Thermo-Kinetic Model of Creep Controlled by Thermally Activated Detachment of Dislocations from Nano-Oxides Revisited

Abstract

Coarse-grained ferritic oxide dispersion strengthened (ODS) alloys represent top creep and oxidation-resistant materials for applications at 1000-1300 °C. The long-term shape stability of loaded parts is conditioned by extremely low creep rates achieved by sticking of dislocations at the back side of nano-oxides. At such temperatures, however, the detachment of dislocations from the nano-oxides can take place by thermal activation. The existing models of this phenomenon assume that dislocations move by slip and only oxides have to be overcome by a combination of slip and climb. Thus, such models are applicable for temperatures significantly below 1000 °C. To fill this gap, we have developed a new model applicable to high temperatures where dislocation climb in the matrix is also considered. The simulations based on the model indicate that the energy barrier for dislocation detachment is up to five times higher than according to the state-of-the-art models. The creep strength benefits from an increased volume fraction and lower size of nano-oxides and increased dislocation line-energy reduction at the particle/matrix interface. An optimal size of the nano-oxides was found between approx. 4 and 6 nm. The dislocation line-energy reduction parameter can serve as a key parameter for the further development of improved oxide dispersion-strengthened alloys.

Keywords: Creep, ODS ferritic steel, Theory and modeling (kinetics, transport, diffusion), Dislocations, Thermally activated detachment