Microstructure formation mechanism of rapidly solidified ternary Co-Cu-Pb monotectic alloys

The metastable phase separation and rapid solidification of ternary Co-Cu-Pb monotectic alloys have been investigated under free fall condition. With the decrease of droplet diameter, the microstructures of Co₅₁Cu₄₇Pb₂ and Co₄₇Cu₄₄Pb₉ alloys display a "dendrite→core-shell→dendrite" transformation and a morphology transition from core-shell to homogeneous microstructure, respectively. X-ray diffraction analysis indicates that the solidified microstructures are composed of α(Co), (Cu) and (Pb) phases. α(Co) and (Cu) phases grow mainly in dendritic manner, and (Pb) phase is distributed interdendritically among (Cu) phase. Both experimental results and theoretical calculations reveal that the interfacial energy between (Co)/(Pb) liquid phases is larger than thoses of (Co)/(Cu) and (Cu)/(Pb) phases. The weak wetting ability between (Co) and (Pb) liquids results in the distribution of (Pb) phase inside the Cu-rich zone instead of Co-rich zone. Three possible solidification routes are deduced according to the solidification microstructure, in which the solidification process consists of phase separation L→L₁(Cu)+L₂(Co), peritectic transformation α(Co)+L→(Cu) and monotectic transformation L(Cu)→S(Cu)+L(Pb).