3.3 Evolution of dislocations
Dislocation is a kind of line defect in crystal, which plays a decisive role in the plastic deformation, strength and fracture of materials.46 Fig. 9 shows the details of dislocation evolution under monotonic loads. Fig. 9a and b show the dislocation distribution when loads are applied along the y -axis and thex -axis, respectively. Fig. 9c provides the initial structure for comparison. In Fig. 9a, dislocations begin to nucleate from the interface between cementite and bcc-Fe when the load reaches d3, and then grow with the increase of shear load. Under periodic boundary conditions, dislocations gradually connect the left and the right interfaces, which manifests as dislocations initiating from one interface and growing through the boundary to the other interface. This indicates that dislocations cannot annihilate and continue to remain in the material, marking the beginning of plastic accumulation of the material. Fig. 9b shows that with the increase of shear load, the quantity of dislocation gradually increases, and the more significant feature is the appearance of defect meshes near the interface. Defect mesh is a triangulated mesh which represents the bad crystal regions that have not been classified as dislocations.37,47 In the present work, it represents the residual shear strain in the bcc-Fe matrix due to cyclic loading, and further indicates the residual shear stress in the model. The appearance of defect mesh corresponds to the increase of local micro-plastic flow and residual stress of the material in stage I of RCF, which is not conducive to the fatigue life of the bearing.11 Note that when the monotonic load applied along the x -axis reaches 24.0Å, the defect meshes on the left and right interfaces have the same orientation, as shown in the red dashed boxes in Fig. 9b. This pattern usually indicates the presence of large shear slip bands in the model. Meantime, it also refers to the transfer of the slip bands from bcc-Fe matrix to cementite phase, indicating that shear deformation begins to occur inside or on the surface of the cementite phase. When shear slip occurs in the brittle cementite phase, the fatigue failure of the material begins to develop.