Phase Selection in Undercooled Melts of Peritectic Cu-Ge Alloys

Phase selection in undercooled melts of Cu-14.8Ge and Cu-18Ge compositions was investigated using the electromagnetic levitation technique in combination with substrate quenching and a drop tube. The results showed that the levitated and gas-cooled samples were all solidified into a microstructure consisting of primary -Cu plus peritectic -Cu5Ge. However, the samples quenched onto a copper substrate showed a segregation-free microstructure in the chilled zone, suggesting direct crystallization of the peritectic -Cu5Ge phase from a highly undercooled liquid. The Cu-18Ge samples quenched onto a glass substrate as well as those solidified in the drop tube also showed a segregation-free microstructure. An analysis of the nucleation kinetics revealed that the -Cu5Ge phase had a larger nucleation barrier than that of -Cu for all accessible undercoolings. It was suggested that phase selection in the undercooled Cu-Ge liquid might be controlled by transient nucleation kinetics.