The structure and origin of boundaries between antiphase regions in L10 intermetallics

Abstract

Recent observations using transmission electron microscopy have shown that defects which show contrast similar to that expected from Anti Phase Domain Boundaries (APDBs) are commonly observed within the massive γ phase in quenched samples of alloys based on TiAl. It has also been shown that there is a thin region of a 90° domain associated with all such boundaries in the TiAl-based alloys which have been examined. In the present paper a mechanism for the formation of such complex boundaries is put forward and image calculations based on this model are shown to be in agreement with the experimental observations. It is concluded that these domains nucleate from a (metastable) f.c.c. phase and are not formed directly from the high temperature equilibrium hexagonal α phase. Upon annealing these boundaries become crystallographic before individual domains shrink so that the APDB is eventually removed. Observations of the structure of APDBs in CuAu, FePd and in a range of TiAl-based alloys are also reported and the structure of APDBs is shown to vary from pure APDBs in CuAu to complex APDBs of varying widths in the other alloys examined. It is argued that the structure of APDBs in the L10 structure is a function of the ca ratio and of the ordering energy.