Depletion and voids formation in the substrate during high temperature oxidation of Ni-Cr alloys

A numerical model to treat the kinetics of vacancy annihilation at the metal/oxide interface but also in the bulk metal has been implemented. This was done using EKINOX, which is a mesoscopic scale 1D-code that simulates oxide growth kinetics with explicit calculation of vacancy fluxes. Calculations were performed for high temperature Ni-Cr alloys oxidation forming a single chromia scale. The kinetic parameters used to describe the diffusion in the alloy were directly derived from an atomistic model. Our results showed that the Cr depletion profile can be strongly affected by the cold work state of the alloy. In fact, the oversaturation of vacancies is directly linked to the efficiency of the sinks which is proportional to the density of dislocations. The resulting vacancy profile highlights a supersaturation of vacancy within the metal. Based on the classical nucleation theory, the possibility and the rate of void formation are discussed

STRUCTURE OF METAL-OXIDE INTERFACES

Ce travail présente quelques résultats relatifs à la structure des interfaces Ni/NiO et Ti/TiO2 formées lors de l'oxydation thermique des deux métaux étudiés. Ces deux systèmes métal-oxyde ont été choisis car ils correspondent aux deux principaux modes de croissance des couches d'oxyde contrôlés soit par la diffusion cationique (Ni/NiO) soit par la diffusion anionique (Ti/TiO2).This work presents some results on the structure of Ni/NiO and Ti/TiO2 interfaces formed by thermal growth of an oxide scale. These two metal-oxide systems were chosen because they correspond to the main mechanisms of oxide growth controlled by cationic diffusion (Ni/NiO) or anionic diffusion (Ti/TiO2)

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

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

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