Rapid solidification behaviour of Ag-Cu-Ge ternary eutectic alloy

Ag_38.5Cu_33.4Ge_28.1 ternary eutectic alloy was undercooled and solidified by glass fluxing method and drop tube technique, respectively, and its growth mode and phase selection in these two processes were investigated. By using B₂O₃ as a denucleating agent, this ternary eutectic alloy is undercooled by a maximum of 185 K (0.228 T_m). It is found that with the increase of undercooling, a transition from the cooperative growth of three eutectic phases, namely the semiconducting phase (Ge), the solid solution phase (Ag), and the intermetallic compound phase η(Cu₅Ge₂), to the preferential growth of (Ge) phase followed by that of the (Ag) and η phases occurs. The fact that the latter two phases easily form lamellar structure indicates that these two phases have a good affinity. In addition, as undercooling increases, the macrosegregation of semiconducting phase (Ge) becomes weak and the solubilities of three eutectic phases are all extended. Under containerless processing conditions in drop tube, the solidification behaviour is significantly influenced by droplet size which can usually determine the actual magnitude of undercooling. The cooling rate and undercooling level in this process are estimated and their effect on growth kinetics and structural morphology of semiconducting phase (Ge) is discussed.