Micromechanics of high-temperature damage in dual-phase stainless steel

High-temperature ductility of dual-phase stainless steels is investigated using a micromechanics approach of damage and fracture. Two different microstructures are studied with either a lamellar or a globular morphology of the ferrite phase, the latter being twice as ductile as the former at 1150 °C. The high-temperature damage evolution is characterized at different loading rates using notched round cylindrical bars producing different stress triaxialities, supplemented by fractographic analysis. The experimental observations have generated an advanced elasto-viscoplastic micromechanical damage model for both microstructures. With a detailed account of the process of nucleation, growth and coalescence of voids, the model properly captures the effect of stress triaxiality, strain rate, and morphology of the ferritic phase on the high-temperature ductility. The key factor controlling the loss of ductility in the lamellar microstructure is the constraint induced by the harder austenite layer on the softer ferrite zones.