CONCURRENT NUCLEATION, GROWTH AND SEDIMENTATION DURING SOLIDIFICATIONOF AL-BI ALLOYS

The liquid-liquid decomposition in hypermonotectic alloys was investig ated experimentally by chill casting of Al-Bi alloys. From the metallo graphic sections the following could be quantitatively taken: The volu me fraction of dispersed Bi droplets as a function of position in the samples, the size distribution and the largest observable radius. The experimental results were compared with a new theoretical description of concurrent nucleation, growth and sedimentation of droplets during the cooling process. Generally, the agreement between the analytical t heory and experimental results was satisfactory especially if the addi tional process of coagulation of drops of different size and thus sett ling speed was taken into consideration. The experimental results indi cate that for given cooling conditions the settling, and thus spatial phase separation of Bi and Al, occurs the faster and more pronounced t he higher the Bi content and thus the larger the time the melt stays w ithin the liquid miscibility gap. Therefore, when producing hypermonot ectic alloys for advanced bearings the amount of soft phase needed in order to prevent strong sedimentation effects during casting proccesse s should be carefully checked.