The microstructure and compression strengths of Ir-15 at. pct X (X=Ti, Ta, Nb, Hf, Zr, or V) binary alloys at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. The fcc and L12 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 °C and about 200 MPa at 1800 °C. The strengths of these alloys under 1000 °C 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 L12 precipitation hardening. The potential of the Ir-base alloys for ultra-high temperature use is discussed.

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Ir-base refractory superalloys for ultra-high temperatures

Elastic constants and moduli of face-centered cubic Ir and its L12 intermetallic compounds Ir3X (X=Ti, Ta, Nb, Zr, Hf, V) have been determined using ab initio density functional theory calculations within the generalized gradient approximation. With the tetragonal, trigonal, and isotropical lattice distortions, elastic constants C11, C12, C44, and bulk modulus B are derived from the second derivative of the total energy as a function of volume. The calculated Young’s modulus E, shear modulus G, Poisson’s ratio ν, and the ratio RG/B of G over B are then used to examine mechanical properties of Ir and Ir3X compounds. By analyzing RG/B and Cauchy pressure C12–C44, the brittle-ductile behavior of the materials is assessed. Based on the modulus difference ΔG between the γ matrix (Ir) and γ′ precipitates (Ir3X), the γ′ strengthening effect in the γ matrix is studied.

Ab initio study of elastic properties of Ir and Ir3X compounds

Ni-based superalloys are the dominant material class in the hot section of gas turbines. This is so, because they uniquely combine three key properties at a favourable performance to cost ratio: toughness, strength at elevated temperature and oxidation resistance. However, there is also an important limitation. Given gas temperatures in excess of 1500 °C and peak material temperatures of up to 1100 °C, the latter are already very close to the melting point of superalloys at around 1300 °C. It is very likely that material development will continue to push the maximum allowable service temperature of superalloys upwards. However, there is a limit and given the long term nature of material development, it is essential to search now for solutions beyond the temperature capability of Ni-based superalloys. For this reason, substantial international research is underway, focussing on ceramics, intermetallic materials such as NiAl, refractory metals such as Nb-based and Mo-based alloys, as well as noble metals mimicking the c/c′-microstructure of superalloys. The difficulty faced in all these new developments is that these materials have at least one pronounced weakness regarding the above mentioned key properties, and the challenge is to find acceptable solutions. In fact, there is substantial progress. For example, much improved oxidation resistance has been reported for certain refractory alloys and significant patent activity is noticeable in this field. Nevertheless, none of the evaluated material systems has come to the point were an acceptable property mix has been reached, allowing for demanding applications in the hot section of gas turbines. For this reason, it seems sensible to further search for alternative solutions, which have not been explored so far. One, namely the development of Co-Re-based high temperature alloys, is addressed here and the article is structured as follows. Firstly, the rational for focussing on Co-Re-based alloys is given. Four alloys are then evaluated to gain first insight in the microstructure evolution of this material class. Finally, these results are summarized and directions for further alloy development are discussed.

Co‐Re‐based Alloys: A New Class of High Temperature Materials?

Superalloys based on platinum-group metals are being developed for hightemperature applications. These alloys have two-phase structures comprising either ordered precipitates in a matrix analogous to the nickel-based superalloys or a fine dispersion of oxide particles in a matrix analogous to oxide-dispersion-strengthened nickel-based alloys. Currently, alloys based on iridium, rhodium, and platinum have been obtained. This article reviews the rationale of developments and the progress made in this area. Oxidation and compression tests as well as characterization with scanning electron microscopy and transmission electron microscopy were undertaken. These tests showed encouraging results, and further work is being done on new alloying additions and tensile testing.

Development of Platinum-Group-Metal Superalloys for High-Temperature Use

Research on platinum-group-metal (PGM)-based intermetallic compounds, which are candidates as high-temperature structural materials, is reviewed in this article. The research reviewed focuses on PGM-based L12 and B2 compounds. Alloys with a face-centered cubic and L12 or B2 phase are also reviewed.

Y. Yamabe

Papers

Development of Ir-base refractory superalloys