Phase Equilibria in Prototype Nb-Pd-Hf-Al Alloys

The phase equilibria in two prototype alloys with nominal compositions 60Nb-20Pd-10Hf-10Al and 40Nb-30Pd-15Hf-15Al (in at. pct) are investigated using scanning electron microscopy and X-ray diffraction. The alloys were heat treated at 1200 °C and 1500 °C for 200 hours each. The phase analysis revealed that the alloys were, for the most part, in the three-phase equilibrium between (Nb), Pd2HfAl, and Pd3Hf. The compositions of these three phases along with other observed phases such as PdAl and (a-Hf) provide important data for establishing the Nb-Pd-Hf-Al quaternary phase diagram. A preliminary Nb-Pd-Hf-Al phase diagram, with pertinent tie-tetrahedra, was constructed based on the available composition data. The lattice parameters of (Nb), Pd2HfAl, Pd3Hf, and the coefficient of thermal expansion of Pd2HfAl were measured, and models were developed to predict the composition dependence of the mean atomic volumes/lattice parameters of (Nb) and Pd2HfAl and the temperature dependence of the lattice parameter of the (Nb) phase. The validity of the models was confirmed by good agreement between predicted and experimental values.