EBSD-Like Orientation Measurements on Selected Grains by Optical Microscopy

Abstract

Mapping crystallographic quantities on the surface of polycrystalline metals has always been the prerogative of electron-based microscopy techniques, such as electron backscatter diffraction (EBSD). EBSD plays a central role in the study of structure-property relationships of crystalline solids, owing to its ability to correlate lattice orientation and misorientation with the geometry of the material's constituent grains at a high spatial resolution. In this work, we demonstrate the capability of performing EBSD-like orientation and misorientation measurements using an microscopy technique. This technique, which we call directional reflectance microscopy (DRM), allows circumventing the diffraction barrier of lenses by quantifying the orientation-dependent reflectance of metal surfaces. We demonstrate DRM on chemically etched polycrystalline aluminum samples. The etching process yields the formation of etch-pits comprising facets with known crystallography, which only reflect light under certain illumination angles. By analyzing the surface directional reflectance, we infer the orientation of the etch-pit facets and thus that of the underlying crystal lattice. Because of the complex reflection of light at etch-pits, DRM measurements are currently limited to a subset of grains that share the same (111) out-of-plane orientation. We compute an average difference of ~4° in the 3D crystal orientation of these grains when we measure it by EBSD and DRM. Our results set the stage for an optical version of EBSD, which will enable high-throughput and low-cost grain orientation mapping of polycrystalline solids.