Containerless rapid solidification of Ag_28.1Cu_41.4Ge_30.5 ternary alloy

Droplets of Ag_28.1Cu_41.4Ge_30.5 ternary alloy with different sizes are solidified during containerless processing in a 3 m drop tube. The experimental result shows that the microstructural evolution depends mainly on droplet size. Generally, droplet undercooling is inverse proportional to its diameter. When the droplet diameter is large, the (Ge) primary phase and intermetallic compound η phase show granular and strip shapes, respectively. In the medium-sized droplets, the (Ge) phase grows in a dendritic mode whereas η phase has little change. Meanwhile, both the grain size and interphase spacing also decrease with the reduction of droplet size. Moreover, it is found that η phase can nucleate directly from alloy melt and does not depend on primary (Ge) phase during the formation of binary eutectic structure. If the droplet diameter is smaller than 250 μm, a spherical anomalous eutectic grain structure forms, which consists of three very fine phases and is surrounded by coarse (Ge) and η phases. There is no evident feature of macrosegregation of semiconducting (Ge) phase and it distributes quite homogeneously, which is ascribed to the reduced gravity condition and rapid cooling in the drop tube. ũ 2000 Elsevier Science S.A.