Optical anisotropy of a color-etched AZ91 magnesium alloy

In this paper, the optical anisotropy of a color-etched AZ91 magnesium alloy grain is studied. In the first part, the rinsing and drying conditions after etching were varied in order to improve the contrast between grains. A rinsing solution was selected and ellipsometric measurements were carried out to characterize the optical anisotropy of the etched surface. The wavelength, grain orientation and angle of incidence were varied. It was found that the reflection intensity at oblique incidence and the phase shift between parallel and perpendicular polarizations depend on the orientation of the etched surface of the grains. The optical contrast under diffuse light is explained by the morphology of the surface film deposited upon etching. The optical contrast under polarized light is attributed to form birefringence induced by the film texture. The birefringence, the fast axis, the slow axis, and the optical axis of the etched surface were also determined with the polarizing microscope.