Uniaxial ratcheting behavior and molecular dynamics simulation evaluation of 316LN stainless steel

In this study, a series of uniaxial ratcheting experiments under different mean stresses with a constant stress amplitude were performed on a 316LN stainless steel at room temperature. Meanwhile, molecular dynamics (MD) simulation was carried out to reveal the microstructure evolution mechanism at the initial stage of ratcheting deformation at atomic scale. The results showed that the initial stage of ratcheting deformation could be divided into three stages: the stage of beginning (ratcheting strain growth rate (RSGR, dεr/dN) >0.01%), the stage of decreasing ratcheting rate (RSGR at 0.00001–0.01) and the stage of elastic/plastic shakedown (RSGR <0.00001) based on the changes of ratcheting strain rate. The shakedown ratcheting strain was linearly increased with increase of the mean stress in both MD simulation and experiment. And the ratchet deformation at different stages strongly depended on dislocation evolution. At the stage of beginning, the dislocation originated from coincident site lattice (CSL) boundaries and formed pile-up structures at random angle grain boundaries (RAGBs); while at the stage of decreasing ratcheting rate, the dislocation tangled at CSL grain boundaries or inside the grain, the dislocation density gradually increased and tended to be stable. When the given stress was insufficient to produce more plastic strain, the dislocation density balanced dynamically and the ratchet deformation entered the stage of elastic/plastic shakedown.