Phase equilibria in prototype Nb-Pd-Hf-Al alloys
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8/8/03
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Phase Equilibria in Prototype Nb-Pd-Hf-Al Alloys A. MISRA, R. BISHOP, G. GHOSH, G.B. OLSON 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 (-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.
I. INTRODUCTION
THE operating temperature currently limits the efficiency of turbine engines, prompting a great deal of research in an effort to design new high-temperature alloys. Nickel-based superalloys are nearing their operating limit as they approach their melting point, thus triggering the need for new alloys that can operate at around 1300 °C. Ideally, such an alloy should have a substantially higher melting point (or low homologous temperature (0.5Tm) at 1300 °C), and at the operating temperature, it should have high creep strength and high oxidation resistance. For oxidation resistance, the material should be capable of passive oxidation, making it capable of forming a protective oxide scale. The refractory metal niobium has a melting temperature of 2467 °C and also has a low density, thus making it an attractive candidate for replacement of nickel.[1] However, Nb has poor oxidation resistance[2] and only moderate strength at high temperatures.[3] A systems-based approach[4] is underway to develop a new niobium-based alloy for usable performance at 1300 °C or above. The principles of strengthening in classical / Nibased superalloys[5] is extended to design a Nb-based superalloy strengthened by an ordered bcc aluminide phase. In bcc alloys, such strengthening can be achieved by precipitating either B2 or L21 (Heusler) phase.[6,7] The ordered intermetallic needs to have high thermodynamic stability to promote microstructural stability at high temperatures. Furthermore, to maintain coherency over a long period of time and to obtain a uniform distribution of the precipitates, the lattice mismatch should be very small (preferably 0.1 pct). Quantum mechanical total energy calculation has been carried out using the full potential–linear muffin–tin orbital (FLMTO) method to
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