Containerless rapid solidification of Ag28.1Cu41.4Ge30.5 ternary alloy

Droplets of Ag28.1Cu41.4Ge30.5 ternary alloy with different sizes are solid ified during containerless processing in a 3 m drop tube. The experimental result shows that the microstructural evolution depends mainly on droplet s ize. Generally, droplet undercooling is inverse proportional to its diamete r. When the droplet diameter is large, the (Ge) primary phase and intermeta llic compound eta phase show granular and strip shapes, respectively. In th e medium-sized droplets, the (Ge) phase grows in a dendritic mode whereas e ta phase has little change. Meanwhile, both the grain size and interphase s pacing also decrease with the reduction of droplet size. Moreover, it is fo und that eta phase can nucleate directly from alloy melt and does not depen d on primary (Ge) phase during the formation of binary eutectic structure. If the droplet diameter is smaller than 250 mu m, a spherical anomalous eut ectic grain structure forms, which consists of three very fine phases and i s surrounded by coarse (Ge) and eta phases. There is no evident feature of macrosegregation of semiconducting (Ge) phase and it distributes quite homo geneously, which is ascribed to the reduced gravity condition and rapid coo ling in the drop tube. (C) 2000 Elsevier Science S.A. All rights reserved.