Tensile behaviour and strain hardening characteristics of constrained groove pressed nickel sheets

In this study commercially pure nickel sheets are severe plastically deformed at room temperature by constrained groove pressing (CGP) technique and the effect of pass number on the room temperature mechanical behaviour is investigated. Increase in strength observed after first pass is much higher than the increase observed during subsequent passes. Mechanical behaviour of constrained groove pressed sheets indicated negligible strain hardening ability during initial passes: gain in strain hardening ability is observed during latter passes resulting in enhanced ductility. The observation of shortened uniform elongation phase during tensile testing of CGP processed sheets could be linked to the lack of strain hardenability and change in deformation mechanism. Constitutive mechanical behaviour in uniform plastic deformation regime of nickel sheets in annealed condition obeys Hollomon relation whereas severely deformed sheets obey Voce relation closely. Strain hardening characteristics of groove pressed sheets analysed by Kocks-Mecking approach revealed stage-III hardening behaviour associated with high initial hardening rate when compared to annealed sheets. The influence of pass number on dislocation density evolution is assessed by Taylor's expression. The synergistic effect of dislocation generation and recovery on the evolution of constitutive mechanical behaviour in the uniform elongation regime is described by applying single parameter based Kocks model. The kinetics of dislocation storage and dislocation annihilation in severely deformed nickel sheets during deformation corroborated with mechanical properties and dislocation density indicates the dominance of dislocation generation during earlier passes. (C) 2011 Elsevier Ltd. All rights reserved.