Rapid dendritic solidification and structure hardening mechanism of substantially undercooled quaternary nickel alloys

The high undercooling and rapid solidification of liquid quaternary Ni-5%Cu-5%Sn-5%Si and Ni-5%Cu-5%Sn-5%Ge alloys were achieved by glass-fluxing technique to explore their dendritic growth kinetics, microstructural evolution and hardening mechanisms. The dendrite growth velocity of α-Ni phase was found to increase with the rise of bulk undercooling, which could be described by power law functions for both alloys. With the increase of liquid alloy undercooling, the morphology of α-Ni phase transformed from developed dendrites into equiaxed grains. Both the fine grain hardening and solute hardening effects resulted in the elevated Vickers hardness for these two alloys. Nano-sized Ni₃Si precipitations were detected within the first alloy, which distributed homogeneously in the α-Ni phase matrix and transformed from faceted to nonfaceted morphology at sufficiently large undercoolings. The precipitate hardening effect dominated the Vickers hardness enhancement for this Si containing alloy. Rapid solidification could modulate the formation of ordered precipitate phase and thus improve the hardening effect by changing dislocation motion conditions.