All lath boundaries were not clearly detected at DC region because these lath boundaries were the primary sites for the annihilation of dislocations during PWHT process. Furthermore, some sub-grains resulting from the divided laths through the annihilation of dislocations were observed at DC region, as indicated by blue arrows in Fig (a), (c) and (e) . For ID region, the lath boundaries gradually became more pronounced with the increase of the V contents in PWHT specimens, which indicated that the pinning effect of the nano-scale M2C carbides on lath boundaries was weaker than that of the nano-scale MC carbides. A considerable number of nano-scale MC carbides decorating the lath boundaries could provide strong resistance to the boundaries motion during PWHT process, as shown in the red rectangular area in Fig (g)–(h). The pinning effect appeared to be stronger at ID region in V20 PWHT specimen than that in V10 PWHT specimen because a higher density of nano-scale MC carbides was probably present on the lath boundaries as a result of more dissolved V atoms and higher dislocation densities. Furthermore, the more intra-lath nano-scale MC carbides at ID region in V20 PWHT specimen may act as obstacles for dislocations motion, which was responsible for preventing the formation of sub-boundaries. Thus, the dislocation density at ID region was the highest in the V20 PWHT specimen, as shown in Fig (f).
