based on dislocation dynamics. The creep curves display an incubation period and dislocation density increases with strain, consistent with the model. It is established that the secondary-creep rate, ɛs, is independent of initial dislocation density and is a property of the material, as is also the creep rate, ε*, at the point of inflection marking the end of the incubation period. These properties are expressed in the forms εs = M′ exp(-H c/RT) σp sinh(Nσ) and ε* = P exp(-H c/RT) eασ, where σ is stress, T temperature, R the gas constant, H c the apparent activation energy for creep of the material, and the other coefficients are constants. The value of H c is 133 kcal/mole, which is twice that for self-d1ffusion in nickel. It is proposed that this is because creep is controlled by the product of two terms, namely dislocation velocity and density, which are themselves controlled by an activation energy equal to that for self-diffusion in nickel. Finally, transmission electron micrographs show that the dislocations are situated mainly in the γ solid solution of the γ-γ′ structure. This is explained in terms of the interaction of the dislocations in the γ matrix with the γ/γ′ interface.