Strengthening of γ-TiAl-Nb by short-range ordering of point defects

The origin of the superior high temperature strength of Ti-rich γ-TiAl with addition of Nb is a highly controversial issue. In this paper, we calculated the interaction energies between point defects in these alloys using a first-principles plane-wave pseudopotential method. It is found that, the nearest neighbouring Nb atoms on Al-sublattice (NbAl) repel whereas the next nearest neighbouring NbAl atoms attract each other, indicating that NbAl atoms may form short-range ordered structure. Based on this finding, we propose that the short-range order may be responsible for the high strength of the Ti-rich TiAl-Nb alloys.

► With low Nb concentration, Nb atoms occupy solely Ti sublattice in gamma-TiAl, however, with high Nb concentration in Ti-rich TiAl, Nb may take Al-sublattice due to the strong attraction between the next nearest neighboring Nb atoms on Al-sublattice.
► The short-range ordering of the Nb atoms on Al-sublattice due to their nearest neighboring repulsion and next nearest neighboring attraction may be critical for attaining the high strength of gamma-TiAl-Nb alloys.
► The addition of Nb strengthens the Al deficient gamma-TiAl alloys instead of the Ti deficient ones as demonstrated by experiments is because of the weak interaction between Nb atoms on Ti sublattice in Ti deficient alloys.