Influence of growth direction on the microstructure of unidirectionally solidified Cu-Pb monotectic alloy using zone-melt technique

The influence of growth direction on the monotectic structure of the Cu-Pb alloy is studied. In order to examine the influence under a 1 g environment, both the upward (opposite to the direction of gravity) and downward (the direction of gravity) unidirectional solidifications (UDS) are carried out. In the case of the upward UDS, a banded structure, which consists of Pb-rich and Cu-rich layers, is observed. The L₂ droplets pile up in front of the solid/liquid interface. On the other hand, in the downward UDS, the irregularly shaped L₂ phase uniformly disperses in the specimen and no banded structure is found. The gravity macrosegregation of the L₂ liquid is observed at the bottom of the molten alloy in the downward solidified specimen. This is caused by the difference in the density between the L₁ and L₂ phases. Furthermore, a mechanism for the formation of a banded structure is suggested. This mechanism suggests that the coalesced L₂ phase covers the solid/liquid interface by producing a Pb-rich layer that permits an increase in the undercooling of the L₁/L₂ interface compared to the monotectic temperature. As nucleation of the α-Cu phase occurs on the Pb-rich layer, the coexisting three phases are then restored. The temperature at the growth front is also returned to the monotectic temperature. The repetition mentioned above will result in the banded structure found in the upward UDS.