Ir-base refractory superalloys for ultra-high temperatures
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I.
INTRODUCTION
THE temperature capabilities of Ni-base superalloys have been improved by more than 300 7C over the last 50 years[1] and are approaching 1100 7C, with single crystal superalloys having ideally designed microstructures composed of fcc and L12 phases.[2] In spite of these efforts, however, it is fair to say that a drastic improvement in the temperature capability of superalloys is becoming more difficult due to the rather low melting temperature of Ni, 1453 7C. Considering the ever-increasing demands for materials with higher temperature capabilities for use in gas turbines with higher efficiency, it is of vital importance to seek new alloys to substitute for Ni-base superalloys. An approach to develop new alloys with capabilities beyond those of Ni-base superalloys is being made with intermetallics or refractory alloys, for example, NiAl-base alloys strengthened by coherent Ni2AlTi precipitates,[3] Wbase HfC dispersion hardening alloys,[4] and Nb-base alloys precipitation hardened by Nb3Al.[5] However, problems remain with the poor room-temperature ductilities of intermetallics and the poor oxidation resistance of refractory alloys. As an alternative, platinum group metals are now being considered. From the point of view the intermetallics, AlRu and RuSc with the B2 structure, IrNb and RuTa with the L10 structure,[6] and Ir3Nb and Ir3Zr[7] with the L12 structure have been noted. We have proposed a new class of alloys using platinum group metals, which we term ‘‘refractory superalloys.’’[8,9] We define this new concept as alloys with fcc and L12 two-phase coherent structures similar to those in Ni-base superalloys, and yet with considerably higher melting temperatures, since it is well known that L12 singlephase alloys, as well as fcc single-phase alloys, are less resistant to creep deformation compared with the fcc and L12 two-phase alloys in Ni-base superalloys.[10] Platinum group metals are ideal as base elements in refractory suY. YAMABE-MITARAI, Researcher, Y. RO, Senior Researcher, and H. HARADA, Group Leader, are with the 3rd Group, National Research Institute for Metals, Ibaraki 305, Japan. T. MARUKO, Engineer, is with the Furuya Metals Co. Ltd., Ibaraki 308, Japan. Manuscript submitted February 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
peralloys due to their melting temperatures (Os: 3045, Ir: 2443, Ru: 2250, Rh: 1960, Pt: 1769, and Pd: 1552 7C, all higher than that of Ni: 1453 7C[11]) and superior oxidation resistance to refractory metals, i.e., Nb, Mo, Ta, and W. Of the platinum group metals, Ir, Rh, Pt, and Pd have the fcc structure, while Os and Ru have the hcp structure. In the Ir-Nb (Figure 1) and Ir-Ti systems, the fcc structure can be equilibrated with the L12 structure according to binary phase diagrams.[12] The compression strengths of these alloys at 1800 7C were previously investigated.[8] The strengths of these alloys were about 200 MPa at 1800 7C. These data show that Ir-base alloys have a good potential as materials for use at ultra-high temperatures for which Ni-
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