LOCAL-STRUCTURE OF L1(2)-ORDERED NI75(AL1-XFEX)25 ALLOYS

We present an Fe and Ni K-edge extended x-ray-absorption fine structur e study of the L1(2)-ordered ternary alloy Ni75(Al1-xFex)25. We find t hat Fe enters predominantly the Al sublattice of Ni3Al (gamma' phase). The distribution of Fe and Al atoms on this sublattice follows the ex pected distribution for an L1(2) structure. The Fe nearest-neighbor di stances are consistent with the measured lattice parameters. The first coordination shell around Ni is composed of Ni, Fe, and Al atoms. The Ni-Ni and Ni-Fe bond lengths are consistent with diffraction data. On the other hand, an unexpected strong (almost-equal-to 0. 2 angstrom) decrease of the Ni-Al distance with increasing Fe content is detected, the Ni-Al bond being also characterized by a large disorder. This res ult provides a local structural explanation to the large lattice strai ns recently measured in similar samples by x-ray diffraction.