The effect of supercooling on the microstructure of Al-Nb alloys

Electromagnetic levitation was used to study supercooling effects on solidi fication characteristics of Al-Nb alloys containing 15-53 at.% Nb. The resu lts indicate that Al3Nb, with a melting temperature of 1980 K, is not a sto ichiometric line compound but has a homogeneity range on the Nb side. These results are consistent with the more recently assessed Al-Nb phase diagram , but not with those assessed previously. Significant changes in the micros tructural morphology of Al-40 at.% Nb were also observed as a function of t he bulk supercooling. These include metastable compositional extensions of Al3Nb, preferential nucleation of Al3Nb in the supercooled liquid and corre sponding changes in the solidification path. For example, spherical domains comprising of a mixture of Al3Nb and AlNb2 were revealed in the eutectic m icrostructure, their volume fraction being an increasing function of the de gree of supercooling. Moreover, the Al3Nb and AlNb2 exhibited metastable co mpositional extensions up to 31 at.% for the former and down to 48 at.% Nb for the latter. (C) 2000 Kluwer Academic Publishers.