Short Range Order and Concentration Fluctuations in Regular and Compound Forming Molten Alloys

Abstract

First, a statistical mechanical model is developed for compound forming molten alloys with the aim that it explains both the concentration dependence of the thermodynamic quantities-free energy of mixing G_M, concentration fluctuations S_cc (0), etc.-and the short range order (SRO) parameter x ₁ for the nearest neighbour shell obtained from neutron and X-ray scattering. The model assumes the existence of appropriate privileged groups or chemical complexes A _μ B _v(μ, v small integers) and that the energy of an AB, AA or BB bound depend on whether that bond is part of the complex or not. The expression for G _M reduces to the well-known quasi-chemical expression for regular alloys when no complexes are formed. The theory is next applied to discuss G _M, S _cc(0), etc., for three compound forming alloys, MgBi, LiPb and AgAl. The bond energies once chosen to explain thermodynamic data enable one to evaluate SRO α₁. The calculated α₁ are, in general, good agreement with the available data in both the compound forming and regular alloys. The theory also provides a natural explanation of change of α₁ on melting in such alloys as Cd-Mg.