Local atomic order and individual pair displacements of Fe46.5Ni53.5 and Fe22.5Ni77.5 from diffuse x-ray scattering studies.

Abstract

We have recovered the local atomic structures in a fcc ${\mathrm{Fe}}_{46.5}$${\mathrm{Ni}}_{53.5}$ solid solution near an equilibrium temperature of 600 \ifmmode^\circ\else\textdegree\fi{}C with an improved version of the recently demonstrated three x-ray energy (3\ensuremath{\lambda}) method. We have also reevaluated the 3\ensuremath{\lambda} data from a ${\mathrm{Fe}}_{22.5}$${\mathrm{Ni}}_{77.5}$ crystal which significantly changed the results. Both the Warren-Cowley short-range order parameters \ensuremath{\alpha} and the first moment of the mean static displacements for Fe-Fe, Fe-Ni, and Ni-Ni pairs are recovered. Comparisons of the \ensuremath{\alpha}'s from ${\mathrm{Fe}}_{22.5}$${\mathrm{Ni}}_{77.5}$ with neutron-scattering results show good agreement, and both compositions have \ensuremath{\alpha}'s with a strong similarity to those from ${\mathrm{Cu}}_{3}$Au quenched from above the ordering temperature. Displacements from the average lattice of less than 0.001 \AA{} can be recovered. For both ${\mathrm{Fe}}_{46.5}$${\mathrm{Ni}}_{53.5}$ and ${\mathrm{Fe}}_{22.5}$${\mathrm{Ni}}_{77.5}$ the average Fe-Fe first-neighbor distance of 2.564(5) \AA{} is much larger than the average derived from the lattice spacing for the alloys and is independent of concentration. However, the Ni-Ni, first-neighbor distance follows the average lattice, and the Fe-Ni first-neighbor distances are smaller than average. Both the Ni-Ni and Fe-Ni first-neighbor distances increase with Fe concentration. The relatively large and strongly chemically specific bond distances observed here may be a more meaningful concept for understanding the magnet properties of these Ni-rich alloys than the relative lesser change in volume of the Fe atom. \textcopyright{} 1996 The American Physical Society.