Fig. 8 Proportion of irregular structures in the model. Loading
directions are along (a) the y -axis and (b) the x -axis,
respectively. Magenta, green and red lines refer to d1, d2 and d3,
respectively. Base line refers to the proportion of cementite phase.
Plastic deformation of materials generally means that regular structures
tend to be chaotic, and it also corresponds to the increase in the
proportion of irregular structures. Statistical analysis of the
proportion of irregular structures within the model can explain the
mechanism of plastic deformation to some extent.45 The
bcc-Fe matrix in the atomic model can be identified as regular BCC
structure by CNA algorithm, while cementite phase, point defects and
surface defects such as stacking faults are identified as irregular
structures. Fig. 8 shows the statistical results of the proportion of
irregular structures in the model under cyclic loads. The loading
directions in Fig. 8a and b are along the y -axis and thex -axis, respectively. The
base line in Fig. 8 represents the proportion of cementite phase in the
model. Some of the same variation rules can be drawn from Fig. 8a and b.
When the load is d1, the proportion of the irregular structures in the
first three periods is close to the base line and then remains at a high
level. When the load is d2, the proportion of irregular structures
gradually increases after the first period and then remains at a high
level. The proportion of irregular structures is always kept at a high
level under d3. And all the curves shown in Fig. 8 have obvious
periodicity. The above rules are consistent with the variation rules of
dislocation density drawn from Fig. 7. However, when the load is d3, the
average value and amplitude fluctuations of the proportion of irregular
structures shown in Fig. 8a are larger than these shown in Fig. 8b,
which is different from the conclusion drawn from Fig. 7. A relatively
reasonable explanation is that different loading directions correspond
to different mechanisms of dislocation nucleation and annihilation, and
this will be discussed in detail in the following sections.