Time-varying concentration-dependent interdiffusion coefficient in the Cu-Ni system

Abstract

Solid-state diffusion has attracted substantial research attention due to its key role in metallic materials processing and performance analysis. The isothermal interdiffusion coefficient is one of the crucial parameters for characterizing this process. Although the interdiffusion coefficient is commonly acknowledged as a function of temperature and concentration (𝐷=𝐹(𝐶,𝑇)), its time dependence cannot be neglected due to the possible effect of diffusion-induced stress (DIS). The aim in the present work is to experimentally verify the occurrence of time effect on the interdiffusion coefficient in a copper-nickel (Cu-Ni) system and then study its most suggestive underlying factor. To avoid the non-trivial errors that arise from the assumption that the initial concentration profile is a “step-function” in space, a new method that utilizes two concentration profiles is used in this work to calculate the concentration-dependent interdiffusion coefficients. The results verify that, in contrast to what is commonly recognized, the interdiffusion coefficient is not only a function of temperature and concentration but can also significantly vary with diffusion time due to the presence of DIS. This can considerably affect the accuracy of theoretical analysis and predictions of the diffusion process in practical applications such as welding, coating, and heat treatments.