Microstructure evolution and orientation analysis of hypereutectic Al-Al2Cu alloy under directional solidification

Dendrite is a fundamental growth pattern in alloy solidification. Normally, dendrites with a specific growth orientation which can remarkably influence casting properties have attracted many great interests. The previous investigations mainly focus on the growth of simple monophase dendrites solid solution. For complex intermetallics in solidification, the correlation between processing parameters and microstructure morphologies with the preferred growth directions has not been shed a light. In this work, considering intermetallic Al₂Cu phase has crystalline anisotropy and can exhibit colorfully complicated growth morphologies with specific growth direction in different solidification conditions, the primary Al₂Cu phase growth behavior of Al-40%Cu (mass fraction) hypereutectic alloy was investigated by using a high thermal gradient directional solidification apparatus. The Al₂Cu phase growth behavior in the experiments included its change in growth morphology and orientation transition. With solidification distance increasing, due to the alloy liquid composition ahead of the solid/liquid interface approaching the eutectic point, the primary Al₂Cu dendrite transited from regular faceted V-shaped morphology to the entirely coupled eutectic at 10 μm/s. Its growth direction changed from [110] direction to the normal direction of (121) plane. The EBSD result indicated that the [001] direction of Al₂Cu phase was along the direction of heat flux. As the growth rate was changed abruptly from 10 to 2 μm/s, the alloy liquid composition ahead of the solid/liquid interface increased firstly and then decreased to the eutectic point. It caused that the primary Al₂Cu dendrite transited from regular faceted V-shaped morphology to long smooth lath morphology, and finally disappeared to form the entirely coupled eutectic. Moreover, the growth direction of Al₂Cu phase changed from [110] direction to [001] direction. The experimental results show that the directional solidification process parameter is the dominant factor affecting the final morphology and growth direction of dendrites.