R. N. Singh

Compound formation in Sn-based liquid alloys

Abstract The phenomenon of compound formation in Sn-based liquid alloys (MgSn and CuSn) has been discussed through the study of concentration-dependent thermodynamic and microscopic functions such as activity, excess free energy of mixing GMXS, concentration fluctuations in the long-wavelength limit Scc(0) and chemical short-range order (CSRO) parameters. The expressions for these functions have been obtained by using statistical mechanical theory based on compound formation. All the above-mentioned properties of mixing of MgSn and CuSn liquid alloys have been found to be asymmetric around equiatomic compositions. The larger negative values of GMXS and maximum deviation of Scc(0) from ideal values (Scc(0,id) at CMg = 0.66 in MgSn and at CCu = 0.75 in CuSn suggest that Mg2Sn and Cu3Sn compounds are formed in MgSn and CuSn systems, respectively. Our study of CSRO reveals that maximum order exists at compound-forming concentrations, i.e. CMg = 0.66 in MgSn and CCu = 0.75 in CuSn. It is also observed that the tendency for compound formation in MgSn is greater than that of the CuSn system, although the interactions in both systems are not very strong.

Stability of HgNa and HgK liquid alloys

Abstract The alloying behaviour of HgNa and HgK liquid alloys has been investigated by assuming that the chemical complexes Hg2Na and Hg2K exist near the melting point on the molten amalgams. This has been used to obtain the concentration dependence of the free energy of mixing, activity, long-wavelength fluctuations and the volume of mixing. The results reveal that HgNa liquid alloys are more stable than HgK at all concentrations. There is every likelihood of the existence of Hg2Na and Hg2K complexes in the liquid alloys.

Temperature-dependent structure and electrical transport in liquid metals

A simple model has been considered to obtain the reversible temperature dependence of the static structure factors S(q) for liquid metals. The measured values of S(q) near the melting point have been utilised to obtain the same at elevated temperatures for Na, K, Mg, Zn, Al and Pb. The notion of Debye temperature in liquid metals has been discussed. The principal peak heights of S(q) computed at the melting temperature have also been discussed in the light of Verlets freezing rule. The electrical resistivity and thermo-electric power of these metals have been computed as a function of temperature. The success and failure of the Ziman formula have been critically assessed.

Local order in Cd-based liquid alloys

A simple approach based on a statistical mechanical model is used to discuss the nature of atomic ordering in Cd-based (Cd-Bi, Cd-Mg and Cd-Ga) liquid alloys in the framework of the chemical short-range order parameter, concentration fluctuation in the the long-wavelength limit and the excess free energy of mixing. It has been observed that the nature of atomic order in binary liquid alloys could be understood with the help of their melting curves.

A Quasi-lattice Model for the Thermodynamic Properties of Au-Zn Liquid Alloys

Abstract The quasi-lattice model developed by Bhatia and Singh1–2 has been applied to understand the alloying behaviour of Au[sbnd]Zn liquid alloys. The large negative values of ordering energy and chemical short range order parameter indicate heterocoordination in Au[sbnd]Zn melt. The dip in concentration fluctuations in long wavelength limit and larger negative value of excess free energy of mixing at C = 1/2 suggest the existence of 1–1 type of complexes (AuZn) in the liquid alloys. This makes us believe that Au[sbnd]Zn liquid alloys with symmetrical properties of mixing belong to strong interacting system.

Conditional Probabilities and Thermodynamics of Binary Molten Alloys

Abstract A simple statistical mechanical model based on pairwise interaction has been considered to obtain conditional probabilities for the occupation of neighbouring atoms in binary molten alloys. This has been extended to obtain simple analytical expressions for activity, free energy of mixing, concentration fluctuations in long wave limit and the chemical short range order parameter. Application is made to compute ordering energy and the concentration dependent thermodynamic properties for CuPb, NaK, LiMg and CdMg. The study reveals that self coordination among nearest neighbour atoms exist in CuPb and NaK whereas heterocoordination exist in LiMg and CdMg.

Thermodynamic investigation of atomic order in AlMg liquid alloys

The grand partition function has been used to obtain analytic expressions for the excess free energy of mixing, the activity, the small-angle partial structure factors and the chemical short-range order parameter for weak interacting compound-forming molten alloys. It has also been applied to explain the asymmetry in the properties of mixing of AlMg molten alloys by assuming that the chemical complex Al3Mg2 exists in the liquid phase. It is concluded that a reasonable degree of chemical order may exist in the concentration range 0.2 or=0.8), however the melt is close to the ideal mixture. The topological order, at least at the long-wavelength limit, has been found to depend only weakly on concentration.

Thermodynamic properties of molten LiMg alloy

Abstract The activity, the heat of mixing, the concentration fluctuation in the long wavelength limit (Scc(0)) and the chemical short range order parameter (CSRO) have been studied as a function of composition in LiMg molten alloys by using first order quasi-chemical (QC) theory. The interchange energy has been considered a function of temperature and thus various thermodynamic quantities are calculated at elevated temperatures. The study reveals that though LiMg is considered a simple regular alloy, its various thermodynamic properties deviate from the ideal solution behaviour. Scc(0) computed at different temperatures from QC-theory are found to be smaller than the ideal values and heterocoordination in the nearest neighbour shell is found to exist through the entire concentration range. On the other hand, Scc(0) computed directly from the measured activity data indicates that the LiMg liquid alloy behaves as a segregating system for smaller content of Li (CLi⩽0.3). The building of heterocoordination starts around CLi⩾0.3 and remains for the rest of the compositions.

The segregation-order transformation in CdNa liquid alloy

The large asymmetry observed in the properties of mixing of CdNa liquid alloys is discussed on the basis of a simple theory based on the complex formation model. The results for long wavelength concentration-concentration fluctuations, Scc(0), number-number fluctuations, SNN(0) and number-concentration fluctuations, SNc(0) are presented as a function of concentration. This has been further used to obtain a chemical short-range order parameter ( alpha 1). Scc(0) and alpha 1 suggest that the segregation-order transformation occurs in the CdNa melt at around 60 at.% of Cd.

Pseudopotential method for the thermodynamic properties of liquid alkali metals

Abstract A pseudopotential method has been considered to study the enthalpy and compressibility of liquid alkali metals (Na, K, Rb and Cs) as a function of temperature. The diameters of the hard sphere reference system were determined by minimising the Helmholtz free energy to ensure thermodynamic stability. The enthalpy, which depends weakly on temperature, has been successfully explained. The investigation suggests that the structure dependent part of energy is sufficient to explain the compressibility of liquid alkali metals near and above the melting temperatures.