The nature and occurrence of the point defects in L12 ordered alloys were studied by using 2 stoichiometric alloys, with 9 or 12at%Cr, and 2 non-stoichiometric alloys with 10at%Cr, and 23 or 27at%Ti. Measurements of the lattice parameter, density and composition were combined so as to calculate the average number of atoms per unit cell. No variation in that ratio was detected as a function of alloy composition; thus suggesting that the point defects in the ordered phases were antisite defects rather than constitutional vacancies. An absence of constitutional vacancies was further supported by comparing measured and calculated superlattice peak intensities. The usual method of regression-fitting calculated and measured X-ray diffraction intensities at a single wavelength was incapable of unambiguously defining the 2 independent order parameters; thus casting doubt on the results of previous studies on these ternary alloys. Only if the assumption were made that 1 type of site substitution predominated in antisite formation, could the method be used to estimate the solution for the dominant behavior. Assuming that 1 type of substitution dominated antisite formation on ß-sites, the present measurements suggested that a small amount Cr, but not Al, substituted at ß-sites and that most of the Cr was on a-(Al) sites. Increasing the Cr content of stoichiometric alloys caused more Cr to occupy ß-sites, thus creating more antisite defects. The Ti deficiency in hypostoichiometric alloys was alleviated by an enhanced substitution of Cr at ß-sites. However, excess Ti in hyperstoichiometric alloys greatly decreased Cr substitution at ß-sites.

Site Occupancy in Ternary L12 Ordered Alloys as Determined by Diffraction. J.P.Nic, D.E.Mikkola: Intermetallics, 1999, 7[1], 39-47