The atomic mechanism of the body-centred cubic to σ-phase transformation

Abstract

An atomic mechanism for the body-centred cubic to σ-phase transformation is suggested. Atomic movements over small distances in the [1 1 {\bar 1}]b.c.c. direction take place leading to the formation of a new layer structure. The transformation is completed by rotation of alternate layers of hexagons within zones related to kagomé tile structures. The mechanism suggests that certain groups of atoms are more strongly bonded in the [1 1 {\bar 1}] direction than others. The mechanism also suggests that the body-centred cubic phase exhibits partial long range order prior to the transformation. The ductility of β-uranium and the brittleness of FeCr and 2NbAl alloys is discussed in terms of the ordering and coordination numbers of the atomic positions in the σ structure.