Novel ordering transitions in off-stoichiometric γ-TiAl intermetallics and the similarities between these and the special point ordering processes in Ni-Mo alloys

Ordering on Ti-rich (002) planes of Al-rich gamma-TiAl (L1(0)) phase can be described in terms of evolution of superlattice structures consisting of space filling arrangements of two-dimensional (2D) motifs of different shapes. These 2D structural units (tiles) comprising unimolecular clusters of Ti2Al, Ti3Al and Ti4Al stoichiometries and having the shapes of lean and fat rhombi and squares respectively, either singly or in combinations, constitute the building blocks of this variety of superlattice structures. The Al5Ti3 and the Al11Ti7 derivatives of the off-stoichiometric gamma-TiAl phase, for instance, can be viewed as identical periodic arrangements of lean rhombi and squares and fat rhombi and squares, respectively. Previous investigations, involving projections from higher dimensional lattices, have shown that these two rather simple structures can be regarded as "unmodulated" periodic approximants of a hypothetical quasiperiodic superlattice (QPSL) structure and that these can transform continuously into the QPSL through an infinitum of long period antiphase boundary (LPAPB) modulated structures. Structures, which identify themselves with some of the intermediate stages of the periodic to quasiperiodic transformation, have been observed in some investigations including the present one. Periodic arrangements of the three types of tiles have also been used to describe the nature of Short Range Order (SRO) and various Long Range Ordered (LRO) phases in Ni-Mo and other f.c.c. alloys which show SRO maxima at 11/20 and equivalent points in the reciprocal space. A remarkably similar state of SRO, albeit, with SRO maxima at incommensurate positions, has been observed in this transmission electron microscopy investigation on rapidly solidified Al60Ti40 and Al65Ti35 alloys. Occurrence of curved streaks of diffuse intensity during the early stages of the evolution of some of the Long Range Ordered (LRO) phases is another point which underscores the similarity between the two ordering systems. The results of the present investigation lend additional support to the hypothesis of continuous periodic to quasiperiodic superlattice transformations. (C) 1998 Acta Metallurgica Inc.