Analysis of gradient structured materials processed using severe plastic deformation

Abstract

To achieve both high strength and ductility is the ultimate goal in the structural material community. However, it is well known that homogeneous ultrafine-grained (UFG) and nanocrystalline (NC) metallic materials have high strength but low ductility. To overcome the strength and ductility trade-off, gradient structure has been suggested as the solution. Gradient structured materials have heterogeneous microstructures with varying grain size (from a few nanometers or micrometers to a few millimeters) from the surface to inner regions. In this study, for generating gradient structure the two types of deformation processes as surface abrasive torsion and surface severe plastic deformation were suggested and analysed. At the first, a novel process of discrete surface abrasion during simple torsion (ST) named surface abrasive torsion (SAT) is proposed to overcome the limitation of ST, i.e., insufficient strain for severe plastic deformation due to cracks initiated on the surface, by removing the roughened surface region. The effect of SAT on delayed crack initiation was explained using the finite element simulations. Larger shear deformation applicable to the specimen in SAT than ST was demonstrated experimentally. Additionally, to create a more suitable gradient structure a new surface severe plastic deformation process called the ultrasonic nanocrystalline surface modification (UNSM) technique introduced. In this study, the effects of surface grain refinement and residual stress on the local and global properties of pure Cu processed using ultrasonic nanocrystalline surface modification (UNSM) was investigated. The distinct contributions of residual stress and surface grain refinement to the tensile property of UNSM treated Cu were determined and discussed.