Rapid crystallization and growth mechanisms of highly undercooled Ag-Cu-Ge ternary alloy

Crystallization from undercooled melt of Ag-33.4wt% Cu-28.1wt% Ge ternary alloy was carried out with glass fluxing method under different undercoolings and the nucleation and growth characteristics of the primary phase, two-phase eutectic, and ternary eutectic are investigated. It is found that both the solidification microstructures and the size of primary phase (Ge) and intermetallic compound eta vary significantly with undercooling, whereas the ternary eutectic structure has no apparent variation. With the increase of undercooling, macrosegregation of primary (Ge) phase becomes weak and this phase experiences a "large block/strip --> small particle" morphology transition. Moreover, the ((Ge) + eta) two-phase eutectic can nucleate in preference to the primary (Ge) phase within a large undercooling regime. The intermetallic compound phase eta grows competently with (Ge) phase and it can directly form alloy melt. The experimental results of the containerless processing by a 3 in drop tube indicate that in order to make the ternary eutectic nucleate in preference to the (Ge) primary phase and the ((Ge) + eta) two-phase eutectic with the glass fluxing method, the undercooling should be at least larger than 195 K. (C) 2002 Elsevier Science B.V. All rights reserved.