The reaction of liquid metals to form intermetallics

Abstract

The formation of intermetallic compounds of high melting point, by reaction between liquid metals at low temperatures, was studied for a number of binary systems, in particular Al-Sb, Mg-Sb, Mg-Bi and Na-Bi. Two regimes of behaviour on mixing were found: Non-catastrophic mixing was investigated by means of equipment which produced a horizontal interface between the two liquid metals. The structures of the interfacial compound layers, and the kinetics of the interdiffusion process, were examined by x-ray microanalysis and metallography. The results are compared with a simple model of the diffusion process. It is concluded that two distinct types of rate controlling intermetallic layer may exist between the molten reactants; either a thin stable layer through which reactants may pass by solid state diffusion, or a much thicker but perforated layer undergoing continuous local remelting and reformation. It is suggested that an important factor determining the layer type adopted by a particular binary system is the composition range of the intermetallic compound. The factors influencing whether catastrophic or non-catastrophic reaction occurred were also determined by thermal and metallographic investigation of different systems under different mixing conditions. It is concluded that catastrophic reaction is only possible in systems for which the enthalpy of mixing is sufficient in the liquid state to raise the temperature of the product alloy above its liquidus.[/p][/p][/p][/p]