Thermodynamics of the α- and β-phases equilibria and ordering in CuZn system

For both the solid solutions α and β, composed of Cu and Zn, approximated by the regular solutions, the interaction parameters E^α and E^β of components in the a and β phases are calculated with the experimental activities. By the application of lattice stability parameters ΔG_Cu^β→α and Δ_Zn^β→α of Cu and Zn, which are obtained from the phase diagram of CuZn system in this paper, a general formula for ΔG^β→α is derived. On the basis of Inden formula for the change of free energy ΔG^β→β′ in the β→β′ ordering transition, it is proposed, through discussing the critical temperature T_c of ordering and ordering degree as a function of temperature, that the theoretical maximum ordering degree can not be obtained for an alloy with given composition. The maximum ordering degree attained is approximately independent of composition for alloys with X_Zn = 0.35–0.65, so an approximate equation for ordering degree as a function of temperature is suggested. This equation is used to calculate the $\text{α}\text{(α + β′)}$ and $\text{β′}\text{(α + β′)}$ phase boundaries of CuZn system, and the results are in good agreement with the phase diagram.