Long range order and defect concentrations in NiAl and CoAl

Abstract

X-ray diffractometry has been used to determine both the concentrations of vacancies and the fractions of lattice sites occupied by anti-site atoms in both CoAl and NiAl as a function of aluminum concentration. It is shown that both compounds can achieve the maximum possible order both at their stoichiometric compositions and when Al-poor. However, Al-rich compositions are less ordered than possible, indicating the presence of constitutional vacancies, a feature consistent with the maxima in the lattice constants at the stoichiometric compositions. Assuming that both constitutional vacancies and Al anti-site atoms exist, it is shown that in Co-52 Al, 2% of the Co-sites were occupied by vacancies and 3% by Al atoms, whilst in Ni55Al, the vacancies and anti-site atoms occupied 7 and 6% of the Ni sites, respectively. Thus, it is shown that CoAl has similar defect characteristics to NiAl.