Tensor and spatially resolved analysis of microscopic stress fields in polycrystalline alumina by polarized Raman spectroscopy

A spectroscopic method is shown to resolve the full set of components of the residual stress tensor, as it locally develops on the microstructural scale in polycrystalline alumina. A polarized/confocal Raman probe is employed. As a first step, the local crystallographic orientation of alumina grains is extracted from the angular dependence of the intensities of polarized Raman bands ( i.e ., upon in‐plane rotation experiments) and expressed in terms of three Euler angles. Then, the stress tensor components for an unknown multiaxial stress state, expressed in their most general form through the Euler transformation matrix, are retrieved from solving the system of linear equations linking them to the observed spectral shifts of different (polarized) Raman bands. In order to clarify the sub‐surface characteristics of the Raman probe and their effect of the assessment of the stress field stored among the alumina grains, both in‐depth (defocusing) and in‐plane (focal shift) analyses were first carried out to quantitatively determine the three‐dimensional response of the Raman probe. Then, a computer‐aided data restoration procedure was applied to the experimental data, which minimized the averaging effect of the laser probe and brought about the actual magnitude of the residual stress components. From the physics viewpoint, the most relevant feature of the newly proposed stress analysis is that stress tensor components could be individually determined without a direct knowledge of the secular equation pertaining the corundum crystal.