Local Structure of L1_2-Ordered Ni_75(Al_1-xFe_x)_25 Alloys

We present an Fe and Ni K-edge extended x-ray-absorption fine structure study of the L1_2-ordered ternary alloy Ni_75(Al_1-xFe_x)_25. We find that Fe enters predominantly the Al sublattice of Ni_3Al(γ' phase). The distribution of Fe and Al atoms on this sublattice follows the expected distribution for an L1_2 structure. The Fe nearest-neighbor distances 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 (≈0,2A) decrease of the Ni-Al distance with increasing Fe content is detected, the Ni-Al bond being also characterized by a large disorder. This result provides a local structural explanation to the large lattice strains recently measured in similar samples by x-ray diffraction.