Creep Deformation by Dislocation Movement in Waspaloy

Abstract

Creep tests of the polycrystalline nickel alloy Waspaloy have been conducted at Swansea University, for varying stress conditions at 700 °C. Investigation through use of Transmission Electron Microscopy at Cambridge University has examined the dislocation networks formed under these conditions, with particular attention paid to comparing tests performed above and below the yield stress. This paper highlights how the dislocation structures vary throughout creep and proposes a dislocation mechanism theory for creep in Waspaloy. Activation energies are calculated through approaches developed in the use of the recently formulated Wilshire Equations, and are found to differ above and below the yield stress. Low activation energies are found to be related to dislocation interaction with γ′ precipitates below the yield stress. However, significantly increased dislocation densities at stresses above yield cause an increase in the activation energy values as forest hardening becomes the primary mechanism controlling dislocation movement. It is proposed that the activation energy change is related to the stress increment provided by work hardening, as can be observed from Ti, Ni and steel results.

A Modern Philosophy for Creep Lifing in Engineering Alloys

“…Subsequent application of this varying activation energy was found to improve fits in a range of other materials [49,50], whilst remaining consistent with the observed micromechanical behaviour.…”

“…As soon as Figure 11. TEM of Waspaloy microstructure after creep at 700 C at (a) 500 MPa, (b) 600 MPa, and (c) 800 MPa [50].…”

“…It was found that the dislocation Figure 9. TEM of Waspaloy microstructure in original, un-tested state [50]. …”

“…Below the yield stress, dislocation densities are relatively low and increase Figure 10. TEM of Waspaloy microstructure after tensile testing [50].…”

“…As such a study was carried out on the nickel superalloy Waspaloy. Details of the material and creep experiments performed can be found in Whittaker et al [50]. Constant stress creep tests were performed from 550 to 800 C and 140-1150 MPa.…”