Zr-poisoning of grain refiner particles studied in Al-Ni-Zr amorphous alloys

The poisoning mechanism of Zr was investigated using a metallic glass technique. Grain refining particles from a 5:1 TIBAL rod were embedded into a metallic glass produced by melt-spinning an Al87Ni10Zr3(at.%) alloy. Within the as-quenched samples, dendrites of α-Al were found on TiB2particles and on primary dendrites of Al3Zr competing to nucleate α-Al. On boride particles a thin diminishing tri-aluminide layer was found to nucleate α-Al. Zr was found to preferentially diffuse into TiB2particles rather than to form a thick layer of Al3Zr on these particles. The epitaxial relationship found between the tri-aluminide layer and the (Zr,Ti)B2particle is limited by the expanding lattice parameter of the boride with increasing Zr content. The nucleation mechanism on the boride particles and the influences of Zr are discussed with respect to conventional casting practice.