A series of long-period superstructures (LPSs) formed in an Al-rich TiAl single crystal with a gradient composition has been observed by transmission electron microscopy. A Ti-54.7 at. percent Al single crystal was heat treated within molten Al at 1234 deg C, and a composition-gradient layer with an Al concentration from 55 to 75 at. percent was formed at the surface of the single crystal. The composition-gradient layer exhibits ordering into different LPSs depending on the Al concentration as follows: short-range order, h-Al_2Ti and one-dimensional antiphase domain structure. These LPSs have an alternate stacking of Al(002) and Ti-Al(002) layers and the ordering of the LPSs proceeds in the TiAl matrix, maintaining an orientation relationship in which Al(002) layers of the LPSs and the TiAl matrix are parallel to each other. The ordering of this type of LPS and the phase transition between these LPSs are thus interpreted as structural changes in Ti(002) layers of the Al-rich TiAl crystal. It was also observed that ordering of the orthorhombic h-Al_2Ti exhibits anisotropic features depending on the direction of the composition gradient. This suggests that the long-period ordering in the TiAl crystal depends on diffusion processes as well as the local concentrations of Ti and Al atoms.
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