OXIDATION OF BETA-NIAL, UNDOPED AND DOPED WITH CE, Y, HF

The high temperature oxidation of beta-NiAl undoped and doped with Ce. Y and Hf was studied in situ by thermogravimetry in He with p(O-2) = 5.10(-6) bar at 1000 degrees C and by high temperature X-ray diffracti on at 950 and 1000 degrees C in air. After the in situ experiments the samples were analysed by optical microscopy and SEM with EDX. It was observed by thermogravimetry that the weight gain for beta-NiAl+Hf is lower than for undoped beta-NiAl, whereas the weight gain for beta-NiA l+Y was similar to that for beta-NiAl. For beta-NiAl+Ce an enhanced in crease of mass gain was observed. The in situ X-ray-experiments show t hat in the first hours of the oxidation process the metastable Theta-A l2O3 is formed, which transforms to the protective alpha-Al2O3 after d ifferent periods of time, depending on the dopant element and the temp erature. The analysis of the samples shows that the ternary phases alo ng the grain boundaries were oxidized, whereas the ternary phase preci pitates within the grains remained unoxidized. The extend of the grain boundary oxidation increases from beta-NiAl+Hf and beta-NiAl+Y to bet a-NiAl+Ce.