Thermodynamic Properties of the Glass-Forming Ternary (Fe, Co, Ni, Cu)–Ti–Zr Liquid Alloys II. Temperature–Concentration Dependence of Thermodynamic Mixing Functions and Chemical Ordering in Liquid Alloys

Abstract

This paper considers the dependence of the thermodynamic properties of glass-forming liquid alloys of the (Fe, Co, Ni, Cu)–Ti–Zr systems on composition and temperature. The associate solution model (ASM) was used as a calculation tool. The results of the calculations correspond to the experimental data on the integral mixing enthalpy, presented in the first part of the work, and reveal the regularities of changes in other thermodynamic functions and the features of interaction between components in these liquid alloys. It was established that the excess thermodynamic mixing functions in each system have negative values, which are determined by pair interactions between Fe, Co, Ni, and Cu as electron acceptors and Ti and Zr as electron donors. The trend of changes in the minimum values of excess thermodynamic mixing functions of the systems shows an increase in their absolute values along the 3d-series from iron to nickel and a significant decrease for copper, which corresponds to a change in the acceptor capacity of metals along the transition series. The temperature dependence of the thermodynamic mixing functions consists in an increase in negative deviations from ideality and an increase in the intensity of interaction between components with a decrease in temperature. The formation of glass-forming liquid alloys from pure metals is accompanied by an increase in the thermodynamic stability of the liquid phase, which is reflected in negative values of the Gibbs mixing energy. In the range of 800–1873 K, the Δ_mG function of liquid equiatomic alloys of the systems considered shows values at the level of –20...–35 kJ/mol. Within the framework of ASM, using the total mole fraction of associates as a quantitative estimate of the degree of short-range chemical order, it is shown that liquid alloys of the Me–Ti–Zr system are characterized by significant chemical ordering, which increases with decreasing temperature. Using the empirical rule, the experimentally known compositions of amorphous alloys for the Cu–Ti–Zr and Ni–Ti–Zr systems were interpreted and the composition regions of liquid alloy amorphization were predicted for the Fe–Ti–Zr and Co–Ti–Zr systems.