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. (C) 2007 Elsevier B.V. All rights reserved.