The specific roles which were played by interface misfit and misorientation dislocations, in creating or annihilating the point defects which mediated diffusion during scale growth, were considered. Anion point defects (vacancies, interstitials) which mediated scale growth by anion diffusion were annihilated or created by the climb of misorientation dislocations or disconnections in the scale at the interface. In the case of scale growth via cation diffusion, cation point defects (vacancies, interstitials) could be annihilated or created by the climb of interfacial misfit or misorientation dislocations in the metal. Because of their necessarily high density in most cases, the climb of misfit dislocations was expected to be favored. As expected from experimental observations of the so-called reactive element effect, large reactive element cations segregated to the metal/scale interface and exerted a pinning force on interface dislocations. This was especially true of misfit dislocations in the metal. An approximate elastic binding energy calculation suggested that a fraction of a monolayer of segregated reactive ions was sufficient to pin the misfit dislocations and thereby retard the oxidation kinetics or change the predominant diffusion mechanism. When the interfacial reaction step blocked the kinetics, a major fraction of the Gibbs energy change was localized across the blocked interface, with a smaller concentration gradient left to drive diffusion in the scale.

On the Role of Interface Dislocations and Ledges as Sources/Sinks for Point Defects. J.P.Hirth, B.Pieraggi, R.A.Rapp: Acta Metallurgica et Materialia, 1995, 43[3], 1065-73