An X-Ray Test of Superstructure in FeNi3

A search has been made for superstructure in permalloy of composition near Fe${\mathrm{Ni}}_3$. Curves of x-ray scattering power near the absorption edges of Ni and Fe show that Fe $K\beta$-radiation should give the strongest superstructure lines, and calculations indicate that reflection from (321) planes of a perfectly ordered structure should have about one-seventh the intensity of the ordinary (222) reflection. The specimen was a powdered alloy containing 70 percent nickel, annealed at 1000°C and baked at 425°C. It was placed in a focusing camera in a beam of Fe $K\beta$-rays which were selected and concentrated by a curved rocksalt crystal, and exposed for one hundred hours. No superstructure lines appeared, although superstructure in $\beta$-brass was easily detected with similar technique. This negative result, which is supported by a considerable amount of indirect evidence, indicates that no long range order exists in Fe${\mathrm{Ni}}_3$.