ABSTRACT
Invariant reactions in ternary systems are associated with the occurrence of four phases co-existing in equilibrium. Rhines has classified four-phase equilibria into three types or classes. This classification is followed in this chapter and can be illustrated by considering the three classes in terms of a liquid phase (L) and three solid solution phases(α, β and γ): ternary eutectic reaction, ternary peritectic reaction, and an intermediate between eutectic and peritectic types. The three classes can be defined in terms of the number of three-phase equilibria above and below the invariant temperature. There are many possible combinations of type of phase giving rise to the three classes of invariant equilibria and reactions: for example, involving a liquid phase (or phases) with solid phases; solid phases only; combinations including a gas phase. The chapter also discusses the solidification sequences for alloys that undergo the three classes of invariant reactions.
