Oxidation of beta -NiAl, undoped and doped with cerium, yttrium, hafnium

The high temperature oxidation of intermetallic beta -NiAl, undoped and doped with cerium, yttrium, and hafnium was studied in situ by thermogravimetry in helium with p(O sub 2 )=5bullet10 exp -6 bar at 1000 deg C and by high temperature x-ray diffraction at 950 and 1000 deg C in air. After the in situ experiments, the samples were analyzed 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 -NiAl+Y was similar to that for beta -NiAl. For beta -NiAl+Ce, an enhanced increase of mass gain was observed. The in situ x-ray-experiments show that in the first hours of the oxidation process, the metastable Theta -Al sub 2 O sub 3 is formed, which transforms to the protective alpha -Al sub 2 O sub 3 after different periods of time, depending on the dopant element and the temperature. The analysis of the samples shows that the ternary phases along the grain boundaries were oxidized, whereas the ternary phase precipitates within the grains remained unoxidized. The extent of the grain boundary oxidation increases from beta -NiAl+Hf and beta -NiAl+Y to beta -NiAl+Ce.