Three-dimensional microstructure reconstruction and the eutectic spacing adjustment during directional solidification of Al-40%Cu hypereutectic alloy

Three-dimensional (3D) microstructures can clearly reveal the size, shape and distribution of the phases, providing a novel way to deeply understand the formation mechanism of the solidified phases. In this paper, by using the serial sectioning technique, the 3D microstructure of the primary Al ₂Cu phase was reconstructed and the eutectic spacing adjustment was investigated during directional solidification of Al-40%Cu hypereutectic alloy. The results show that the primary Al ₂Cu phase pattern was observed faceting due to the growing faceted angle and plane parallel to the solidification direction at the pulling rate of 5 μm/s. Further, there existed some hopper-like cavities in the 3D microstructure of the primary Al ₂Cu phase that caused by remelting owing to a large amount of the latent heat difficult to be extracted from the solidification interface. For the 3D eutectic microstructures of the Al-40%Cu alloy, the growth direction of the Al and Al ₂Cu phases in the coupled eutectics had a deviation angle of 5.1° with the heat flux direction and the phase volume fractions of the Al and Al ₂Cu phases were measured directly to be 56.8% and 43.2%, respectively. In addition, at the abrupt change in pulling rate from 2 μm/s to 500 μm/s, the lamellar-to-rod eutectic transition was observed and the continuously splitting and branching occurred in different planes responsible for the eutectic spacing adjustment in the 3D microstructure of the Al-40%Cu alloy. Meanwhile the renuleation mechanism in the 2D microstructure for the adjustment of the eutectic spacing did not work in the 3D eutectic microstructure of the Al-40%Cu alloy.