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

Abstract

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. Some long-period superstructures appear accompanied by obscure coherent boundaries in the L1₀ matrix. It was found that the AL₅Ti′₃ structure distinguished from the Al₅Ti′₃ structure is composed of only a Ti′₃Al-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 Al₅Ti′₃ to h-Al₂Ti (ZrGa₂-type) through the imperfectly ordered Al₅Ti′₃ structure. Peculiar streaks of electron diffraction appeared during the transition between the AL₅Ti′₃ 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 L1₀ matrix.