THE STRUCTURE AND ORIGIN OF BOUNDARIES BETWEEN ANTIPHASE REGIONS IN L1(0) INTERMETALLICS

Recent observations using transmission electron microscopy have shown that defects which show contrast similar to that expected from Anti Ph ase Domain Boundaries (APDBs) are commonly observed within the massive gamma phase in quenched samples of alloys based on TiAl. It has also been shown that there is a thin region of a 90 degrees domain associat ed with all such boundaries in the TiAl-based alloys which have been e xamined. In the present paper a mechanism for the formation of such co mplex boundaries is put forward and image calculations based on this m odel are shown to be in agreement with the experimental observations. II is concluded that these domains nucleate from a (metastable) f.c.c. phase and are not formed directly from the high temperature equilibri um hexagonal alpha phase. Upon annealing these boundaries become cryst allographic before individual domains shrink so that the APDB is event ually removed. Observations of the structure of APDBs in CuAu, FePd an d in a range of TiAi-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 str ucture of APDBs in the L1(0) structure is a function of the c/a ratio and of the ordering energy. Copyright (C) 1996 Acta Metallurgica Inc.