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.