EBSD-Like Orientation Measurements on Selected Grains by Optical Microscopy

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 optical microscopy technique. This technique, which we call directional reflectance microscopy (DRM), allows circumventing the diffraction barrier of optical 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.