A novel explanation of the "reactive element effect" in alloy oxidation

The precise measurements of oxidation kinetics for an undoped Fe-25Cr alloy by Hussey et al. [1] have been analyzed to determine values for both the parabolic rate constant for diffusion and the linear rate constant for the interfacial step. The greatly reduced scaling kinetics for Fe-25Cr in response to a CeO2 surface doping are consistent with a blocking of the interfacial step for creation of cation interstitials at the metal/scale interface. For sufficient doping (Ce ion segregation) at the metal/scale interface, cation transport is replaced by anion diffusion over vacancies. The poisoning of the interfacial cationic reaction step results from pinning of the misfit edge dislocations whose climb action otherwise creates interstitial cations. But the creation of the anion vacancies at the metal/scale interface is not blocked by the segregated reactive element ions. This model provides a consistent interpretation for the commonly observed REE consequences in the growth of chromia scales on alloys, especially the changes in growth direction and scale growth kinetics

Interface migration and the Kirkendall effect in diffusion-driven phase transformations

An analysis is presented for the mass and vacancy fluxes within the reacting phases and at the interface for a diffusion-driven phase transformation between disordered binary solid solutions. As decided by the intrinsic diffusion coefficients and the initial and interface concentrations for each phase, vacancies must either be created or annihilated in the bulk phases but also at their interface. A general analysis is formulated for the growth or recession of a phase in terms of the Kirkendall reference frame and this result is compared to the analysis based on the reference frame centered on the number of moles. Specific example calculations demonstrate the various possiblities for vacancy creation/annihilation and for the growth/recession of phases. The growth rate expressed in the Kirkendall reference frame can be found by taking into account the associated vacancy exchanges for the interface reaction

Fluxes and interface migration in diffusion driven phase transformations.

An analysis is presented for the mass and vacancy fluxes within the reacting phases and at the interface for a diffusion-driven phase transformation between disordered binary solid solutions. As decided by the intrinsic diffusion coefficients and the initial and interface concentrations for each phase, vacancies must either be created or annihilated in the bulk phases but also at their interface. A general analysis is formulated for the growth or recession of a phase in terms of the Kirkendall reference frame, and this result is compared to the analysis based on the reference frame centered on the number of moles. Specific example calculations demonstrate the various possibilities for vacancy creation/annihilation and for the growth/recession of phases

Reaction and diffusion in multiphase systems : phenomenology and frames

The Kirkendall effect is well-known and documented for the diffusion in a single-phased substitutional solid soln. Also in multiphased systems, much exptl. evidences for the occurrence of the Kirkendall effect can also be found in the literature. However, the relevant observations are not always analyzed in terms of differing vacancy fluxes in the differing parts of such diffusion couple. The anal. recently proposed by the authors shows that the interface must be able both to create or annihilate the point defects involved in the diffusion process within the two adjoining phases. The implication of this model on the relative motion of the phase lattice with respect to the vol. center (Matano frame) and to the interface will be described and discussed. The specific case of the growth of an intermediate phase in binary systems is also considered. Some specific aspects for the case of ternary systems are discussed

Interfacial dynamics in diffusion-driven phase transformations

The migration of ledges in a semicoherent a/ß interface is considered to participate in solid-state transformations driven by diffusion. The advance of the ledge and/or the progress of the transformation can require the climb of misfit dislocations both in the ledge and in its path. The creation or annihilation of vacancies required for the transformation and the legde advance is provided by a combination of three vacancy sources or sinks: (a) the net vacancy flux to/from the interface resulting from the difference in lattice plane shift (Kirkendall effect) within the two contacting phases, (b) the climb of misfit dislocations from the interface into the bulk of the a and ß phases, and (c) the climb of misorientation dislocations within the interface. Thus, the dynamic action of the interface during the phase transformation would include: (i) climb of misfit dislocations out of the interface, with ensuing dissociation into glissile dislocations which resupply the interface by return glide, and (ii) climb of misorientation dislocations in the interface necessitating a continuing arrival of such dislocations from sources in the bulk or in the interface