Unidirectional solidification of irregular silver-germanium eutectic alloys

Abstract

Unidirectional solidifications of silver-germanium eutectic alloys have been undertaken at solidification rates ranging between 5 × 10 -5 and 3 cm sec -1. At low growth rates, germanium grows ahead of the average solid-liquid interface, in the 〈100〉 direction. It then gives a complex regular structure (“starred” fibers and “ferns”), while above 2 × 10 -2 cm sec -1, the solidification mode changes and germanium grows in the 〈211〉 direction with a flake-like structure. Electron microscopy shows that germanium crystals are always multiple twinned (〈210〉 type at low growth rates and 〈111〉 type at high growth rates). The same transition in growth twin type was also found in aluminium silicon eutectic alloys. An explanation is proposed for the change in twin types: it takes into account the different structures of the reentrant twin corners, where nucleation of a new atomic layer takes place during growth.