Morphology Evolution of Alumina Scales Formed on NiAl with and without 0.5 at. percent Ag at 1273K

摘要

The transient oxidation behavior of beta-NiAl and NiAl-0.5Ag in 1 atm O_2 at 1273K was studied by thermogravimetry, giving special emphasis to the evolution of the morphology of the oxide scale, especially for what concerns the transformation of transient aluminas into the stable a-Al_2O_3 form. The oxidation kinetics can be described as being composed of three different parabolic stages for NiAl-0.5Ag and of two stages for P-NiAl. Three different oxide morphologies are observed for the two alloys at different oxidation times. The SEM and XRD results show that the transient oxidation can be divided into two main stages, including the initial formation of y-A1_2O_3, which transforms quickly into 0-Al_2O_3, and the subsequent transformation of 0-Al_2O_3 into a-Al_2O_3. A combination of the results by XRD, EDAX and SEM shows that silver promotes the transformation of 6-Al_2O_3 into a-Al_2O_3, which is faster on NiAl-0.5Ag than on p-NiAl.