A primary creep model for Class M materials

In many high temperature applications the accumulation of creep strain during the primary stage cannot be ignored since most of the allowable design strain occurs in this stage. An appropriate modelling is therefore needed. In this work, a mechanism based model for primary creep has been derived assuming that the creep rate in the transient regime can be given as a function of the steady state creep rate and the internal stress. Taking into account that the apparent activation energy varies with the internal stress and that the internal stress kinetics can be given as a function of strain, an exponential form of the creep rate versus creep strain has been obtained. The proposed model has been applied to high chromium steel P91 and René 80 nickel based superalloy. The decay constant and scaled activation volume have been found to vary only with the applied stress and not with temperature. The evolution of the these two parameters with stress seems to indicate the stress at which the transition from diffusional flow to dislocation climb creep occurs.