The present results showed that the creation of an Al vacancy occurred preferentially in the vicinity of the stoichiometric composition. The question of why Al vacancies were created in preference to Ni vacancies was considered by comparing Al-defect and Ni-defect models. The former model assumed that no Ni anti-structure defects formed, while the latter model assumed that no Al anti-structure defects formed. The defect concentrations were then given by:

xNi = 2cv - xAl, yNi = 0 and yAl = xAl - cv

yNi = xNi - cv, xAl = 2cv - xNi and yAl = 0

respectively, for the 2 models. In the former model, upon increasing the Al vacancy fraction (xAl) from 0.5 to 1.0%, the Al anti-structure defect density (yAl) also increased from 0 to 0.5% while the Ni vacancy fraction (xNi) decreased from 0.5% to 0. Similar changes occurred in the other model. The calculated results for the first model indicated a minimum in xAl at about 0.8%. It was concluded that the defect structure which gave preference to Al vacancies was more appropriate for this alloy.

M.Kogachi, T.Tanahashi, Y.Shirai, M.Yamaguchi: Scripta Materialia, 1996, 34[2], 243-8