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. The ternary eutectic reaction is represented by a plane in the space model; the composition of each of the four phases participating in the reaction is fixed, and the liquid composition point lies within the triangle formed by the compositions of the three solid phases. To illustrate the nature of the invariant reaction it is helpful to calculate the percentages of the phases present in a typical alloy just above and below the peritectic temperature. The ternary eutectic reaction involves three three-phase regions that approach the eutectic plane from higher temperatures, and one three-phase region existing below the plane. With decreasing temperature the positions of the tie-triangles change as the compositional changes occur in the phases involved in the reaction.
