Al-Mn quasicrystal aggregates with icosahedral morphological symmetry

Abstract

A scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigation of icosahedral quasicrystals in rapidly cooled Al-Mn alloys with two different bulk compositions Al₆Mn (14·3 at% Mn) and Al₁₇Mn (5·6 at% Mn) has revealed two different morphologies with icosahedral symmetry. In Al₆Mn specimens the pentagonal dodecahedron is the basic morphological form and in Al₁₇Mn specimens flower-like quasicrystal units (‘flowers’) with icosahedral point symmetry are the basic forms. Three types of flowers could be differentiated which can be interpreted as representing growth stages. The flowers can adequately be described by a star polyhedron model based on a {5, 6} Archimedean polyhedron. Indices of the facets have been determined on the basis of TEM and electron diffraction measurements. Most of the facets have index (100000), while those at the growth front have index (211211).