Reduced gravity processing of Cu-Co metastable monotectic alloy via drop-tube processing

The metastable monotectic alloy Co-Cu has been subject to rapid solidification under reduced gravity conditions via drop-tube processing, with the resulting particles being classified into 9 size fractions between 850 µm and 38 µm, giving cooling rates between 850 K s-1 and 85,000 K s-1. Two alloy compositions were studied, Co-50 at.% Cu and Co-31.5 at.% Cu, with the undercooling required for binodal decomposition estimated as 41 K and 143 K respectively. Liquid phase separation was observed in both alloys, as were fully spherical core-shell morphologies. A statistical analysis of > 3000 particles across both compositions and all size fractions was undertaken. It was found that for both alloys the incidence of liquid phase separation increased with increasing cooling rate (decreasing particle size). For the Co-31.5 at.% Cu alloy the incidence of spherical core-shell morphologies also increased with increasing cooling rate but for the Co-50 at.% Cu alloy it was found that the incidence of spherical core-shell morphologies peaked in the 106-75 µm size fraction, wherein  40 % of all particles displayed this morphology. However, with further increases in cooling rate the incidence of core-shell structures decreased rapidly. This is thought to be due to the limited time available for migration of the two melts following liquid phase separation.