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 microscopic origins of the energetic destabilization and the transformation into metastable phases during the milling process. II was found that the intermetallic phases were chemically partially disordered on milling followed by the transformation into solid solution phases after long milling. In detail, for the gamma phase, the formation of numerous deformation 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 D0(22). TiAl3 phase occurred inhomogeneously in the material via the formation of antiphase boundaries on (001) planes, resulting in an f.c.c. solid solution in the final state. In summary, it can be concluded that the formation of the observed metastable phases results from chemical disordering, whereas the excess enthalpy of grain boundaries plays only a minor role for the energetical destabilization of the intermetallic compounds during milling in this case. (C) 1997 Acta Metallurgica Inc.