4.3. Ordered Structure Evolution
Figure 9 shows the diffraction pattern with [011] zone axes (
Figure 9a), and the dark-field images of the B2 (
Figure 9b) and DO
3 (
Figure 9c) domains in the as-forged Fe-6.5 wt. % Si alloy. The diffraction pattern indicated the presence of both B2 and DO
3 ordered phases in the as-forged sample, as represented in
Figure 9a. The B2 and DO
3 domains were visible in the dark-field images using (200) and the (111) superlattice diffraction spots, respectively [
31,
32,
33]. The sizes of the coarse B2 domains were 300‒500 nm, and those of the fine DO
3 ordered domains were 20‒40 nm, as shown in
Figure 9b,c, respectively. Furthermore, a smoothly curved a’/4 <111> anti-phase boundary (APB) was clearly observed in
Figure 9b [
34].
Figure 10 shows the diffraction pattern with the [011] zone axes (
Figure 10a), the dark-field image for B2 domains (
Figure 10b), and the bright-field image for the substructure (
Figure 10c) in the bar after rough rolling. Diffraction spots could be indexed to the ordered DO
3 and B2 phases, as well as the disordered A2 phase. The DO
3 spots were too weak to obtain a dark-field image. The amount of the DO
3 phase was reduced by rough rolling. The superlattice spots corresponding to the B2 structure seemed weak and obscure, signifying some variation in the B2 structure, as shown in
Figure 10a. In the dark-field image (
Figure 10b), the size of the B2 domains decreased to 100–150 nm, differing from that of the forged specimen. Dislocations and substructures were also observed, as shown in
Figure 10c. Dislocation lines were visible, and subgrains appeared at the grain boundaries. Specifically, discontinuous dynamic recrystallization (DDRX) occurred during the rough rolling at a high deformation rate [
35]. Ferrite recrystallization occurs through a mechanism analogous to DDRX at high strain rates, and new grains nucleate from the subgrains at the boundaries. The dynamic migration of grain boundaries is considered to be an indication of DDRX. Enough stored energy is accumulated during deformation at high strain-rates to promote the DDRX mechanism, i.e., the formation of new recrystallized grains through nucleation and growth [
36].
Figure 11 shows the diffraction pattern with the [011] zone axes (
Figure 11a), the dark-field image for B2 domains (
Figure 11b), and the bright-field image of the substructure (
Figure 11c) of the bar after finish rolling. Only B2 diffraction spots were observed in the diffraction pattern. The characteristic diffraction spots of the DO
3 ordered phase became so weak (as shown in
Figure 11a) that a corresponding dark-field image could not be obtained. Moreover, the B2 ordered domains were refined to a size of 40‒80 nm, as represented in
Figure 11b. This indicates that the degree of order decreased after the finish rolling. Subgrains were observed inside the grains (
Figure 11c) during the finish rolling, at a low deformation rate. Furthermore, the subgrain boundaries partly evolved into grain boundaries. This is consistent with the mechanism of continuous dynamic recrystallization (CDRX) [
35,
36]. The occurrence of CDRX in the ferrite under low strain-rates is in excellent agreement with the model suggested by Gourdet and Montheillet, who claim that the low angle boundaries (LABs) created by DRV, and increasingly transformed by strain, become high angle boundaries (HABs) through the continuous absorption of dislocations during CDRX [
37].
These observations demonstrate the transition from CDRX to DDRX with increasing strain rate, which agrees with the recent findings made by Haghdadi et al [
36]. These authors proposed that deformation at lower strain rates provides sufficient time for significant DRV, thus leading to CDRX, whereas the magnitude of DRV is reduced under deformation at higher strain rates, instead promoting the DDRX mechanism with the formation of new recrystallized grains. However, the current work implies that DDRX in the Fe-6.5 wt. % Si alloy (considered a single-phase steel because of the low content of the B2 ordered phase) may exhibit some important differences when compared with two-phase steels. In fact, the initial HABs, which are the preferred sites for nucleation of new recrystallized grains, are always present in the Fe-6.5 wt. % Si alloy at high strain rates, but are rarely present in two-phase steels. Nonetheless, in two-phase steels, the interphase can assume this role through the formation of highly-strained regions [
35]. Therefore, DDRX is probably occurs at ferrite-ferrite boundaries in single-phase steels (such as the Fe-6.5 wt. % Si alloy), but at interphase mantle regions in two-phase steels.
In conclusion, DDRX can occur at grain boundaries in the Fe-6.5 wt. % Si alloy with high stacking fault energy, through the nucleation and growth of subgrains at high deformation rates. However, CDRX appears inside the grains by continuous absorption of dislocations at low deformation rates.