The formation of ordered domains during crystal growth of γ-TiAl

The growth of single crystals of γ-TiAl by a floating zone method is described. It is not possible to grow crystals of the stoichiometric composition from the melt so all single crystals are aluminium rich. The composition range that gives good quality crystals is very limited. Although it is relatively easy to grow γ crystals if the composition is about 56a/0 Al, on cooling the crystals appear to undergo additional short-range ordering of the excess Al atoms which makes them less suitable for deformation studies. At the other extreme, alloys containing <50a/0 Al, including the public domain alloy 48a/0 Al 2 Mn 2a/0 Nb, form PST crystals. Binary alloys with 53.2a/0 Al appeared to give good crystals according to X-ray Laue photographs but always contained a domain structure, i.e. [110] crystals contained regions with a [011] growth direction. With >54a/0 Al it is possible to suppress domain formation by careful control of the growth conditions. A mechanism for domain formation is suggested and the results have implications for the phase diagram.