Concentration of point defects in binary NiAl

A mean-field-type model was established to describe the concentration of point defects in binary NiAl by taking into account the effects of crystal expansion and off-stoichiometry. Different from some recent studies, this analysis shows that constitutional vacancies are present only in Al-rich NiAl at T = 0 K but that the vacancy concentration decreases with increasing temperature until diffusion is activated and thereafter becomes nearly independent of temperature. Such vacancy formation behaviour in Al-rich NiAl belongs to the category of metastable constitutional type according to a method of classifying vacancy formation in B2 crystals proposed in this letter.