Formation of point defects in TiAl and NiAl

A mean-field model is proposed to estimate the atom–atom and atom–vacancy bond energies of intermetallic compounds, with the results of first principles calculations as input. The variation of point defects with temperature and chemical composition estimated with the model agreed with experimental measurements for TiAl and NiAl alloys. In particular, off-stoichiometric effect considered in this treatment was found to increase vacancy concentration in Al-rich but decrease it in Ti-rich TiAl alloys. As a result, the differences in vacancy concentration between Al-rich and Al-lean TiAl alloys are larger than those reported in previous studies. For NiAl alloys, the consideration of off-stoichiometric influence yields lower vacancy concentration on the Al-rich side but higher concentration on the Ni-rich side. The present model suggests that constitutional vacancies are present, but anti-site defects are forbidden in Al-rich NiAl at 0 K. For concentrated Al-rich NiAl, the vacancy concentration has its maximum at 0 K, and gradually decreases with rising temperature until diffusion is activated. The reason why constitutional vacancies are more difficult to form at low temperatures in TiAl than in NiAl is discussed.