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

摘要

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 η 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) +η) two-phase eutectic can nucleate in preference to the primary (Ge) phase within a large undercooling regime. The intermetallic compound phase η grows competently with (Ge) phase and it can directly form alloy melt. The experimental results of the containerless processing by a 3 m drop tube indicate that in order to make the ternary eutectic nucleate in preference to the (Ge) primary phase and the ((Ge) +η) two-phase eutectic with the glass fluxing method, the undercooling should be at least larger than 195K.