Thermodynamic properties of melts and phase equilibria in the copper-zirconium system

The vapor composition over and thermodynamic properties of Cu-Zr melts (5.1-85.0 at % Zr) are studied by Knudsen cell mass spectrometry between 1191 and 1823 K. The data set obtained comprises more than 1100 activity values for various compositions and temperatures. The thermodynamic behavior of Cu-Zr melts is described in terms of the associated-solution approach with an accuracy no worse than the experimental accuracy. The melts are shown to contain two molecular species: CuZr and Cu2Zr. The contributions of different types of chemical bonding to the Gibbs energy and enthalpy of formation of Cu-Zr melts are asymmetrical and shifted from the equiatomic composition in opposite directions: the extremum of covalent bonding is shifted to the Cu-rich side, while metallic bonding is more significant in Zr-rich alloys. The rapid temperature variation of covalent bonding leads to a large excess heat capacity C-p(E) of the melts and a negative excess entropy Delta(f)S(E), which rapidly drops with decreasing temperature. It is shown that not only C-p(E) and Delta(f)S(E) but also their temperature variations are governed by the parameters of association reactions and depend more strongly on the entropy than on the enthalpy of complex formation. This indicates that, in the general case, the glass-forming capabilities of melts are independent of the interparticle interaction and accounts for the pronounced tendency of Cu-Zr melts toward amorphization.