DETERMINATION OF LONG-RANGE ORDER AND VACANCY CONTENT IN THE NIAL BETA'-PHASE ALLOYS BY X-RAY-DIFFRACTOMETRY

The long range order in the beta'-phase alloys Ni1-cAlc (0.46 less-than-or-equal-to c less-than-or-equal-to 0.54) with L2(0)-type structure was investigated by powder X-ray diffractometry. Integrated intensities for eight reflections distributed over a wide angle range were measured and a least-squares calculation was performed based on the profile fitting method. In the Al-rich side, the concentration of vacancies and their preferred lattice site were determined. The results were summarized as: (1) the atomic arrangement on the Ni-rich side attained practically the maximum ordered state; (2) the vacancy content on the Al-rich side increased linearly with Al content; (3) the vacancies preferentially occupied the Ni-sites (with a probability of about 3/4), while a major portion of Ni and Al atoms occupied their own sites. These conclusions were supported by performing a Rietveld analysis for all the samples.