ACCELERATED OXIDATION, INTERNAL OXIDATION, INTERGRANULAR OXIDATION, AND PESTING OF INTERMETALLIC COMPOUNDS

The compounds MoSi2, NiAl, and NbAl3 all form protective oxide films, particularly at high temperatures where the diffusion of Si or Al is more rapid and, for the case of MoSi2, the transient oxides evaporate. However, at low temperatures, all three can undergo accelerated oxidation. The mechanisms of degradation are unique to the particular compound although there are some similarities. The accelerated oxidation of MoSi2, occurs at temperatures below 600 degrees C by the rapid growth of Mo oxides which prevent development of a continuous silica film. Internal or intergranular oxidation does not occur. rf the specimen contains cracks or pores, the rapid oxidation in these defects leads to fracture of the specimen or ''pesting.'' The accelerated oxidation of NiAl occurs at temperatures below 1000 degrees C at reduced oxygen partial pressures as the result of internal oxidation and rapid intergranular oxidation. The intergranular oxidation does not lead to pesting. Special circumstances are required for the accelerated oxidation of NiAl as it does not appear to occur in flowing gases unless sulfur is present. The accelerated oxidation of NbAl3 also occurs at temperatures less than 1000 degrees C and at reduced oxygen partial pressures and takes the form of intergranular oxidation of Al. The intergranular oxidation results in pesting of NbAl3. The phenomena of accelerated oxidation, internal oxidation, intergranular oxidation, and pesting have not been investigated in detail for most other intermetallic compounds but one or more of these phenomena seems to afflict mast aluminides and silicides.