Abstract
A new diffusional model of disintegration of intermetallics during oxidation was developed. The model is concerned with intermetallics in which the pest phenomenon occurs as a result of intergranular oxidation. An example of such intermetallics is NbAl 3. It was assumed that, in these cases, oxidation leads to an excess of vacancies at grain boundaries. The migration of the excess vacancies to the surface facilitates the diffusional growth of grain boundary cracks. The steady state vacancy concentration ahead of the crack tip was found from the diffusional equation compromising the vacancy sources and sinks. The rate of crack propagation was found through the vacancy flux to the crack tip. Two different steady state regimes of crack growth (fast and slow) were established, and the temperature of transition from one regime to the other was determined. The fast regime was related to the disintegration of intermetallics containing a substantial number of structural vacancies. The slow regime corresponds to the case when no disintegration occurs. The rate of diffusion crack propagation is only slighly affected by the gradients of the internal stresses and of the equilibrium vacancy concentration in the oxidation zone. The decisive role of the vacancy excess generation in the fast crack diffusion propagation was revealed. Activation energies of the process were estimated.
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