Site occupancy in ternary Ll < inf> 2 ordered alloys as determined by diffraction: Observations on (Al,Cr) < inf> 3 Ti alloys

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The nature and occurrence of point defects have been studied in Ll2 ordered (Al,Cr)3Ti alloys. Four compositions were investigated, two stochiometric alloys with either 9 or 12 at. pet. Cr, and two nonstoichiometric alloys with 10 at. pet. Cr and either 23 or 27 at. pet. Ti. Measurements of lattice parameter, density, and composition were combined to calculate the average number of atoms per unit cell. No variation in this quantity was detected with alloy composition, suggesting that the dominant point defects in these ordered phases are antisite defects rather than constitutional vacancies. A lack of constitutional vacancies was further supported by comparisons of measured and calculated superlattice peak intensities. The common method of regression fitting calculated and measured X-ray diffraction intensities using a single wavelength is shown to be unable to define the two independent order parameters unambiguously, calling into question the results of such previous studies on these ternary alloys. Only if an assumption is made that one type of site substitution dominates the antisite formation can the method be used to estimate the solution for the dominant behavior. Assuming that one type of substitution dominates the antisite formation on β (Ti) sites, the measurements performed in this study suggest a small amount of Cr, but not Al, substitution on β sites, and that most of the Cr is on α (Al) sites. Increasing the Cr level in stoichiometric alloys causes more Cr to occupy β sites, displacing some Ti to α sites, thus creating more antisite defects. The Ti deficiency in hypostoichiometric alloys is alleviated by enhanced substitution of Cr on β sites. However, the excess Ti in hyperstoichiometric alloys greatly decreases any Cr substitution on β sites. © 1998 Elsevier Science Ltd. All rights reserved.

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