Fig. 9 Dislocation distribution in the model under monotonic
loads. Loading directions are along (a) the y -axis and (b) thex -axis, respectively. The initial structure is provided for
comparison in (c). Green and magenta lines in bcc-Fe matrix correspond
to 1/2<111> and <100>
dislocations, respectively. Gray planes in bcc-Fe matrix are defect
meshes. Red dashed boxes contain defect meshes with the same
orientation.
Cyclic softening and elastic-plastic transformation in Fig. 6 are
closely related to the evolution of dislocations during cyclic shear
deformation. Fig. 10 shows the
dislocation distribution in the model underlying cyclic deformation.
Fig. 10a-c correspond to the loading conditions of d1, d2 and d3
respectively, and the loading direction is along the y -axis. Fig.
10a shows that dislocations begin to appear at the end of the third
cycle, and then increase and remain at a low level in the following
cycles. Fig. 10b shows that dislocations begin to appear at the end of
the first cycle. When the load is d3, a large amount of dislocations and
defect meshes exist in the bcc-Fe phase at the end of the first cycle.
The proportion of irregular structures is large, and the quantity of
dislocations and defect meshes remains high throughout the whole cyclic
loadings. The dislocation evolution under these three conditions
corresponds to the variation of dislocation density shown in Fig. 7. The
defect meshes shown in the red dashed boxes in Fig. 10c have the same
orientation, which is similar with the results shown in Fig. 9b,
indicating that there exists a lot of residual strain in the model. This
also points out that the material has undergone severe plastic
deformation and damage accumulation, and the cementite phase tends to be
damaged.20 Note that there are more defect meshes in
Fig. 10c than those in Fig. 9a, indicating that cyclic shear deformation
is more destructive to the structure than monotonic shear deformation,
and is more likely to result in fatigue failure of the material.
Fig. 10d-f correspond to the loading conditions of d1, d2 and d3
respectively, and the loading direction is along the x -axis. When
the load is d1, the interfaces are free of dislocations at the end of
the third cycle, after which the dislocations increase and remain at a
low level. When the load is d2, dislocations begin to appear at the end
of the first cycle. It is noted that under these two loading conditions,
most of the dislocations in the model are annular, germinating from the
interface and eventually remaining in the bcc-Fe phase. When the load
reaches d3, dislocations and obvious defect meshes are generated at the
end of the first cycle. In the subsequent cycles, although there are
still many dislocations in the model, no obvious defect mesh could be
found on the interface. The less residual shear strain in the model can
be attributed to that the direction of the cyclic loadings is parallel
to the interface. This indicates that the model produces less plastic
accumulation, or the rate of plastic accumulation is slower, which may
result in a relatively longer fatigue life of the material. It is worth
noting that, under cyclic deformation, fewer defect meshes are generated
in the model when loads are along the x -axis, which is contrary
to the rule drawn from Fig. 9. This suggests that the plastic
accumulation mechanism of the material is ascribed to the synergistic
effects between the cyclic deformation and the loading direction.