Complex phase separation of ternary Co–Cu–Pb alloy under containerless processing condition

The macrosegregation and solidification processes of ternary Co43Cu40Pb17 alloy have been investigated under the containerless processing condition in drop tube. The addition of Pb element is found to be quite effective to promote the phase separation of Co–Cu alloy, resulting in a macroscopic segregation pattern in ternary Co43Cu40Pb17 alloy. The rapidly solidified alloy droplets are characterized by core–shell structures due to the combined effects of reduced gravity and containerless state during free fall. When the droplet diameter is smaller than 450 μm, a kind of three-layer core/shell structure is formed, which consists of a Co-rich core, a Cu-rich intermediate layer and a Pb-rich surface shell. EDS analyses show that the Pb solute has a stronger affinity with Cu-rich liquid and a repulsive interaction with Co-rich liquid. The solidification process of Co43Cu40Pb17 ternary alloy involves two successive phase separation events, a subsequent peritectic transformation and a final monotectic transformation.

► The addition of Pb element can promote the phase separation of Co–Cu based alloy.
► Two successive phase separation events happen forming a three-layer core/shell structure.
► The solidification pathways of Co43Cu40Pb17 immiscible alloy in different alloy droplets are proposed.