Experimental assessment and simulation of surface nanocrystallization by severe shot peening

Surface nanocrystallization is an effective approach to bypass the difficulties of synthesizing bulk nanocrystalline material and yet exploit its unique advantages in service. This study uses air blast shot peening over a wide range of coverage (peening time), from conventional to severe, to generate nanostructured surface layers on high strength low alloy steel. Electron microscopy observations were carried out to systematically study the degree and the mechanism of grain refinement as the severity of deformation increases. A model linking finite element simulation of severe shot peening to dislocation density evolution due to the accumulated plastic strain was developed to predict the resultant grain/cell size gradient in the surface layers. The proposed framework establishes a physical connection from processing parameters such as media size, velocity and peening coverage to the resultant structure, opening the possibility of designing a severe surface peening process to achieve a desired nanostructure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.