MICROSCOPIC MECHANISMS OF METASTABLE PHASE-FORMATION DURING BALL-MILLING OF INTERMETALLIC TIAL PHASES

Powders of the intermetallic equilibrium phases alpha(2)-Ti3Al, gamma- TiAl and TiAl3 were ball milled in order to investigate the microscopi c origins of the energetic destabilization and the transformation into metastable phases during the milling process. II was found that the i ntermetallic phases were chemically partially disordered on milling fo llowed by the transformation into solid solution phases after long mil ling. In detail, for the gamma phase, the formation of numerous deform ation twins, thin h.c.p. lamellae and lamellae of the 9R phase formed by an antitwin operation was observed by TEM. The disordering of the D 0(22). TiAl3 phase occurred inhomogeneously in the material via the fo rmation of antiphase boundaries on (001) planes, resulting in an f.c.c . solid solution in the final state. In summary, it can be concluded t hat the formation of the observed metastable phases results from chemi cal disordering, whereas the excess enthalpy of grain boundaries plays only a minor role for the energetical destabilization of the intermet allic compounds during milling in this case. (C) 1997 Acta Metallurgic a Inc.