Site selectivity and bonding in the beta-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 beta-phase NiAl. For both transition metal rich and poor compositions, the Ru or Pd do pant 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 c omposition. 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 r esistance for Ru dopants in NiAl relative to Pd dopants in NiAl. These resu lts are discussed in terms of calculated elastic moduli and cohesive energi es of PdAl and RuAl.