4 Conclusion
The atomic simulations based on the two-phase model of bcc-Fe and cementite in this work characterize the microscopic mechanism of cumulative damage in bearing steels under RCF. The main conclusions can be summarized as follows.
1. The stress responses of the atomic model appears cyclic softening under all three kinds of cyclic load, and a larger load leads to a greater reduction in stress amplitude and a time-advanced cyclic softening.
2. Dislocations nucleate and annihilate on the interface when shear load amplitude and cycle index are small. When the amplitude and index are large enough, regular structures of the material are destroyed, and massive dislocations retain in the model, resulting in the gradual accumulation of plasticity.
3. Larger shear load amplitude leads to higher average density of dislocation and larger proportion of irregular structure in the model. As loading direction changes, the stress responses and dislocation density of the model is basically unaffected, but the proportion of irregular structures, as well as the initiation and movement of dislocations changes significantly.
4. Large shear load amplitude causes a mass of defect meshes in the model, further resulting in lots of residual shear strains and stresses in the model. When the cementite phase tends to be damaged under severe shear deformation, the material is usually in a dangerous state, which indicates the origin of failure. Since cementite is brittle and difficult to deform, it can represent a series of brittle inclusions in bearing steels. Therefore, plastic accumulation and defect initiation are more likely to occur on the interface between inclusions and bearing steel matrix under RCF.