Formation and stability of transitional long-period superstructures in Al-rich Ti-Al single crystals

The phase transition and thermal stability of long-period superstructures were examined in Ti-58.0 at.% Al. Ti-60.0 at.% Al and Ti-62.5 at. %Al single crystals focusing on reflections based on sets of lattice planes parallel to {310)(fcc). come long-period superstructures appear accompanied by obscure coherent boundaries in the Ll(o) matrix. It was found that the Al5Ti3' structure distinguished from the Al5Ti3 structure is composed of only a Ti3Al-type unimolecular cluster with the shape of fat rhombus on the Ti(002) layers. As the annealing temperature and Al concentration rose, transition of the long-period superstructures proceeded from the Al5Ti3 to h-Al2Ti (ZrGa2-type) through the imperfectly ordered Al5Ti3' structure. Peculiar streaks of electron diffraction appeared during the transition between the Al5Ti3' and h-Al2Ti structures, suggesting the coexistence of at least two types of microcluster with fat and lean rhombi shapes corresponding to fourfold and threefold more periodicity respectively than the a axis of the Ll(o) matrix.