Quantitative Mapping of Stress Heterogeneity in Polycrystalline Alumina using Hyperspectral Fluorescence Microscopy

The microstructurally-induced heterogeneous stress fields arising in a series of Cr-doped polycrystalline alumina materials are mapped with sub-micrometer sub-grain size resolution using fluorescence microscopy. Analysis of the hyperspectral data sets generated during imaging enabled both the amplitude and position of the characteristic Cr R1 fluorescence peak to be determined at every pixel in an image. The peak amplitude information was used to segment the images into individual grains and grain boundary regions. The peak position information, in conjunction with measurements on single-crystal controls, was used to quantify overall stress distributions in the materials and provide stress scales for maps. The combined information enabled spatial variations in the stress fields in crystallographic axes to be mapped and compared directly with microstructural features such as grains and grain boundaries. The mean c-axis stresses in these materials were approximately 200 MPa with stress distribution widths of about 70 MPa, both increasing with average grain size. Greatest variations in stress were observed at grain junctions; no trend in the stress for individual grains with grain size was observed.