Non-equilibrium transformations involving L12-Al3Zr in ternary Al-X-Zr alloys

Abstract

The metastable L12-Al3Zr phase has been obtained as a solidification product on melt-spinning ternary Al-X-Zr (X = Cu, Ni) alloys. Different non-equilibrium effects of the metastable Al3Zr phase (L12) have been observed in the as-solidified and heat-treated alloys. The solidification sequence begins with the formation of the L12-Al3Zr (cubic) phase as a primary phase, followed by heterogeneous nucleation of α-Al. Morphological changes in the primary phase result in a shape transformation from a faceted cube to one with concave interfaces and protrusions along the corners, having a preferential growth along the 〈111〉 direction. This is brought about by a kinetic effect taking place during the growth of the L12-Al3Zr phase into the surrounding liquid, as the alloy is quenched. In another instance, the primary L12-Al3Zr phase nucleates as solid-state precipitates of the same L12-Al3Zr phase on annealing, by dissolution and reprecipitation of solute, under the influence of moving grain boundaries. A third case shows the metastable L12 phase nucleating on the equilibrium DO23-Al3Zr phase, upon solidification. This is attributed to the sluggish growth kinetics of the latter.