13: Critical. evaluation of constitution of ternary systems C - Fe - X (X - Co, Ni, Cu)

Abstract

The phase equilibria of the ternary systems C-Co-Fe, C-Fe-Ni, and C-Cu-Fe are reviewed. The liquid-solid equilibria of the first two systems are now known quite accurately. In each case the liquidus projection shows three surfaces of primary crystallization, bcc (δFe), fcc (γFe, γCo, or (Ni)), and graphite. These surfaces are defined by two univariant reactions. One is of peritectic type where L + δFe ⇌ γFe and the other is of eutectic type where L ⇌ γFe + C. Solid-phase equilibria are less well documented apart from boundaries for the solubility limits in γFe. Metastable equilibria in which the participating phases are αFe, γFe, and (Fe, Ni)₃C have been determined in the C-Fe-Ni system between 730 and 500°C. The third of the three systems, C-Cu-Fe, is very different. There is a wide liquid immiscibility gap, in which iron-rich and copper-rich liquids (denoted L₁ and L₂, respectively) co-exist. In addition, the behaviour on freezing is very sensitive to rates of cooling and a partial metastable diagram has been characterized as well as the equilibrium diagram. The stable liquidus includes two invariant ternary reactions, U₁ at 1172°C, L₁ + L₂ ⇌ γFe + C and U₂ at 1091°C, L₂ + γFe ⇌ (Cu) + C. In the metastable liquidus the U₁ reaction is replaced by a eutectic invariant reaction, probably at 1134°C, where L₁ ⇌ γFe + (Cu) + M₃C. Knowledge of the solid-phase boundaries is, again, confined largely to solubility limits in γFe. A metastable ternary invariant solid-state reaction(γFe ⇌ αFe + (Cu) + M₃C) is well established at 710°C.