9. Multiphase precipitation and metastability trajectories

From a thermodynamic point of view, the number of phases at equilibrium, under constant pressure and temperature conditions, is determined by the variance rule: two in a binary alloy, with the exception of eutectic/eutectoid points (i.e. a single precipitate in a matrix), three in a ternary, and so on. However, the formation of metastable precipitates can facilitate the germination stage. In this case, a wide variety of phases can be present simultaneously, even in alloys with a fairly simple chemical composition.