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

Abstract

Dendrites of the icosahedral quasicrystalline phase in an aluminium matrix were formed on rapid solidification of an Al₉₀Mn₅Fe₅ 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 Al₈₆Mn₇Fe₇ and Al₈₀Mn₁₀Fe₁₀ an icosahedral phase, Al₃Fe and α -Al 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 Al₃Fe and α-Al formed. The relationship of the icosahedral twin to the decagonal phase has been brought out.