The microstructure and compression strengths of Ir-15 at. pct X (X ϭ Ti, Ta, Nb, Hf, Zr, or V) binary alloys at temperatures between room temperature and 1800 ЊC were investigated to evaluate the potential of these alloys for ultra-high-temperature use. The fcc and L1 2 two-phase structures of these alloys were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The strengths of the Ir-Ta, -Nb, -Hf, and -Zr alloys were above 800 MPa at temperatures up to 1200 ЊC and about 200 MPa at 1800 ЊC. The strengths of these alloys under 1000 ЊC are equivalent to or higher than those of the commercially used Ni-base superalloys, MAR-M247 and CMSX-10. The Nb concentration dependence of strength was investigated using a series of Ir-Nb alloys with Nb concentrations from 0 to 25 at. pct. It was found that the Ir-base alloys were strengthened by L1 2 precipitation hardening. The potential of the Ir-base alloys for ultra-high temperature use is discussed.

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Beyond Ni-Based Superalloys OverviewThis article presents an overview of publications on mechanical properties of chromium and chromium-based alloys, with particular emphasis on ductility at low temperature and strength at high temperature. Analysis of rather scattered data suggests that a chromium or chromium-based alloy can be ductilized at ambient temperature and is quite capable of being strengthened to high levels at high temperature. A new composition design and process would open new opportunities for chromiumbased alloys as structural materials used at temperatures up to 1,300°C.

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultrahigh-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1 2 two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria-fcc, Ir 3 Nb-L1 2 , and Ni 3 Al-L1 2 -existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 ЊC; at 1300 ЊC, three phase equilibria-fcc, Ir 3 Nb, and Ni 3 Al-existed in alloys A and C and four-phase equilibria-fcc, Ir 3 Nb, Ni 3 Al, and IrAl-B2-existed in alloy B. The fcc/L1 2 coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 ЊC, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-hightemperature use is also discussed.