Heterogeneous nucleation of α-Al in Al-Ni-Si alloys

In order to elucidate the detrimental effect of high Si contents on the grain refinement of Al alloys, the heterogeneous nucleation of α-Al in an Al70Ni13Si17(at.%) alloy was studied using a novel metallic glass technique. A Ti-B-Al 5:1 grain refiner rod containing TiB2particles, on which Al3Ti layers were detected, was added to an Al70Ni13Si17melt that was subsequently rapidly quenched to form a metallic glass. The nucleation events, which occurred during quenching, were examined using transmission electron microscopy and energy-dispersive X-ray spectroscopy. No α-Al nucleation was detected on added boride particles although nucleation of an unidentified phase occasionally occurred on their non-basal faces. Nevertheless, α-Al was systematically observed to nucleate on hexagonal dendrites present in the melt, which implies that Si poisoning of the boride particles is occurring. The increased Si content appeared to be responsible for the weakening of the Al3Ti layer, originally present on the surface of the boride particles in the grain refiner rod, which resulted in a reduction in the ability of these particles to nucleate α-Al.