Design of quaternary Ir-Nb-Ni-Al refractory superalloys
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THE temperature capability of Ni-based superalloys has been improved by more than 300 8C in the last 50 years[1] and is approaching 1100 8C for single-crystal superalloys consisting of g-fcc and g 8-L12 phases.[2] In Ni-based superalloys, the g 8 phase is formed with a coherent interface. The coherent structures become obstacles to dislocation movement and are therefore one of the important features of Nibased superalloys that allow them to maintain sufficiently low creep rates at high temperatures. Many attempts have also been made to develop alloys superior to Ni-based superalloys, by using intermetallic and refractory alloys.[3,4] Platinum-group metals have been considered because of their higher melting temperatures (Ir: 2447 8C) and superior oxidation resistance.[5,6] We propose a new class of superalloys using platinum group metals, called “Refractory Superalloys.”[7,8] Our experimental results show that Ir-based refractory superalloys have fcc/L12 coherent structure and have a superior high-temperature strength compared to Nibased superalloys. However, the density and expense of Irbased alloys are much higher than those of Ni-based superalloys. The ductility of Ir-based refractory superalloys must also be improved. Increased expense is never desirable, and increased density is particularly undesirable for applications in rotating equipment where inertial stresses are increased and also in airborne equipment due to reduced carrying capacity. II. ALLOY DESIGN Based on the known properties of Ir- and Ni-based superalloys, we developed a series of quaternary Ir-Nb-Ni-Al alloys, which have superior strength compared to Ni-based
alloys and superior properties compared to Ir-based alloys. The objective was to combine the high-temperature strength of Ir-based alloys with the high ductility, low density (about 8.5 g/cm3, compared to 22.65 g/cm3 for Ir), and relatively low cost of Ni-based alloys. Figure 1 shows a sketch of a portion of the Ir-Nb-Ni-Al quaternary phase equilibrium diagram. Initially, two binary alloys (Ir- and Ni-based binary alloys, indicated by I and N, respectively) were mixed to prepare quaternary Ir-Nb-Ni-Al alloys (for example, Irbased:Ni-based 5 25:75, 50:50, and 75:25 are indicated as alloys A, B, and C, respectively). It is expected that fcc/L12 regions exist over the entire range of mixture ratios, from pure Ni-based to pure Ir-based alloys. In particular, the coexistence of the fcc/L12-Ni3Al and fcc/L12-Ir3Nb coherent structures is both desirable and expected. The fcc/L12 coherent structures are required for high strength at high temperature. The coherent structures and high melting temperatures of Ir-based alloys give the quaternary alloys useful hightemperature strength. A decrease in density and cost are achieved by using Ni-based alloys. From the available Irbased binary alloys, we selected the Ir-Nb binary alloy because of its high strength at high temperature (over 500 MPa at 1200 8C).[8] For the Ni-based alloy, the Ni-Al alloy was selected because it contained fcc/L12 coherent
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