The temperatures of airfoil surfaces in advanced turbine engines are approaching the limits of nickelbased superalloys. Innovations in refractory metal-intermetallic composites (RMICs) are being pursued, with particular emphasis on systems based on Nb-Si and Mo-Si-B alloys. These systems have the potential for service at surface temperatures Ͼ1350 °C. The present article will review the most recent progress in the development of Nb-silicide-based in-situ composites for very-high-temperature applications. Nb-silicide-based composites contain high-strength silicides that are toughened by a ductile Nb-based solid solution. Simple composites are based on binary Nb-Si alloys; more complex systems are alloyed with Ti, Hf, Cr, and Al. In higher-order silicide-based systems, alloying elements have been added to stabilize intermetallics, such as Laves phases, for additional oxidation resistance. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, high-temperature creep performance, and oxidation resistance. Recent progress in the development of composite processing-structure-property relationships in Nb-silicide-based in-situ composites will be described, with emphasis on rupture resistance and oxidation performance. The Nb-silicide composite properties will be compared with those of advanced Ni-based superalloys.

A new generation of refractory material systems with significant increases in temperature capability is required to meet the demands of future aerospace applications. Such materials require a balance of properties such as low-temperature damage tolerance, high-temperature strength, creep resistance, and superior environmental stability for implementation in advanced aerospace systems. Systems incorporating niobium-based beta alloys and intermetallic compounds have the potential for meeting these requirements.

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This article reviews the most recent progress in the development of Nb-silicide-based in situ composites for potential applications in turbine engines with service temperatures of up to 1350°C. These composites contain high-strength Nb silicides that are toughened by a ductile Nb solid solution. Preliminary composites were derived from binary Nb-Si alloys, while more recent systems are complex and are alloyed with Ti, Hf, W, B, Ge, Cr, and Al. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, fatigue-crack-growth behavior, high-temperature creep performance, and oxidation resistance over a broad range of temperatures. Nb-silicide-based composites are described with emphasis on processing, microstructure, and performance. Nb silicide composites have been produced using a range of processing routes, including induction skull melting, investment casting, hot extrusion, and powder metallurgy methods. Nb silicide composite properties are also compared with those of Ni-based superalloys.

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Композиционные материалы (КМ) на основе ниобия c функциональными и легирующими добавками (Si, Hf, Ti, Al и др.) имеют перспективу промышленного освоения в авиационном двигателестроении. Ранее авторами было показано, что такие КМ можно синтезировать в автоволновом режиме (режиме горения), используя высокоэкзотермические смеси Nb 2 O 5 с Al, Si, Hf и Ti. Было обнаружено, что в волне горения гафний активно участвует в восстановлении Nb 2 O 5 , что усложняет его введение в КМ. Настоящая работа направлена на изучение возможности синтеза методами центробежной СВС-металлургии композиционных материалов на основе Nb с высоким содержанием Hf. В экспериментальных исследованиях, проведенных на центробежной установке под воздействием перегрузки 40 g, было показано, что замена активного Hf на менее активные его соединения Hf-Al или Hf-Ti-Si-Al в составе смесей Nb 2 O 5 /Al позволяет перевести горение смеси из взрывоподобного режима в режим стационарного горения. С увеличением размера гранул Hf-Al от 0-40 до 160-300 мкм в смеси содержание Hf в КМ возрастает от 1,3 до 3,8 мас.%. Введение в исходную шихту гранул Hf-Ti-Si-Al с размером частиц от 1 до 3 позволяет получать литые КМ на основе силицидов ниобия с содержанием Hf до 8,1 мас.%. Методами электронной микроскопии и рентгенофазового анализа определены интегральный состав и распределение базовых и примесных элементов в структурных составляющих литых КМ, а также их фазовый состав. Композиционные материалы с максимальным содержанием Hf (8,1 мас.%) содержат 3 структурных составляющих: (1)-основу, которая включает Nb, Si, Ti; (2)-межзеренные границы, содержащие Nb, Ti и Al; (3)-включения на основе оксида гафния. На рентгенограмме КМ выявлены 3 фазы: твердые растворы на основе Nb и Nb 5 Si 3 , а также небольшое количество Nb 3 Si.

The present article will describe the science and technology of titanium aluminide (TiAl) alloys and the engineering development of TiAl for commercial aircraft engine applications. The GEnxTM engine is the first commercial aircraft engine that is flying titanium aluminide (alloy 4822) blades and it represents a major advance in propulsion efficiency, realizing a 20% reduction in fuel consumption, a 50% reduction in noise, and an 80% reduction in NOx emissions compared with prior engines in its class. The GEnxTM uses the latest materials and design processes to reduce weight, improve performance, and reduce maintenance costs.GE’s TiAl low-pressure turbine blade production status will be discussed along with the history of implementation. In 2006, GE began to explore near net shape casting as an alternative to the initial overstock conventional gravity casting plus machining approach. To date, more than 40,000 TiAl low-pressure turbine blades have been manufactured for the GEnxTM 1B (Boeing 787) and the GEnxTM 2B (Boeing 747-8) applications. The implementation of TiAl in other GE and non-GE engines will also be discussed.

Authors' Note: All compositions are given in atom percent. -temperature, refractory-metal, intermetallic, in-situ composites consist of highstrength, niobium-based silicides with a niobium-based metallic toughening phase. A variety of processing schemes have been used to generate these in-situ composites, including solidification and vapor phase processes. Secondary processing, such as forging and extrusion, has also been employed. These composites offer an excellent balance of high-and low-temperature mechanical properties with promising environmental resistance at temperatures above 1,100°C. High B.P. Bewlay is a metallurgist with