A transmission electron microscopic study of icosahedral twins- I. Rapidly solidified Al-Mn-Fe alloys

Dendrites of the icosahedral quasicrystalline phase in an aluminium matrix were formed on rapid solidification of an $Al_{90} Mn_5 Fe _5$ alloy. A large number of these particles was found to be twinned with one of the five-fold symmetry axes as the twin axis, as established by careful tilting experiments in the transmission electron microscope. The twin relationship can be expressed in 60 different ways by a rotation around seven different axes of the icosahedron and utilising 12 different angles. In the alloys of composition $Al_{86} Mn_7 Fe_7$ and $Al_{80} Mn_{10} Fe_{10}$ an icosahedral phase, $Al_3 Fe$ and $\alpha-AI$ formed at wheel surface velocities above 23.5 m/s. The icosahedral phase in these alloys is not twinned. At 12 m/s wheel velocity only $A1_3 Fe$ and $\alpha-AI$ formed. The relationship of the icosahedral twin to the decagonal phase has been brought out