Local Structure of Metallic Alloys: Pair-Distribution-Functions For Pu-2at.%Ga From Neutron Powder Diffraction Data. George H. Kwei, Materials Science and Technology Division,Los Alamos National Laboratory, Los Alamos, NM 87545

As part of a program to study structural instabilities in metallic alloys, we have determined atomic pair-distribution functions (PDF) for Pu-2at.%Ga over a wide temperature range. The powder neutron diffraction patterns clearly indicate that the sample is in the fcc d-phase. Several features are notable: (1) the peaks are extremely sharp, indicating that the long-range structure must be pretty well ordered, (2) there is some diffuse scattering evident at the base of some of the peaks which suggests that there must be some disorder despite the sharp peaks, and (3) the presence of approximately 2% 239Pu results in an absorption that needs to be corrected in the data analysis. We find that sharp pair-correlation peaks in the transformed pair distribution functions (PDF) extend well past the transformation range and that the measured PDFs agrees moderately well with the PDFs calculated for fcc Pu-2at.%Ga based on the crystallographic structure. This suggests that the local structure is reasonably well described by the crystallographic structure and that the structure remains fairly well ordered to large length scales. However, significant deviations in peak height and position are observed in the measured PDF, when compared with the fcc model; these suggest that at a more detailed level there must be interesting differences in the local structure that may arise either from disorder or from anisotropic thermal motion. Results from modelling the local structure will be presented.

This work was done under the auspices of the US Department of Energy under contract W-7405-ENG-36 to the Los Alamos National Laboratory.