Zero-Flux Plane Kinetics at Multicomponent Interfaces

Abstract

Zero flux planes (ZFP's) are the locations in a diffusion couple where fluxes of individual components vanish. The conditions required for developing ZFP's between interdiffusing multicomponent alloys are now well understood through the works of Dayananda and Morral. In this paper we analyze the kinetics of multicomponent diffusion near ZFP's. In contrast to the usual mixing that occurs between end-member alloys in a diffusion couple, where the average component compositions are approached through “global” exchange of atoms, the presence of a stationary ZFP requires instead that mixing of the blocked component occurs through the release of a pair of coupled diffusion waves. In classical (thick) couples these waves spread symmetrically away from the Matano plane. The wave pair consists of a “depletion” wave that reduces the blocked component concentration in the component-rich alloy, and a conjugate “repletion” wave that increases the concentration in the adjacent component-poor alloy. Curiously, each of these waves establishes the average (equilibrium) concentration unilaterally on either side of the Matano plane. A stationary ZFP precludes net transport of one component across the Matano plane—a circumstance that has practical and theoretically interesting implications for the design of stable multicomponent films and coatings that must resist diffusive loss of a component.