A model has been presented describing the microstmcture evolution in an immiscible alloy solidified rapidly under the vertical Bridgeman-Stockbarger-type unidirectional solidification conditions. Numerical solutions have been performed for hypermonotectic Al-Pb alloys by coupling a given temperature field and a calculated concentration field with the phase diagram. The numerical results show that at a high solidification velocity a constitutional supercooling region appears in front of the solid/liquid interface and the liquid-liquid decomposition takes place there. A higher solidification velocity leads to a higher nucleation rate for a given temperature gradient and, therefore, a higher number density of the minority phase droplets. As a result, the average radius of droplets in the melt at the solid/liquid interface decreases with the solidification velocity. It is indicated that the number density and the average radius of droplets in the melt at the solid/liquid interface can be related to the solidification velocity by N(z=0)=C{sub}1V{sup}(1.01±0.02) and 〈R〉(z=0)=C{sub}2V{sup}(-0.39±0.01) with C{sub}1 and C{sub}2 being constants for an Al-Pb alloy with a Pb content less than 9 weight percent.
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