OXIDATION AND INTERGRANULAR DISINTEGRATION OF THE ALUMINIDES NIAL AND NBAL3 AND PHASES IN THE SYSTEM NB-NI-AL

The oxidation behavior is very different for an aluminide with a wide homogeneity range such as beta-NiAl than for a line compound such as NbAl3. Oxidation of beta-NiAl at temperatures greater-than-or-equal-to 1273 K leads to a slow-growing alpha-alumina layer. The metal phase beneath the scale remains as beta-NiAl; however, cavity formation is observed. The cavity formation may be favored by sulphur surface segregation. Oxidation of NbAl3 at temperatures greater-than-or-equal-to 1273 K initially leads to alpha-Al2O3, but the Al depletion causes the formation of Nb2Al beneath the oxide layer. Cracking of the Al2O3 layer opens Nb2Al to the atmosphere, which oxidizes rapidly to Nb2O5 and NbAlO4. After consumption of the Nb2Al, a layer of Al2O3 formed again on the NbAl3 phase, but failure of the alumina and the fast growth of the other oxides occur as a repeated process. Thus, NbAl3 exhibited rapid linear oxidation kinetics. Multiphase alloys in the system Nb-Ni-Al generally behave better than NbAl3, and the low oxidation rates of beta-NiAl can be approached. In the temperature range below 1273 K, with a maximum at 1000 K, both NiAl and NbAl3 show the "pest" phenomenon, an intergranular disintegration. Preceding the disintegration, oxygen diffuses into the grain boundaries of the material and Al2O3 is formed at the grain boundaries, beginning from the surface region. NiAl is susceptible only in a very limited range of oxygen pressures and temperatures, whereas NbAl3 is much more susceptible.