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.