The key point in this part of work is the novel method to develop new quaternary refractory superalloys for ultra-high temperature uses. As we known, the g-phase is formed with a coherent interface in Ni-based superalloys. The coherent structures become obstacles to dislocation movement, and are therefore one of the important features of Ni-based superalloys that allow them to maintain sufficiently low creep rates at high temperatures.
We propose a novel method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys by mixing two types of binary alloys, Ir-20at.%Nb and Ni-16.8at.%Al, which contain fcc/L12 two-phase coherent structures. Platinum-group metals have been considered because of their higher melting temperatures (Ir: 2447 C) and superior oxidation resistance. The objective is to combine the high-temperature strength of Ir-based alloys with the high ductility, low density (about 8.5g/cm3, compared to 22.65g/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, Ir-based : Ni-based = 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 structure is both desirable and expected.
Figure 1.
For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibrium---fcc, Ir3Nb-L12, Ni3Al-L12---existed in Ir-5Nb-62.4Ni-12.6Al (at.%) (Alloy A), Ir-10Nb-41.6Ni-8.4Al (at.%) (Alloy B) and Ir-15Nb-20.8Ni-4.2Al (at.%) (Alloy C) at 1400C; at 1300C, three phase equilibrium---fcc, Ir3Nb and Ni3Al---existed in alloy A and C and four-phase equilibrium--- fcc, Ir3Nb, Ni3Al and IrAl-B2 existed in alloy B (Figure 2). Figure 3 shows the two kinds of coherent structure, fcc/Ir3Nb and fcc/Ni3Al, taken from another alloy Ir-3.1Nb-68.25Ni-6.75Al (at.%) by using transmission electron microscopy (TEM). At a temperature of 1200 C, the compressive 0.2% flow stress of these quaternary alloys was between 130 and 350MPa (Figure 4), which was higher than that of commercial Ni-based superalloys, such as MarM247 (50MPa), and lower than that of Ir-based binary alloys (500MPa). For reference, the compressive 0.2% flow stress of quaternary Ir-Ta-Ni-Al, which prepared by combing Ir-Ta and Ni-Al binary alloys, were also plotted in Figure 4. Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved.
Figure 2.
Figure 3.
Figure 4.