VACANCY WIND CONTRIBUTIONS TO INTRINSIC DIFFUSION

In the absence of kinetic cross interactions between diffusing compone nts, intrinsic diffusion can be described by a simple atomic mobility model in which intrinsic diffusion is driven solely by the chemical po tential gradient of each component. For systems in which the diffusion al interactions between components cannot be ignored, the interactions can be related to a vacancy wind effect whereby the net vacancy flux makes an additional contribution to the total intrinsic flux of a comp onent. New relations are derived to describe interdiffusion fluxes in terms of atomic mobilities and a vacancy wind parameter. These relatio ns are applied at selected composition points on the diffusion paths o f gamma-phase U-Pu-Zr diffusion couples investigated at 750 degrees C. The calculated atomic mobilities and the vacancy wind parameter are e mployed to assess the contribution of the vacancy wind effect to the i ntrinsic diffusion of the individual components. The results show that the vacancy wind contribution to intrinsic diffusion can be greater t han the contribution arising from chemical potential gradients. When t hese two contributions are in opposite directions, the net intrinsic d iffusion of a component can be against its own chemical potential grad ient. This paper reports, for the first time, experimental evidence fo r the intrinsic diffusion of U up its own chemical potential gradient.