An Electron Microscopy Study of the Icosahedral Phase in the Aluminum-Manganese System

Abstract

One of the most interesting structures to evolve from rapid solidification processing (RSP) methods is the solid metallic phase of the Al-Mn system first reported by Shechtman et al to have icosahedral symmetry. With six 5-fold rotational axes, an icosahedron defies classification as a crystallographic point lattice: it simply has no translational symmetry. Earlier work by Levine and Steinhardt showed that “quasicrystals” with 5-fold axes could be defined by extension of the Penrose tiling idea to three dimensions. These results and their recent history prompted the present attempt at structural analysis of this novel solid by a combination of convergent beam electron diffraction (CBED) and phase contrast imaging. Specimens of 74.7 at % A1 and 25.3 at % Mn were prepared by arc melting and splat quenching (hammer and anvil) followed by jet electropolishing in a nitric acid-methanol electrolyte. They were examined in both the JEM 200CX analytical electron microscope and the JEM ARM 1000 atomic resolution microscope at the National Center for Electron Microscopy.