Rapid solidification of ternary Ni-Pb-Cu monotectic alloy under free fall conditions

The phase separation and rapid solidification of ternary Ni₆₂Pb₃₀Cu₈ monotectic alloy were investigated under free fall conditions. X-ray diffraction analysis indicates that the solidified microstructure consists of (Ni) and (Pb) phases. With the decrease of droplet diameter, the morphology transforms from a triple-layer core-shell microstructure into (Ni) dendrite microstructure. In the triple-layer core-shell microstructure, the surface layer is (Pb) phase, which is favourable to reducing the surface Gibbs free energy. EDS analyses reveal that both (Ni) and (Pb) phases exhibit conspicuous solute trapping effect during rapid solidification. The calculated cooling rate and undercooling increase exponentially with the decrease of droplet diameter. Furthermore, the Maragnoni migration of the second liquid (Pb) phase was studied, which indicates that the L(Pb) phase droplets can migrate to the central part. Under free fall conditions, the cooling rate, surface segregation, Maragnoni migration and dendrite growth are responsible for the microstructure evolution.