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.
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.
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.
More than 5 years ago, wrought processing was first used to produce fully lamellar (FL) microstructures in TiAl alloys having grain sizes less than Ϸ400 m. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (500 to 1000 MPa at low temperatures) while maintaining attractive high-temperature properties. The improvements appeared to be related to an unusually high apparent sensitivity of strength to grain size. Studies reported an apparent value for the slope of the Hall-Petch (HP) plot approaching 5 for FL gamma alloys, while that for single-phase or ͌ MPa m duplex microstructures is near unity. The present investigations examine the slope of the HP plot for FL microstructures, paying particular attention to the lamellar microstructural variables. Results show that the ␣ 2 lamellar thickness and spacing and the ␥ lamellar thickness can vary over more than two orders of magnitude with typical process methods. These spacings influence the value of k y in the HP (grain size) relationship. Since they often change concomitantly with grain size in processing, they can give rise to a large scatter in the HP plot. The investigations also examine the flow behavior, glide barriers, and slip multiplicity for polysynthetically twinned (PST) crystals (the single-grain analogue of FL material), and then map this behavior into an explanation of the yield behavior of high-strength FL gamma alloys.
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