Size effect affected mechanical properties and formability in micro plane strain deformation process of pure nickel

Abstract

The plastic deformation behaviors are difficult to predict because of the size effects induced by the surface effect and inhomogeneous distribution of microstructure when the specimen dimension scales down to micro/meso scale. It is a non-trivial issue to study the size effect on deformation behaviors in micro-forming process. In this study, the flow stress size effects in micro plane strain compression of pure nickel micro rods of 0.7 mm in diameter with different annealing processes were investigated experimentally. It is found that the flow stress decreases with the increasing grain size under a low strain stage but tends to increase at a higher strain level. A dislocation density theory based constitutive model considering the coupling influence of free and constrained surface effects with strain was developed to simulate the flow stress size effect. The experimental and calculated results matched well. The micro coining of array micro channels results indicated that the height of micro rib decreased from the center to the edge along the transverse direction of the specimen. The protrusion feature height was strongly dependent on the ratio of cavity width to grain size. When there were about 2.4 grains across the cavity width, the filling ability was the worst.