Mathematical framework for a semi-physical model for the prediction of creep life of Fe-Ni-Cr alloys addressing solid-solution hardening

Incoloy 800H is an austenitic Fe-Ni-Cr alloy whose creep behaviour is characterized by the onset of two minima within the creep strain rate-time curve. The underlaying physical phenomena inducing this mechanical response are still unclear. (Guttmann and Bürgel, 1983) attribute the first creep rate minimum to a creep mechanism transition induced by dislocation-pinning, whereas the second one is said to be consequence of internal nitridation. The creep hardening effect of nitridation in 800H is evidenced in (Young et al., 2023). Meanwhile, studies on austenitic steels exhibiting similar creep responses attribute both creep rate minima to two separate precipitate strengthening phenomena (Hatakeyama et al., 2022). it is thereby clear that the accurate prediction of the creep behaviour in 800H alloy requires the assessment of creep micromechanics and microstructure evolution.
In this work, we provide a numerical framework for the implementation of a mean-field creep model adapted to predict the high-temperature creep deformation response of Incoloy 800H. As a semi-physical model, microstructure evolution is considered in the form of dislocation density distributions, average grain and sub-grain size, and primary precipitate kinetics. The latter is calculated separately via thermodynamic simulations. This approach has proved high accuracy for the prediction of dislocation-based creep deformation of P91 steel (Riedlsperger et al., 2020), and diffusion + dislocation creep deformation observed in A617 Ni-alloy (Riedlsperger et al., 2023). To predict the creep deformation response of 800H alloy, we adapt the model to consider all primary precipitates known to play a role in its creep behaviour (namely MC, M23C6 and TiN). On the basis of the work of (Young et al., 2023), further discussion is given on the possibility of the extension of the model to account for the internal nitridation of the alloy by including the precipitate kinetics of chromium (CrN, Cr2N) and aluminium (AlN) nitrides.