Limitation of using infinite-couple interdiffusion coefficient for predicting diffusion in finite diffusion couple

The concentration-dependent interdiffusion coefficient, D(C), is a key parameter governing atomic diffusion-based processes and is often assumed to be constant under isothermal conditions. However, this assumption neglects the influence of diffusion-induced stress (DIS), which can modify D(C). Time-varying surface concentration C s , typical in finite thin-layer systems and practical processes such as homogenization, can strongly affect the development and relaxation of DIS. In this study, using the Cu–CuAl diffusion couple, a comparison of experimental concentration profiles from infinite and finite diffusion couples, based on a numerical criterion from Fick's diffusion equation, reveals that the operative D(C) differs between the two types of couples, contrary to common assumptions. Moreover, D(C) functions obtained from infinite couples fail to correctly predict concentration profiles in finite couples. A DIS theoretical model calculation supports these observations by showing that the pressure gradient, ∂P/∂C, which influences D(C), can differ in a finite couple with time-varying C s compared to an infinite couple with time-independent C s . These results underscore the need for experimental validation, as done in the present work, since D(C) derived from infinite couples cannot always be simply assumed to apply to finite systems for accurate prediction and analysis of diffusion effects.