First-principles investigations were made of the structural stability of all experimentally observed ordered long-period superstructures: r-Al2Ti, h-Al2Ti, Al5Ti3, together with Al5Ti3, Al11Ti7 and Al3Ti2 long-period superstructures which were observed only as short-range ordered clusters at the nanoscale level in Al-rich TiAl-based alloys. A procedure based upon space-filling tiling with ordered Ti2Al, Ti3Al, Ti4Al motifs and their combination – together with a symmetry analysis - was used to determine unit-cell and crystallographic information on Al5Ti3, Al11Ti7 and Al3Ti2 long-period superstructures in terms of the L10 fcc unit cell. Calculations of the single-crystal elastic constants of Al5Ti3, Al11Ti7, Al3Ti2 and Al5Ti3 long-period superstructures showed that all four structures were mechanically stable. Also calculated, using the super-cell approach, were the antiphase boundary formation energies of two types of boundary, type-A and type-C, in ordered Al5Ti3 long-period superstructures. The relaxed antiphase boundary energies for type-A and type-C were 15.44 and 124.16mJ/m2, respectively.

Ab initio Study of Long-Period Superstructures and Anti-Phase Boundaries in Al-Rich γ-TiAl (L10)-Based Alloys. P.S.Ghosh, A.Arya, U.D.Kulkarni, G.K.Dey, S.Hata, T.Nakano, K.Hagihara, H.Nakashima: Philosophical Magazine, 2014, 94[11], 1202-18