Local structural order around dopant atoms in Fe- and Co-doped ternary alloys based on y′ -Ni₃Al

We have utilized x-ray absorption fine structure (XAFS) to study the local structural order of dopants in ternary alloys of the composition Ni₇₅(Fe/Co)_x Al_25-x In γ′ -Ni₇₅Al₂₅, the first coordination shell around Al consists of 12 Ni atoms, whereas Ni is surrounded by 8 Ni and 4 Al atoms. Cobalt absorbers exclusively occupy the Ni sublattice, yet the Co XAFS shows only Co-Ni bond contributions in the first peak of the Fourier transform, with the Al backscattering being either completely absent or so weak that it is masked by the strong Ni backscattering background. A model based on a highly distorted Al sublattice, created by dopant atoms occupying the Ni sublattice, gives a possible explanation for the absence of Al backscattering contributions, and furnishes an explanation for our results on Fe dopants in γ′ -Ni₃Al, as well as results from similar studies by others on doped γ′ nickel aluminides.