Hall–Petch and dislocation strengthening in graded nanostructured steel

The structure and strength of low carbon steel samples have been analyzed after plastic deformation by shot peening and cold rolling. The fine scale surface microstructure caused by shot peening extends to about 50 micrometers below the surface. The structure is graded and subdivided by dislocation boundaries and high angle boundaries showing a clear resemblance to the lamellar structure which evolves during conventional rolling of bulk metallic materials from medium to high strain. As the surface is approached the boundary spacing decreases to about 50nm at the surface. In parallel the misorientation angle across boundaries increases to about 65% of high angle boundaries. The cold rolled steel shows a low hardening rate at high strain and by assuming additive strength contributions from Hall-Petch and dislocation strengthening, the flow stress has been .