Interdiffusion in Oxidized Multicomponent Alloys

The Wagner model of oxidation of binary Ni-Pt type alloy and the generalized Darken model of interdiffusion (GDM) are combined to describe the selective oxidation of multicomponent alloys. The model allows a study of the evolution of the composition of the oxidized alloy and to approximate the rate of its oxidation. We show the usefulness of the presented model in estimating the changes of density of elements at the alloy*scale interface. The simulations of the interdiffusion in selectively sulphidized ternary alloy (Fe-18Cr-9Ni in wt. %) show the qualitative agreement with the experimental results. The results indicate the potential of the GDM in the modeling of the oxidation, the high temperature surface treatments and in general the simulation of interdiffusion driven by the reactions at interfaces. THEORY The first model of selective oxidation of an alloy was proposed by Wagner for a binary Ni-Pt alloy [1]. The Wagner model is based on the postulate that the reaction rate is controlled by reactive diffusion in the growing layer and by interdiffusion in the oxidized alloy, Fig. 1. The oxidation of pure metal (A) can be written as: When the Wagner assumptions are fulfilled this process is governed by the parabolic rate law: where the parabolic rate constant, k & , is a following function of oxidant partial pressure: where denote the equilibrium oxidant partial pressure in gas atmosphere and at the AX*(s)|alloy interface respectively, are self diffusion coefficients of metal and of the oxidant in AX*. When diffusivity of the oxidant is negligible and single type of cationic defects dominates in metal sublattice then, the parabolic rate constant is expressed by simple function [1]: where (is positive in a case of negatively charged defects), z denotes their effective charge.