MICROSTRUCTURE EVOLUTION IN RAPIDLY SOLIDIFIED IMMISCIBLE Al-BASED-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 in a supersaturated matrix and collision and coagulation of drops induced by Stokes and /or Marangoni motion. 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. The main target of the present investigations is to achieve a physical understanding of the liquid-liquid phase separation by studying the microstructure evolution in different alloys, cooled with various cooling rates through the miscibility gap and comparing the experimental results with modelled microstructures.