Plastic deformation size effects in micro-compression of pure nickel with a few grains across diameter

In microscale, it is hard to predict the plastic deformation behaviors because of size effects. In this study, size effects on plastic deformation behaviors in micro-compression were investigated experimentally at room temperature, including flow stress and inhomogeneous deformation. It is found that the flow stress decreases with the increase of grain size when there are more than 6.2 grains across the specimen diameter. The flow stress tends to increase when there are only 3–4 grains across the specimen diameter. Then, a flow stress size effect model considering both free surface and constrained surface effects is developed to model the flow stress size effect in microforming. The calculated results are in good agreement with experimental ones. The influence of the ratio of specimen diameter to grain size on the flow stress size effect is revealed. The experimental results also indicate that the inhomogeneous deformation in micro-compression increases with the increase of grain size. The surface folding occurs on the surfaces of some specimens with a few grains across the diameter. Then, the surface folding phenomenon is studied through Electron Backscattered Diffraction (EBSD) and the proposed model considering the microstructure distribution on the specimen surface. The reported results and models provide a basis for understanding of the size effects on plastic deformation behaviors in microforming.