HETEROGENEOUS NUCLEATION OF SOLIDIFICATION OF SI IN AL-SI AND AL-SI-PALLOYS

Heterogeneous nucleation of solidification in melt spun Al-Si and Al-S i-P has been studied using differential scanning calorimetry, and tran smission, scanning transmission and high resolution electron microscop ies. The microstructures of the heat treated melt spun alloys all cons ist of an Al matrix, Al-Si eutectic distributed along the Al grain bou ndaries, and Si embedded in the Al matrix. The Si microstructure depen ds 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 s urrounding Al matrix at a high undercooling in Al-Si containing 0.25 p pm P. The Si nucleation onset temperature remains approximately consta nt while the peak and end temperatures both decrease with increasing c ooling rate, in agreement with classical nucleation theory. Kinetic an alysis, using the spherical cap model gives contact angles of 10 degre es, 43 degrees and 10 degrees for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.