Mean-field modeling of the creep deformation response of alloy 1.4959 (800H) addressing precipitate kinetics

High temperature creep deformation of austenitic steel alloys is ccompanied by a complex process of microstructure evolution induced by internal and environmental factors. Processes such as aging and corrosion (i.e., ripening and precipitation of primary and secondary precipitates) result in non- conventional creep hardening and softening trends that are found beyond the limitations of conventional creep models. In this work, a mean-field creep model adapted to recover the non-classical creep response of austenitic steel 1.4959 alloy is formulated, implemented and validated. The microstructure evolution is considered in terms of precipitate kinetics, and is evaluated by means of ex-situ CALPHAD simulations conducted in ThermoCalc TC-PRISMA. This approach allows to recover the creep hardening and softening effects attributed to phenomena such as solid-solution hardening, precipitate strengthening and precipitate damage accumulation. The numerical implementation of the model follows a fully-implicit 1-level Newton-Raphson resolution algorithm. Physical parameters are obtained from either literature or in-house experiments, whereas fitting parameters are optimized to fit the creep response observed during our creep experimental campaign. Creep curves obtained using the validated set of parameters show reasonable accuracy with experiments. Further extrapolation of results is physically consistent, and reliability is limited to loading regimes sharing the same creep mechanisms. An additional phenomenological cavitation damage formulation is implemented to achieve accurate fracture prediction.