Effect of an abrupt growth rate on primary Al₂Cu phase in directional solidification of Al-40%Cu hypereutectic alloy

Intermetallics combining metal and ceramic properties exhibit colorfully complicated growth morphologies in solidification. Different from the solids phase solidification, intermetallic solidification remains less understanding of the correlation between processing parameters and microstructure morphologies. In this paper, considering that the intermetallic compound Al₂Cu phase has well-known thermo physical properties available for the theoretical and experimental research, we carried out the directional solidification experiments and focused on an Al-Al₂Cu hypereutectic alloy, where the Al₂Cu phase was solidified as a primary phase. The primary Al₂Cu phase growth behavior in the experiments included phase competition growth between coupled eutectic and primary phase, faceted phase transition and its change in growth morphology. By using a high thermal gradient directional solidification apparatus, the directionally solidified microstructures of Al-40%Cu (mass fraction) hypereutectic alloy were investigated and discussed based on the competition growth model. As the growth rate was changed abruptly from 10 μm/s to 2 μm/s, the microstructure transition from a primary Al₂Cu dendrite plus interdendritic eutectic to an entirely coupled eutectic occurred due to the interface growth temperature of the coupled eutectic exceeding that of the primary Al₂Cu dendrite. Also, the alloy liquid composition ahead of the solid/liquid interface approaching the eutectic point caused this microstructure transition. Simultaneously, in the changing-growth rate experiments, the primary Al₂Cu dendrites were firstly broken into small ones and then became the eutectic microstructure; it was interpreted rather by the decrease in liquid solute concentration ahead of the solid/liquid interface tan by the effect of the thermal-solute convection. Moreover, the morphology change phase growth from faceted primary Al₂Cu phase to non-faceted phase was observed by reducing abruptly the growtrate from 10 μm/s to 2 μm/s, which can be explained by the Jackson factor decreasing with increasing the interface growth temperature of primary Al₂Cu phase.