3.4 Morphology of cementite phase
The present work has shown that although cementite is much harder than matrix in bearing steels, it still participates in shear deformation. Fig. 13 shows the morphologies of the cementite phase under different loading conditions after ten cycles. As the load applied along the y -axis increases, the surface morphology of the cementite phase is destroyed more seriously, and the regular lattice structure tends to be chaotic (shown in Fig.13a-c). When the load applied along the x -axis increases, the volume of the cementite phase change inconspicuously (shown in Fig.13d-f). This is due to the fact that the loading direction is parallel to the interface of the two phases. Nevertheless, the regular crystal structure of the cementite phase is destructed and tends to be disordered when the shear deformation is large enough as shown in Fig. 13f. The above description is consistent with the mechanism of dislocation evolution drawn from Fig. 10. The residual shear strain in the model cumulates, as the shear load increases, and the cementite phase prefer to be damaged.
Plastic deformation results in dislocations initiating and growing from one interface to another until they are absorbed by the cementite phase. And the absorbed dislocations may then issue in a local shear deformation in the cementite phase.23 In addition to that, when the shear zone in bcc-Fe matrix is thick enough to penetrate the interface, the shear zone is formed in the cementite phase. It is known that a breaking of brittle phase such as cementite works as a trigger of cleavage fracture initiation.50 Under the joint induction of RCF and the inclusion defects within the subsurface, severe cyclic shear deformation will destroy brittle phase such as cementite inclusion in bearing steels, and thus impair the fatigue life of bearing.