" Study of the Thermodynamic and Structural Behaviour of Binary Liquid AIIoys of Tlpe ApBu

Abstract

Due to the numerous commercial applications and anomalous concentration-dependent thermo-physical properties of binary alloys, physicists, chemists, and metallurgists have long been interested in understanding the mixing behavior of two metals that form alloys. Most alloys have different interaction energies between like and unlike atoms. Therefore, the mixing property cannot be described as the concentration average of the properties of constituent metals. The study focuses on the thermodynamic properties of four different binary liquid alloys (Na-Hg, Pb-Mg, Bi-Tl and Cu-Al) of type 𝐴𝜇𝐵𝜈 by assuming NaHg2, PbMg2, BiTl3 and Cu3Al2 as their most stable intermetallic compound respectively using the ‘Quasi-Chemical Approximation’ model given by Bhatia & Singh (1982). The Gibbs free energy of mixing, enthalpy of mixing, entropy of mixing, chemical activity, concentration fluctuation in long wavelength limit and chemical short range order parameter are among the properties that are the subject of study of this thesis. For this purpose, the interaction energy parameters are optimized at the melting temperatures of the aforementioned alloys. The study shows that the free energy of mixing is minimum at about compound forming concentration for all the alloys. Similarly, the theoretical analysis of structural properties indicate that all the preferred liquid alloys have ordering tendency at their melting temperature, which is found more near the vicinity of the compound forming concentration. Next, the viscosity of preferred alloys is studied by using Kaptay model (Kaptay, 2003). The computed viscosities for Na-Hg, Pb-Mg and Bi-Tl liquid alloys are positively deviated whereas it is negatively deviated for Cu-Al liquid alloy from ideal behavior at their melting temperatures. The surface properties of the aforementioned liquid alloys have been studied by the refined Butler model (Kaptay, 2019). Surface tension and surface concentration calculations have been used to analyze the patterns of surface segregations in these liquid binary alloys. The segregation of the alloy component having a lower surface tension is confirmed by theoretical analyses. Finally, the Quasi-Chemical Approximation model has been extended to study the thermo-physical properties of preferred liquid alloys at different temperatures. For this purpose, the interaction energy parameters for each of the alloy system have been computed at different temperatures by making the assumption that the concentrations or mole fractions and the temperature derivative interaction energy parameters are independent on temperature. The study shows that as temperature rises above melting point, the Gibbs free energy and the enthalpy of mixing of alloys gradually become less negative. Further, the ordering tendency of these alloy systems gradually decreases. These findings are supported by increase in concentration fluctuation in long wavelength limit at higher temperatures. The viscosity and the surface tension both decrease with increase in temperature. The liquid alloys thus show the maximum tendency towards complex formation as well as maximum values of viscosity and surface tension at their respective melting temperatures.