The ordering mechanism of long-period superstructures (LPSs) in Al-rich Ti-Al alloys was studied using a TiAl single crystal with a composition gradient. A TiAl single crystal with gradient compositions from 55 to 75 at.% Al was prepared by annealing in a molten Al at 1234°C. The single crystal exhibits long-period ordering into different LPSs depending on the Al concentration as follows: an Al5Ti3 type short-range order, h-Al2Ti and one-dimensional antiphase domain structures. These LPSs show an orientation relationship in which Al (002) layers of the LPSs are parallel to those of the TiAl matrix. The atomic arrangements of the LPSs are characterized in common as the alternate stacking of the Al (002) layers and Ti-Al (002) layers. It is thus concluded that the ordering of this type of LPSs and the phase transition between these LPSs are explained as structural changes in Ti-Al (002) layers of the Al-rich L10-TiAl crystal.
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