TEM investigation of heterogeneous nucleation mechanisms in Al–Si alloys

Heterogeneous nucleation was studied in an Al80Ni10Cu8Si2 glass containing hexagonal TiB2 particles within the glassy matrix. Distinct orientation relationships were found for different phases on the TiB2 faces. On basal faces copious nucleation of α-Al occurred with the close-packed planes and directions of Al and TiB2 parallel, while on prism faces copious nucleation of tetragonal Al2Cu was observed. Holding for 30 min at 1173 K prior to melt-spinning resulted in the formation of TiSi2 on the basal faces instead of Al. The implications of the observed nucleation mechanism are discussed with respect to industrial casting practice, in particular to Si poisoning.