Site selectivity and bonding in the β-phase aluminides: Studies of RuAl, PdAl, and Pd and Ru dopants in NiAl

We have determined the site selectivity of Ru and Pd dopants in β-phase NiAl. For both transition metal rich and poor compositions, the Ru or Pd dopant goes to the Ni sublattice. The local structural disorder introduced by Pd dopants is significantly greater for the Al-rich than for the TM-rich composition. We have also determined the temperature dependence of the x-ray absorption fine structure Debye-Waller term for Ru atoms in stoichiometric RuAl, Ru in NiAl, and Pd in NiAl. We find evidence for large local shear resistance for Ru dopants in NiAl relative to Pd dopants in NiAl. These results are discussed in terms of calculated elastic moduli and cohesive energies of PdAl and RuAl.