Evolution Of Microstructure In 304 Stainless Steel During Ratcheting Deformation

Ratchetting behavior of the materials has been extensively investigated and simulated in the last two decades, since it is important in the design and assessment of engineering structure components subjected to asymmetrical stress-controlled cyclic loading. However, most of the referable literature concerned only the macroscopic phenomenon of ratchetting and its phenomenological models. In order to reveal microscopicmechanism and Physical nature of ratchetting, the microstructure evolution of SS304 stainless steel during uniaxial stress-controlled cyclic loading with the ratchetting deformation was observed by using OM, SEM, XRD and TEM methods. Several specimens with the same applied stress level were first tested under the stress-controlled cyclic loading with different numbers of cycles macroscopically, and then the thin-filmed samples obtained from the specimens subjected to different numbers of cycles were microscopically observed. The results show that:(1):When the ratchetting strain reaches a certain value high enough, the strain-induced martensite transformation occurs in the process of ratchetting deformation. The resultant phase isα-martensite and the amount of the induced martensite increased gradually with the number of cycles, which is proved by the quantitative analysis of XRD.(2):The plastic deformation caused by the martensite transformation has a un-negligible contribution to the total value of ratchetting strain. The total ratchetting deformation should be considered as a superposition of two parts, the one caused by applied cyclic stress and the other caused by the strain-induced martensite transformation.(3):The density of dislocation in the material increases gradually with the increase of ratchetting strain. The dislocation substructures change from the form with low dislocation density such as dislocation lines and dislocation nets to that with higher dislocation density such as dislocation walls and dislocation cells and the number of grains containing dislocation cells increases with the increasing number of cycles during the ratchetting deformation. Simultaneously.(4):The evolution and configuration of dislocation sub-structures depends on cyclic loading way. The dislocation configurations and evolution law are varied in different cyclic loading ways such as asymmetric tension-compression loading, symmetric tension-compression loading and asymmetric tension-tension loading, which cause different ratchetting deformation characteristics. There is no sub-grain formed in the whole deformation process.