Effect of thermosolutal convection on microstructure of directionally solidified Al-Al₂Cu hypereutectic alloy

Thermosolutal convection has a remarkable effect on solidified microstructure. The microstructures of directionally solidified Al-38.5%Cu hypereutectic alloy were investigated by using the apparatus with a high temperature gradient. The results show that the solute composition ahead of the solid/liquid interface (copper element) decreases along the solidification direction due to the thermosolutal convection in the melt. The theoretical calculation results indicate that the primary θ-Al₂Cu phase of faceted dendrites disappears when the alloy composition decreases to 37%Cu for a pulling velocity of 5 μm/s, in agreement with the experimental results. The microstructure of directionally solidification is eutectic structure of entirely coupled growth without any faceted dendrites of primary θ-Al₂Cu when the solid fraction reaches 0.49 and the alloy composition is less than 37%Cu.