Assessment of a powder metallurgical processing route for refractory metal silicide alloys
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TRODUCTION
THE development of ultra-high-temperature structural materials for turbines is a challenge set to the materials science and engineering community. The establishment of nickel-based superalloys in the 1950s opened an era of 50 years of growth and development of gas turbines. Today’s engines expose superalloys to temperatures approaching 1150 °C, while the turbine inlet temperature can be raised to about 1500 °C by distributing cooling air through the airfoils and protecting the metal with thermal barrier coatings. However, when looking at the maximum service temperature of superalloys over the last 20 years, only a few major improvements such as the introduction of single crystals (including the addition of Re) and the application of thermal barrier coatings, respectively, have been achieved (e.g., References 1 through 3). The technological requirements of the 21st century demand a revolutionary improvement, which can only be achieved by the development of a new ultrahigh-temperature material class, allowing operating temperatures beyond those of Ni-based superalloys. The outstanding mechanical properties at service temperatures in excess of 1000 °C make refractory metals and alloys P. JÉHANNO, H. KESTLER, A. VENSKUTONIS, and M. BÖNING are with the Technology Center, Plansee AG, A-6600 Reutte in Tirol, Austria. M. HEILMAIER is with the Institute of Materials Engineering and Testing, Otto-von-Guericke Universität Magdeburg, Institut für Werkstofftechnik und Werkstoffprüfung, D-39016 Magdeburg, Germany. B. BEWLAY and M. JACKSON are with the Global Research Center, General Electric, Schenectady, NY 12301. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
first choice candidates for ultra-high-temperature applications. For example, molybdenum alloys such as TZM[4] are well established as high-temperature structural materials in protective environments. However, the high density of molybdenum with 10.2 g cm3 is a first drawback, which should be assessed as a function of the specific mechanical properties, while the use of niobium alloys with a density below 8.5 g cm3 promises significant advantages even over Ni-base superalloys. Besides the limited room-temperature ductility and moderate fracture toughness, the main barrier for turbine applications in an oxygen-containing environment is the formation of nonpassivating oxides of niobium or molybdenum. One possible way out of this dilemma is alloying of these refractory metals with silicon: for instance, the intermetallic compound MoSi2 exhibits an outstanding oxidation resistance up to 1600 °C due to the formation of a passivating silica glass layer. However, the mechanical p
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