Atomic force microscopic study on surface morphology of ultra-fine grained materials after tensile testing

The surface morphology of tensile tested ultra-fine grained copper and nickel produced by severe plastic deformation is investigated on different scale with a help of atomic force microscopy. A key role of grain boundaries in surface patterning and plastic flow is emphasized. The presence of two mechanisms contributing to a total strain was found (1) microscopic dislocation slip limited to the grain interior and (2) grain boundary shearing (sliding) of the mesoscopic scale in the plane of maximum shear stresses. This grain boundary sliding which appears at room temperature, results in a macroscopically observed characteristic shear band relief that is often observed on the surface of deformed ultra-fine grain metals.