When an inhomogeneous binary solid solution or alloy is annealed at constant temperature and constant pressure, it will become homogeneous to lower the free energy. Conversely, when a homogeneous binary alloy is annealed at constant pressure but under a temperature gradient, i.e. one end of it is hotter than the other, the opposite will happen: the alloy will become inhomogeneous, and the free energy increases. This de-alloying phenomenon is called the Soret effect, as mentioned in Chapter 10. It is due to thermomigration or mass migration driven by a temperature gradient [1–3]. Since the inhomogeneous alloy has higher free energy than the homogeneous alloy, thermomigration is an energetic process which transforms a phase from a low-energy to a high-energy state. It is unlike a conventional phase transformation which occurs by lowering Gibbs free energy.
In thermodynamics, under homogeneous external conditions defined by a constant temperature and constant pressure (for example, if T is fixed at 100 °C and p is fixed at atmospheric pressure), a thermodynamic system will minimize its Gibbs free energy, and it will move toward the equilibrium condition at the given T and p. Both enthalpy and entropy are state functions, so the Gibbs free energy of the equilibrium state is defined when T and p are given. On the other hand, if the external conditions are inhomogeneous, for example, having different temperatures at the two ends of a sample in thermomigration, the equilibrium state of minimum Gibbs free energy is unattainable.