Ordering and Segregation in Liquid Alloys

Abstract

The design and fabrication of new alloys is usually approached by a method that combines theoretical analysis and experimental observation (composition−structure− property), but in many cases, due to the experimental difficulties related to high temperatures, the theoretically predicted property values are of key importance. This is particularly relevant for all industrial processes, such as casting, joining, crystal growth, etc. that involves the presence of the liquid phase, i.e. liquid metals and alloys. Alloying processes have been evolved as one of robust tool to achieve desired materials with required characteristics. The thermal treatments of materials, chemical compositions and operating parameters (temperature, pressure and working atmosphere) overrule the microstructure of an alloy. The in−depth knowledge of thermodynamics, kinetics and thus, the energetic of the alloy prototypic process has great significance in metallurgical science and engineering. In view of the aforementioned, we have studied and explained the mixing behaviour of two Al−based (Al−Fe at 1873 K and Al−Mg at 1073 K) as well as two Bi−based (Bi−Tl at 750 K and In−Bi at 900 K) liquid binary alloys with the help of theoretical modeling. The thermodynamic properties, such as free energy of mixing (G ), activity (a), enthalpy of mixing (H

) and entropy of mixing (S

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), structural properties, such as concentration fluctuation in long wave length limit (S

(0)), chemical short range order parameter (α ) and ratio of diffusion coefficients (D ) for above mentioned liquid alloys at chosen temperatures have been analyzed in the fame work of regular associated solution model. For this purpose, we have determined the model parameters which are the interaction energy parameters (ω , , ω ,

and ω /D ,

), equilibrium constant (k), mole fractions of complex (x

) and free monomers (x

and x ). The compositional contribution from the heat associated with the formation of the complex and the heat of mixing of species to

the net enthalpy change has been studied for each system. The comparative studies of the thermodynamic properties of these systems reveal that the Al−Fe being the most interacting system followed by the Bi−Tl, the Al−Mg and the In−Bi, in which the

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ABSTRACT interactions are the weakest. Theoretical investigations of structural properties show that all the preferred liquid alloys show complete ordering nature at least close to their respective melting temperatures. The interaction energy parameters are found to be temperature dependent. The surface properties of the chosen liquid alloys have been explained with the help of Renovated Butler model. The trends of surface segregations in these liquid binary alloys have been studied by computing surface tension (σ) and surface concentrations (x

and x

). Theoretical investigations confirm the segregation of the alloy component having a lower surface tension, i.e. the extent of segregation of Bi−atoms at the surface layer, which is more pronounced in In−Bi melts with respect to that of liquid Bi−Tl alloys. Moreover, in case of two Al−based melts, the Al−atoms segregate on the Al−Fe surface phase whereas they remain in the bulk region of Al−Mg. Additionally, the regular associated solution model has been extended to study and predict the thermodynamic and structural properties of concerned liquid alloys at different temperatures. For this purpose, ω , , ω , and ω for each of the system have been computed at different temperatures keeping x

, , x

, x and k invariant. The modeling equations obtained by the polynomial fitting of different orders along with the values of

parameters to forecast these properties have also been included in this work. Theoretical computations indicate that the excess Gibbs free energy of mixing (G ) of the alloys gradually decreases with an increase in temperature above melting temperatures. Accordingly, at higher temperatures, the ordering or the compound formation tendencies of these alloy systems gradually decrease and sometimes show segregating nature. These findings are further supported by decrease in deviations between the computed and observed values of S (0) at higher temperatures. The liquid alloys thus show the maximum tendency towards complex formation at their respective melting temperatures, however, these tendencies significantly decreases at elevated temperatures. Thermodynamic properties have been then correlated with the Renovated Butler model to explain the surface properties at different temperatures. The computed values for the surface concentrations of the segregating components of the liquid alloys approach respective ideal values at higher temperatures. Similarly, the surface tension of metallic melts metals and alloys, decrease at elevated temperatures.