Summary

Solid solutions consist of a mixture of components that are completely miscible with one another, and hence are a single solid phase. This Teaching and Learning Package has discussed the concept of solid solutions in two-component systems. By taking an atomistic approach, the processes of exsolution and ordering can be described and even modelled (using a Monte Carlo simulation). The interaction parameter, a quantitative representation of the tendency for bonding in the mixture, has been shown to be a critical factor in determining the equilibrium state of the mixture. In other words, the miscibility of one component with the other is dependent on the underlying thermodynamics.

Relating this TLP to the Phase Diagrams and Solidification TLP, the presence of solid solutions in phase diagrams has been discussed. Achieving a solid solution has been seen to be advantageous in terms of strengthening and hardening (via precipitates). The scale of the precipitates possible from a supersaturated solid solution can be significantly smaller than that possible with solidification from a liquid.

Owing to the difficulty in demonstrating phase separation in solids, a demonstration involving a mixture of cyclohexane and aniline (both in liquid form) has been used instead. The ability of this system to scatter light during the change between one and two phases allows the transition to be easily identified.