Behaviour of vacancies during oxidation

Abstract

The behaviour of point defects during oxidation has been re-examined in terms of the process of vacancy injection, the oxide-induced stress, and the oxygen-carbon gas reaction. In zinc and magnesium, the growth of dislocation loops as a function of oxygen pressure for specimens of different orientation is incompatible with the concept of oxidation-induced stress, but is consistent with vacancy injection. In nickel containing carbon, the amount of voiding on oxidation is prolific and is associated with the production of CO₂. However, the measurement of oxide-induced tensile stress and the metallographic examination of oxidized single crystals both support a process of vacancy injection rather than vacancy generation by oxide-induced stress. In decarburized nickel, the oxidation rate increases dramatically compared with carbon-containing nickel, because the vacancies produced by oxidation are not required to relieve the stresses set up by the gas pressure arising from the carbon-oxygen gas reaction and are annihilated at the oxide/metal interface giving rise to spalling. In the absence of an oxide film, the source of vacancies is dislocations. The role of induced stress, whatever its source, is simply to aid or inhibit the precipitation of vacancies as appropriate.

MST/180