On the structure of the Ti3Al phase in Ti-Al and Ti-Al-Nb alloys

The present work describes the calculation of the atomic shift of titanium atoms (6(h) Wyckoff positions) in Ti-20Al, Ti-25Al, Ti-30Al and Ti-45Al-5Nb alloys (all atom%) in the α2-phase using a Rietveld refinement procedure. The Ti atoms are slightly shifted from their regular cph atomic sites. This is related to the presence of symmetry elements in the α2-phase with D019 structure, which in turn allows local shifts due to different site occupancies resulting in different atomic potentials. The shift in the Ti positions changes the Ti-Ti and Ti-Al first neighbor bond lengths within and out of plane and also the size of tetrahedron.