Heterogeneous nucleation in liquid immiscible alloys

Alloys being immiscible in the liquid state decompose into two liquid phases during cooling through the miscibility gap. In contrast to solidification where the solid phase is generally nucleated at inocculants or container walls, liquid immiscible alloys are thought to decompose via homogeneous nucleation. The process is badly investigated. Therefore we study heterogeneous nucleation in Al-base immiscible alloys (Al-Pb, Al-Bi) induced by inocculants like TiB2, TiC and ZrO2. We present experimental results on Al-Pb alloys cooled with different amounts of TiB2 and ZrO2 through the miscibility gap. The microstructrue is analysed in 2D with SEM and light microscopy and also in 3D using x-ray tomography. The experiments show a pronounced effect of the inocculants. The average Pb drop size changes as well as the size distribution. The experimental results are discussed with respect to classical theories of heterogeneous nucleation and newer models of nucleation and growth by Greer and co-workers