X-ray determination of stresses in alumina scales on high temperature alloys

The resistance of alumina scales to cracking and spalling under the influen ce of growth and thermal stresses is a critical aspect of the environmental resistance of high temperature structural alloys, oxidation resistant coat ings, and bond coats for thermal barrier coatings. However, the relative ma gnitudes of the stresses and their distribution are often not known. In thi s study several X-ray diffraction techniques are being used to measure the strains in alumina scales on a variety of high temperature alloys both duri ng oxidation and after cooling to room temperature. The corresponding stres ses are being calculated using appropriate elastic constants. The results i nclude the observations that: (1) Growth stresses are higher in alumina for med on FeCrAl alloys as compared to that formed on nickel-base alloys, such as NiAl or single crystal superalloys (studies have not yet been performed on NiCrAl or CoCrAl alloys). (2) Yttrium additions to do not result in low er growth stresses in alumina scales on FeCrAl alloys even though the addit ions decrease the amount of lateral scale growth. (3) Growth stresses can b e relaxed by plastic deformation of both the alloy and oxide. The implicati ons of these results with regard to alumina adhesion are discussed.