Strain-induced refinement and thermal stability of a nanocrystalline steel produced by surface mechanical attritiontreatment

A surface layer with a depth-dependent nanocrystalline microstructure was formed on reduced activation ferrite-martensitic (RAFM) steel by means of surface mechanical attrition treatment (SMAT). Equiaxed nano-sized grains in the surface layer were characterized by XRD, SEM and TEM, and the result indicated that the grain size decreased gradually with increasing deformation strain and strain rate. Compared with the size and number of carbides in the matrix, smaller and fewer carbides were detected in the SMAT layer, which indicated the refining and dissolving process of carbide during the drastic deformation in the SMAT process. Sub-grain boundaries (sub-GB) and high dense dislocation walls (DDWs) in ferrite were found in deformation layer, and dislocation accumulation, rearrangement and annihilation during the drastic deformation were discussed in detail. Both TEM images and XRD results demonstrate that the nanocrystalline layer has excellent thermal stability after annealing at 823 K.