The role of interface dislocations and ledges as sources/sinks for point defects in scaling reactions

Abstract

The specific roles of interface misfit and misorientation dislocations, and of disconnections, in creating or annihilating the point defects supporting diffusion during scale growth are considered. Anion point defects (vacancies/interstitials) supporting scale growth by anion diffusion are annihilated/created by the climb of misorientation dislocations or disconnections in the scale at the interface. For scale growth by cation diffusion, cation point defects (vacancies/interstitials) can be annihilated/created by the climb of interfacial misfit or misorientation dislocations in the metal. Because of their necessarily high density, in most cases, the dominant climb of misfit dislocation would be favored. Consistent with experimental observations of the “reactive element effect”, large reactive element cations segregated to the metal/scale interface provide a pinning force on interface dislocations, especially on the misfit dislocations in the metal. An approximate elastic binding energy calculation suggests that a fraction of a monolayer of segregated reactive ions is adequate to pin the misfit dislocations and thereby retard the oxidation kinetics, or change the dominant diffusion mechanism. When the interfacial reaction step blocks the kinetics, a dominant fraction of the Gibbs energy change is localized across the blocked interface with a smaller concentration gradient to drive diffusion in the scale.