The microstructure of unidirectionally solidified Ag-Ge eutectic alloys

Ag-Ge alloys containing 15 to 22 wt% Ge were unidirectionally solidified to investigate the growth conditions for fully eutectic growth (the coupled region) in the range of growth rate 1.4 x 10^–4 to 1.1 cm sec^–1 and at a temperature gradient of 200° C cm^–1. Primary silver was not formed in the hypereutectic Ag-Ge alloys, implying that the coupled region of Ag-Ge alloys may be different from that of the other nf-f alloy systems such as Al-Si, Fe-C, Al-Ge and Al-Fe, whose coupled regions are usually skewed towards the faceted component. It was also observed that the morphologies of primary silver, primary germanium and eutectic structure were changed with increasing growth rate. Lamellar colonies were formed prominently in the fast-grown hypereutectic alloys. As the growth rate increased the tendency for branching in massive primary germanium was so pronounced that a lamellar colony was finally formed.