Solidification of aluminium-germanium alloys at high cooling rates

Abstract

Aluminium-germanium alloys, covering the entire composition range, were subjected to rapid solidification by different techniques ranging from the gun technique of splat cooling to simple quenching of the melt in water. The constitution, microstructure and crystal structure of the alloys thus solidified under cooling rates varying from 10³ to 10⁷°C/sec were studied by standard optical, electron- and X-ray metallographic techniques. The solid solubility of Ge in Al could be increased from the equilibrium value of 2.8 at. % to a maximum of 7.2 at. %. In addition, two new non-equilibrium tetragonal phases γ₁(a=12.91 to 13.11 Å, c=12.00 to 12.10 Å) and γ₂(a=14.98 Å, c=16.03 Å) could be obtained in alloys containing 15 to 40 and 40 to 80 at. % Ge respectively. In relation to the face-centred cubicagr solid solution, these appear to be made up of 30 and 52 unit cells, respectively, with the relations a_γ1≃√10a_α, C_γ1≃3a_α and a_γ2≃√13a_α, C_γ2≃4a_α. A new non-equilibrium constitution diagram connecting cooling rate with constitution at room temperature has been proposed and seems to explain all observations satisfactorily.