Intergranular oxidation and pesting of intermetallic compounds

The intermetallic compounds NbAl3 and NiAl are susceptible to intergranular oxidation in a certain range of intermediate temperatures 700-1000 degrees C and lower oxygen pressures 10(-2) bar < pO(2) < 10(-20) bar. As was shown by AES analyses, oxygen can penetrate into the grain boundaries of these aluminides, in spite of the fact that oxygen is virtually insoluble in the metal matrix. Models were developed to explain the oxygen penetration, by Al-depletion and formation of voids and fissures at the grain boundaries, caused by the selective oxidation of Al at the surface. After the oxygen penetration, formation of Al2O3 also starts in the interior of the aluminides. In the case of NbAl3, this leads to the ''pest'' phenomena, i.e. a rapid disintegration of the material into very fine oxidized particles. In the case of NiAl general internal oxidation may follow the intergranular attack. The mechanism of oxygen penetration into grain boundaries after selective oxidation and depletion of one component and possible void and fissure formation at grain boundaries appears to be important not only in the oxidation of intermetallics but also for other alloys.