Analysis of creep curves of a nickel base superalloy in a wide stress/temperature range

The creep behaviour of the nickel base superalloy Nimonic 263 has been studied at constant load and temperature in the 750–30 MPa/600–950 °C range. The experimental results have shown a very strong dependence of the creep curve shape with the applied stress/temperature. At high stresses, when an instantaneous plastic strain occurs during the initial loading, the creep curves are characterised by an important decelerating/primary creep while, at lower applied stresses, the primary stage becomes very short and small and other strain stages dominate the creep curves. The experimental results can be rationalised supposing that the mobile dislocation density decreases/increases during creep, from an initially high/low value in the tests run above/below the alloy yield stress. With this hypothesis, the creep results have been modelled using coupled differential equations of the Kachanov form.