Scandium Effect on Undercooling and Dendrite Morphology of Al-4.5 Wt Pct Cu Droplets

This paper reports on the undercooling and growth morphology of Al-4.5 wt pct Cu and Al-4.5 wt pct Cu-0.4 wt pct Sc with a focus on the effect of Sc addition. It is found that the addition of Sc reduces the undercoolings of both primary phase and eutectic. In addition, the morphology of the Al-4.5 wt pct Cu-0.4 wt pct Sc dendrites is less favored in the 〈111〉 direction at similar undercoolings as with Al-4.5 wt pct Cu. The development of solidification continuous cooling transformation diagrams that relate the solidification paths to the inherent solidification microstructures is also introduced. The solidification continuous cooling transformation diagrams are obtained based on the measurement of phase fractions of a solidified microstructure. The quantitative data are combined with well-established solidification models and phase diagrams to yield undercooling temperatures of individual phases. The thermal history and undercooling of different phases in the solidified alloy are estimated for a wide range of cooling rates (from 10 −2 to 10 4 K/s). It is found that a minimum cooling rate of about 1 K/s is required to avoid the nucleation of the detrimental intermetallic, W-phase in hypoeutectic Al-Cu-Sc.