Growth mechanisms of dendrites and eutectics within undercooled liquid Al-Ni alloys

Al-4 wt.%Ni (hypoeutectic), Al-5.69 wt.%Ni (eutectic) and Al-8 wt.%Ni (hypereutectic) liquid alloys are highly undercooled and rapidly solidified under free fall condition. Theoretical calculations indicate that the coupled zone of (Al+Al₃Ni) fibrous eutectic is an unclosed region in the composition range from 4.8 to 15 wt.% Ni, which is strongly skewed towards the Al₃Ni phase side. It is found that the cooling rate and undercooling of liquid alloys would increase as the droplet size decreases. Then the microstructural evolution of Al-Ni alloys will be induced by the competitive growth of (Al) dendrite, Al₃Ni dendrite, and (Al+Al₃Ni) eutectic. During the rapid solidification of Al-4 wt.%Ni hypoeutectic alloy, complete solute trapping effect occurs and then causes the formation of metastable segregationless (Al) solid solution phase. When the droplet undercooling exceeds 58 K, the structural morphology of Al-5.69 wt.%Ni eutectic alloy shows a transition from (Al+Al₃Ni) fibrous eutectic to primary phase (Al)-dominated hypoeutectic structure. As the undercooling increases further, the fibrous eutectic becomes the unique microstructure of Al-8 wt.%Ni hypereutectic alloy, and finally evolves into a kind of granular eutectic.