Concept of post-eutectic composition for undercooled alloys

The concept of post-eutectic composition at a fixed undercooling is introduced as the most probable composition in zones of coupled growth. This composition corresponds to the minimum Gibbs free energy release rate in the steady state regime, aligning with Prigogine principle of minimum entropy production for steady states. Geometrically, this is represented by the parallel tangents rule, a generalization of the common tangent rule for transitioning from equilibrium to steady state conditions.