Measurement of the stress in oxide scales formed by oxidation of alumina-forming alloys

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 high-spatial-resolution (similar to 2 mu m) measurements. As illustration, room-temperature measurements of the residual stress in scales formed at 1100 degrees C on single-crystal NiAl, polycrystalline Ni3Al, two Fe-Ni-Cr-Al alloys, and two Ni-Al base superalloys are presented.