Internal oxidation

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Abstract
In the classical models of internal oxidation of binary alloys comprising a more noble solvent metal and a less noble solute metal, it has been assumed that the process is controlled by diffusion of oxygen through the bulk metal lattice. However, the oxidation kinetics observed have not always been consistent with this hypothesis. In this paper, modifications to the classical theories are considered and, in particular, the importance of the morphology of the internal oxide precipitates is discussed. Internal oxidation of the Ni–Al, Ni–V, and Ni–Cr systems in the absence of an external scale is used to demonstrate that, depending on the population density of particles, enhanced diffusion of oxygen along the incoherent internal oxide/alloy interfaces and blocking effects by the particles themselves can influence considerably the rates of penetration of internal oxide. Consideration is also given to the influence of stresses generated at the internal oxide/alloy interface as the oxide particles form with an increase in volume. Under certain conditions, the rate determining step may be deformation of the matrix to accommodate this volume change, rather than oxygen diffusion through the internal oxidation zone. It has been suggested in recent research that transport of .the more noble solvent metal to the strain free surface may occur by a pipe diffusion controlled creep process and this step may become rate controlling at high temperatures and high strain rates. MST/951