Effects of Sc alloying on the evolution of solidification microstructure and formation of W phase in as-cast 2519 aluminum alloys

Abstract

In this study, the effect of Sc content on the phase transformation of the 2519 alloy during solidification is investigated through microstructure characterization and thermodynamic calculations. The results show that the grain size of the 0.3–0.4Sc alloy is reduced by 70% (approximately 28.9 µm) to achieve maximum refinement in the as-cast 2519(Sc) Al alloy. The phase diagram calculation shows that the sequence of intermetallic formation during the solidification of 2519(Sc) Al alloy is as follows: Al3Sc, W phase, θ. The primary Al3Sc particles appear at the center of the α–Al grains and evidently acted as heterogeneous nucleating agents. Transmission electron microscopy shows that the nano-sized bean-like Al3Sc phase precipitated from the saturated solid solution. Moreover, the phase transition analysis of the 0.3–0.4Sc alloy during solidification shows that the formation of the W phase underwent two different types of reactions: the peritectic-eutectic reaction (L + Al3Sc → αAl + W) and eutectic reaction (L → αAl + W, L → αAl + θ + W). The W phase formation mechanism of these two reactions is revealed through experiments. Simultaneously, W phases with different morphologies are observed in the 0.3–0.4Sc alloy, i.e., the W phase of the spherical-shell structure is formed by the peritectic-eutectic reaction, and the long strip W phase (~6 µm in size) is precipitated by the eutectic reaction.