Physically-based model for creep in nickel-base superalloy C263 both above and below the gamma solvus

Abstract

The polycrystalline nickel-base superalloy (C263) is used for stationary components in aero-engines such as combustion chambers, casings, liners, exhaust ducting and bearing housings. It is a fine-precipitate strengthened alloy at 800°C, with a precipitate solvus temperature of 925°C. Below the solvus, the precipitate coarsens at elevated temperature. A critical precipitate size exists below which particle cutting is the rate-controlling creep mechanism. Above the critical size, it is dislocation pinning and climb. Once the solvus temperature has been exceeded, however, the rate-controlling mechanism becomes pinning and climb within a dislocation network. This paper presents physically-based constitutive equations for creep deformation in C263. In the presence of the γ′ precipitate, populations of climbing and gliding dislocations are assumed to exist in a similar way to that proposed by Dyson and Osgerby [3]. Climbing dislocations are assumed to be pinned at precipitates. The dependence of steady state glide dislocation flux can then be obtained as a function of precipitate volume fraction. Above the γ′ solvus, the pinning process is different and results from the establishment of a dislocation network, the size of which determines the pinning distance. The physical constants arising in the equations have been determined by the conventional minimisation of errors between experimental and calculated creep curves. They have also been determined quite independently using fundamental data and experiments. Remarkable agreement between the two sets of physical constants is achieved. The constitutive equations have been shown to capture the material’s creep deformation characteristics over a broad range of temperature, both below and above the γ′ solvus. In addition, the effect of precipitate coarsening on creep rate is correctly captured, together with the effect of prior ageing on subsequent creep rate. The damage processes of cavitation and multiplication of mobile dislocation density have been coupled with the constitutive equations and used to predict failure in constant and variable stress creep.