Formation of nanocrystalline structure in carbon steels by ball drop and particle impact techniques

The formation of nanocrystalline structure on the surface of bulk steel samples was studied using ball drop and particle impact techniques. Nanocrystalline layers with several microns thick were successfully fabricated by both methods. The microstructural observations, hardness measurements and annealing experiments suggest that the nanocrystalline layers have similar characteristics with those produced by ball milling (BM). The nanocrystalline regions have sharp boundaries with work-hardened neighboring regions and no intermediate regions were observed. Pre-strain and low deformation temperature was found to enhance the formation of nanocrystalline regions. Nanocrystalline layers with large shear deformation were sometimes observed. From the measured shear strain, the equivalent true strain necessary to produce nanocrystalline region was estimated to be about 3.