Neutron powder diffraction measurements were carried out  in situ, at temperatures ranging from ambient to about 1400C, on several B2 alloys in order to study the formation of vacancies and anti-structure atoms at high temperatures. It was observed for the first time that both Al vacancies and Al anti-structure atoms formed in appreciable numbers, at temperatures above about 1000C, in nearly stoichiometric alloys. The observed compositional and temperature dependences of these point defect concentrations could be understood qualitatively within the framework of the atomic pair bonding model if it were assumed that the bond energies between Ni-Ni and Al-Al were sensitive functions of the atomic separations.

S.M.Kim, Y.Takeda, M.Kogachi: Scripta Materialia, 1996, 34[12], 1845-50