Dislocation mechanisms of mean stress effect on cyclic plasticity

Abstract

Abstract The dislocation mechanisms behind sagging behavior of autosuspension spring steels are far from being understood due to their complicated microstructures. In this study, systematic cyclic loading tests were carried out on two model materials - industrial pure iron and spheroidized 1045 steel – to understand the mechanisms behind sagging. It is found that it is the different dislocation microstructures that control the cyclic creep behavior of different materials. With iron samples with low mean stress values, higher mean stresses do not trigger cell structure formation mainly due to the little requirement for high plastic strain amplitude and hence the lack of necessity for high dislocation density, and a large number of dislocation interactions through large mobile dislocations. In the case of the 1045 steel samples, the collapse of pre-existent substructures and the movement of newly generated mobile dislocations are the main reasons for the cyclic softening observed. The massive reorganization process of dislocation configurations and the competition between softening and hardening both carry on throughout the entire cycling process, but the softening process is found to be the predominant factor.