HETEROGENEOUS NUCLEATION OF SOLIDIFICATION OF SI IN AL-SI AND AL-SI-P ALLOYS

Heterogeneous nucleation of solidification in melt spun Al-Si and Al-Si-P has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al-Si containing 2 ppm P and Al-Si-P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding Al matrix at a high undercooling in Al-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of 10 degrees, 43 degrees and 10 degrees for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.