High-Temperature Strength of Ir-Based Refractory Superalloys
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High-Temperature Strength of Ir-Based Refractory Superalloys
Yoko Yamabe-Mitarai High-Temperature Materials 21st Project (HTM21), National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
ABSTRACT Ir-based refractory superalloys with an fcc and L12 two-phase structure similar to that in Ni-based superalloys but with considerably higher melting temperatures are proposed. First, the microstructure and strength are shown for several kinds of binary alloys. Then, the effect that the third element and the combination of Ni-Al with an fcc and L12 two-phase structure in binary alloys had on the lattice misfit, microstructure, and strength was investigated. The precipitation-hardening effect in Ir-based alloys was investigated and discussed in terms of precipitate morphology and lattice misfit. The creep properties of Ir-based alloys were also investigated. INTRODUCTION Interest in pure Ir and Ir-based alloys has increased during the past few decades. In March 2000, the International Symposium of Ir was held [1]. There, the mechanical properties, structures, oxidation behavior, fabrication, processing, refining, and chemistry of Ir and Irbased alloys were presented. As application fields, medical implants, ignition devices, electrodes, thermometers, and combustion chambers for rocket propulsion were suggested in addition to the classical applications of these alloys as crucibles and catalysts. In Japan, Ir demand has increased since 1995 due to its increasing variety of applications. Ir is used for automobile catalysts, electrodes in the chemical industry, crucibles, thermocouples, and rocket parts [2]. At the beginning of 2000, a symposium concerning platinum group metals and alloys was held in Tokyo. Here, in addition to the above applications, new attempts, structural materials, coating, and ferroelectric memory capability (so-called F-RAM) were presented [3]. Ir is used in many application fields because it has high melting temperature (2447°C), the most stable element for corrosion [4], a modulus with the highest elasticity (570GPa) at room temperature [5], and the second-highest elevated-temperature strength [6]. On the other hand, the disadvantages of Ir are its highest density (22.7 g/cm3), brittleness (unlike typical fcc metals), and limitation of supply in the world (4 tons/year). Investigation of the mechanical properties of Ir was started in 1960 and showed that polycrystalline Ir cleaves at grain boundaries similarly to brittle materials after insignificant preliminary deformation but unlike typical fcc metals [6]. On the other hand, the mechanical behavior of a single crystal of Ir, which was shown first in 1961 [7], differs from that of brittle materials. Generally, brittle materials cannot be deformed plastically in either polycrystalline and single forms. However, Ir single crystal showed large elongation (80%) at room temperature; moreover, it showed brittle transcrystalline fractures [8]. This suggests that Ir has both a brittle and a ductile manner. Thus, the fracture behav
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