Phase orientation relationships in the TiAl–TiAl2 region

The microstructures of three alloys of nominal compositions of Ti–60 at.% Al, Ti–62.5at.% Al and Ti–64 at.% Al were studied by transmission electron microscopy after the alloys were annealed at 1200, 1000, 800 and 600°C. Four phases based on the face-centered cubic lattice, designated as TiAl, Ti3Al5, TiAl2S (S for short period) and TiAl2L (L for long period) were confirmed. Their crystallographic orientation relationships (ORs) were studied in detail. The stability ranges of these phases are briefly outlined as well. It is found that the observed ORs are independent of the annealing procedures and can approximately be regarded as ‘cubic-to-cubic’ relationships. The structural connection for the observed ORs is that the planes containing pure Al layers are parallel to each other. The possible mechanisms leading to these ORs are discussed with a consideration of atomic tiling of the AlnTim planes.