Constitution and microstructure of rapidly solidified aluminium-germanium alloys

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Abstract
X-ray and electron metallographic techniques, electron microscope micro-analysis and differential thermal analysis have been used to study splat-cooled aluminium-germanium alloys containing between 15 and 50 at. % germanium. The observed microstructure and constitution have been correlated with the solidification process at cooling rates in the range 102–109 K/sec. At very high cooling rates the retention of almost all the germanium in f.c.c. solid solution has been shown possible. At slightly lower cooling rates two metastable intermediate compounds were produced; δ1, which was tetragonal with α=6.59 Å and c=12.01 Å with the possible stoichiometric composition Al5,Go3, and S2, also tetragonal, with a=6.25 Å and c=9.44 Å, and having a composition AlGe or AlGe2. On thermal decomposition the two compounds yielded a further metastable compound δ3, which was hexagonal with a=2.88 Å and c=4.58 Å. On further heating, this compound decomposed to the equilibrium aluminium and germanium solid solutions. Due to the wide range of cooling rates in any foil, the splat-quenched specimens generally contained all the metastable constituents, in addition to small amounts of the equilibrium solid solutions of aluminium and germanium.

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