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

Abstract

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 GM, concentration fluctuations Scc (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.