Thermodynamics of alloys and phase equilibria in the copper-iron system

Together with previous values at 1873 K, new measurements of the enthalpies of formation of liquid Cu-Fe alloys, carried out by high-temperature calorimetry at 1673 K, have revealed a temperature dependence of DeltaH, corresponding to a negative excess heat capacity of mixing. Our calorimetric results were combined with critically selected values of the enthalpies of alloying and activities of components in liquid and solid solutions, as well as parameters of stable and metastable phase transitions, to perform a thermodynamic evaluation of the system. The latter was carried out in the spirit of the CALPHAD approach, using Thermo-Calc software. The thermodynamic model generates a self-consistent description of all thermodynamic properties and phase equilibria, including metastable solidification, in close agreement with reliable experimental data. In particular, a very satisfactory representation of stable (L+delta)/gamma, (L + gamma)/epsilon, gamma/ (epsilon + alpha) and metastable (L-1/L-2), (L-1 + L-2)/epsilon phase boundaries has been achieved. The occurrence of a monotectic reaction L-2 <----> L-1 + delta is suggested in the region of the metastable precipitation of delta-phase. A comprehensive thermodynamic model of the Cu-Fe system is used to explain the widening of the concentration limits of formation of supersaturated bcc and fcc solutions prepared by highly nonequilibrium methods of synthesis.