The formation of superlattices in alloys of iron and aluminium

Abstract

The crystal structures of metallic elements or alloys are built up of individual atoms arranged according to a regular pattern. In the case of a metallic element, such as aluminium, all the atoms being alike, the structure is usually very simple, and all positions are equivalent. In the case of an alloy, geometrical theory would require atoms of different kinds to be sorted out into different sets of positions. For example, in the alloy AlSb, as in NaCl, the atoms as a whole are situated on a simple cubic lattice, but the two sorts of atoms are distributed at alternate positions. There are many alloys which do not behave according to the geometrical theory, and unlike atoms occupy positions which should strictly be occupied by atoms of identical character. In some cases the atoms are distributed entirely at random, but in others there is a partial approach towards an ordered arrangement. On the whole, each type of atom has its appropriate place in the lattice, but, owing to one element being in excess, it partially takes the place of the other.