Effects of crystal structure and grain orientation on the roughness of deformed polycrystalline metals

Surface roughening during tensile deformation of polycrystalline aluminum, iron and zinc is investigated using white light confocal microscopy and orientation imaging microscopy. A height–height correlation technique is used to analyze the data. The surface obeys self-affine scaling on length scales up to a correlation length which approximately equals the grain size and above which no height correlation is present. The self-affine scaling exponent increases initially with strain and saturates at a value around 0.9 for aluminum and at 0.8 for iron and zinc. A linear relation is observed between root mean square roughness and strain. The observed grain scale roughening is explained as arising from orientation differences between neighboring grains and depends on the available number of slip systems in the material. Orientation imaging microscopy is used to investigate the influence of the orientation of the surface grains and subsurface grains on the topography. It is found that the Schmid factor of surface grains alone is not enough to predict local deformation and evolution of surface height. In particular, grains with high Schmid factor may show less deformation than expected. It is shown that subsurface grains influence the roughness and it is hypothesized that a high cumulative Schmid factor on a cross-section below a point at the surface leads to a depression at the surface.