Interfacial analyses for elucidation of the intergranular disintegration upon oxidation of intermetallic phases

Abstract

In a range of intermediate temperatures from 600 to 1000°C and in the presence of oxygen, numerous intermetallic phases show the phenomenon ‘pest’, an intergranular disintegration into small pieces. Thermo-gravimetric investigations and annealings in quartz ampoules have been performed, for annealing the oxygen pressures were established in the range 10−30 to 10−10 bar O2 using metal/oxide mixtures. The specimens after annealing were fractured in UHV and the intergranular fracture faces were analyzed by AES. The Auger peaks of Al are markedly different for the intermetallic phase and for Al2O3, therefore it can be distinguished if oxygen has diffused into grain boundaries and not yet reacted or if Al2O3 was formed. The fracture face of NbAl3 shows oxide precipitates near the surface and oxygen which had penetrated into the interior. Also in NiAl, Al2O3 was detected as well as oxygen penetrated into the grain boundaries. The ‘pest’ obviously is a complex interplay of the processes: 1) penetration of oxygen through the outer oxide layer on the surface into grain boundaries of the intermetallic phase; 2) inward diffusion of oxygen along the grain boundaries into the interior of the intermetallic phase; 3) precipitation of Al2O3, beginning near the surface or (at low oxygen pressure) in the whole cross section, and cracking of the materials by the growth of Al2O3 into grain boundaries and cracks. Depending on the range of oxygen pressure different steps can be rate determining.