Determination of the Eutectic to Peritectic Fold Transition in the Cu(Ni)–Fe–S System by Directional Crystallization of Melts

This paper is concerned with theoretical analysis of the quasi-equilibrium directional crystallization of three-component melts. How the phase composition changes at each site of the sample and how the phase compositions change for various types of phase reactions are considered. It is shown that the eutectic reaction can change to a peritectic reaction as temperature lowers during directional crystallization. A 29.96 Fe + 21.55 Cu + 2.01 Ni + 46.49 S (at %) melt was directionally crystallized. Since nickel is present in the ingot in the form of impurities dissolved in the phases of the Cu–Fe–S system, the behavior of the melt belonging to this three-component system could be involved in data interpretation. As a result, a sample has been obtained with three zones changing one another: single-phase Fe_zS_{1 ± δ} (Poss), a Poss + (Cu,Fe)_{1 + x}S (Iss) two-phase eutectic mixture, and then single-phase Iss. The average composition of the solid phase changes from one discrete value to another in going to the next zone, while the melt and solid solutions in the neighboring zones have their compositions changing continuously. These results are consistent with theoretical ideas.