Abstract
The alumina scales on a variety of high-temperature alloys are found to fluoresce when illuminated with light having a frequency greater than 18,000 cm−1. The fluorescence exhibits two narrow lines characteristic of chromium-doped alpha-aluminum oxide. The frequency shift of the two lines from the room-temperature, stress-free values of 14,402 cm−1 (1.786 eV) and 14432 cm−1 (1.789 eV) provides a noncontact measure of the stress in the alumina scales using the piezospectroscopic effect. In addition, the broadening of the lines is a measure of the stress gradient in the scale. The physical basis for the fluorescence technique is described together with its implementation for highspatial-resolution (∼2 μm) measurements. As illustration, room-temperature measurements of the residual stress in scales formed at 1100°C on single-crystal NiAl, polycrystalline Ni3Al, two Fe−Ni−Cr−Al alloys, and two Ni−Al base superalloys are presented.
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