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.