Microstructure evolution in rapidly solidified immiscible alloys

Abstract

When a hypermonotectic alloy is cooled from the single-phase liquid state into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally the liquid-liquid decomposition of an initially homogenous liquid begins with the nucleation of the liquid minority phase in the form of droplets, which grow by diffusion. Reaching the non-variant monotectic reaction temperature the matrix liquid decomposes into a solid and a second phase, being not distinguishable from the liquid minority phase that emerged in the miscibility gap. The size spectra of the drops of both processes merge the larger the cooling rate. Within a suitable interval of cooling rates they become, however, distinguishable and then the spectra stemming from the liquid-liquid decomposition gives unique access to the nucleation process inside the miscibility gap. Immiscible alloys like Al-Pb offer a great potential as for instance self-lubricating bearings in automotive applications, if a finely dispersed microstructure is achieved. This can be obtained under conditions of rapid cooling. In order to explore the potential of rapid solidification of immiscible alloys, investigations of the microstructure of different Al-Pb alloys were carried out, varying over a wide range of alloy-concentrations and cooling rates. The experimental results, obtained with the help of different methods, are compared with numerical simulations of the decomposition and the microstructure evolution process.