Microstructure and Hardness of Pure Aluminum Surface Nanocrystallization after SMAT

A nanostructured surface layer was formed on pure Al by means of surface mechanical attrition treatment (SMAT).The microstructure of the surface layer of the sample treated by SMAT was characterized by means of X-ray diffraction(XRD) and transmission electron microscopy (TEM).Hardness variation along the depth direction was tested.The results showed that the grain refinement process at the surface layer involves formation of dislocation tangles (DTs),dislocation cells (DCs) and dense dislocation walls (DDWs) in original grains.With the increase of strain the dislocation walls and tangling transformed into dislocation cells,subgrains,dense dislocation walls-microbands (DDWs-MBs) and lamellar boundaries (LBs).With the increase of strain and strain rate further,grains were subdivided on a smaller scale and finally equiaxed nanocrystallites with random crystallographic orientations were formed.The hardness of nanostructured surface layer was enhanced significantly after SMAT compared with that of the initial sample.