Cyclic softening of martensitic steels at high temperature—Experiments and physically based modelling

Martensitic steels subjected to cycling at high temperature usually show softening accompanied by microstructural changes such as the disappearance of low-angle boundaries (LABs). A micromechanical model of the disappearance of LABs due to annihilation between mobile impinging dislocations and LAB dislocations has been recently proposed [M. Sauzay, H. Brillet, I. Monnet, M. Mottot, F. Barcelo, B. Fournier, A. Pineau, Mater. Sci. Eng. A 400–401 (2005) 241–244]. This model is, however, unable to accurately reproduce viscoplastic strain amplitude and hold time effects, while these effects are clearly observed. Considering that there is not a complete renewal of the (edge) mobile dislocations at each cycle, the lifetime of the mobile dislocations is introduced as well as a climb mechanism to describe more accurately the annihilation mechanisms. Using in addition the Hall–Petch modelling proposed by Li, stress softening predictions are discussed with respect to experimental results.