KINETICS OF PHASE-SEPARATION IN A HYPERMONOTECTIC AL-PB ALLOY

A numerical model is presented that describes the microstructure evolu tion in liquid immiscible alloys under the common action of nucleation , diffusional growth and drop motion during continuous cooling through the miscibility gap. The model is satisfactorily verified by comparis on with an analytically solvable case. The numerical results for an Al -5 wt.% Pb alloy cooled with 10, 20 and 50 K/s from a temperature abov e the binodal show that the simultaneous action of Marangoni migration and Stokes settlement governs the macrosegregation of Pb. At low cool ing rates a Pb film establishes by Stokes motion at bottom, at high co oling rates the Pb droplets accumulate in the middle part of a cylindr ical sample due to the strong Marangoni motion opposing gravity induce d sedimentation. The average drop radius does not vary with position i n the sample at high cooling rates. At small cooling rates a slight va riation with position is predicted.