The two diffusing components are not always completely soluble within each other and many diffusion pairs present components that fall into two phase regions. The composition of the two phases will be constant if the pair is truly binary. If the components were constant, there won’t be composition gradients and the diffusion can’t happen in two phase regions. Consequently, the particles of the two existing phases won’t grow via diffusion as a diffusion pair. Diffusion will take place only in the monophase regions because composition gradients happen there, and the composition can vary between the limits of solubility. Figures 2.2.16 and 2.2.17 show examples of diffusion pairs with limited solubility, formed initially by pure solids.
