6: Critical evaluation of constitution of cobalt–chromium–iron and cobalt–iron–nickel systems

The phase equilibria of the systems Co–Cr–Fe and Co–Fe–Ni are reviewed in the light of recent experimental studies. Liquidus and solidus projections are given for each system. Each has one univariant reaction line defining two surfaces of primary crystallization, fcc and bcc. Detailed solid-phase studies of the Co–Cr–Fe system are presented as a series of isotherms from 1300° to 600°C and show, as the temperature is reduced, the development of the simple equilibrium between fcc and bcc into a more complex constitution involving the σ-phase. In the Co-Fe-Ni system there is no a-phase and isotherms between 800° and 400°C chiefly represent fcc/bcc boundaries. No ternary phases are found in either of the two systems. A brief note is made of metastable behaviour and of thermodynamic assessments of each system. In conclusion, some consideration is given to the parallels observable in the constitution of the two systems and that of Cr–Fe–Ni