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AN OVERVIEW OF POTENTIAL TITANIUM ALUMINIDE COMPOSITES IN AEROSPACE APPLICATIONS JAMES M. LARSEN, WILLIAM C. REVELOS, AND MARY L. GAMBONE Materials Directorate, Wright Laboratory, WL/MLL Wright-Patterson AFB, OH 45433

ABSTRACT High-temperature, light-weight materials represent enabling technology in the continued evolution of high-performance aerospace vehicles and propulsion systems being pursued by the U.S. Air Force. In this regard, titanium aluminide matrix composites appear to offer unique advantages in terms of a variety of weightspecific properties at high temperatures. However, a key requirement for eventual structural use of these materials is a balance of mechanical properties that can be suitably exploited by aircraft and engine designers without compromising reliability. An overview of the current capability of titanium aluminide composites is presented, with an effort to assess the balance of properties offered by this class of materials. Emphasis is given to life-limiting cyclic and monotonic properties and the roles of high-temperature, time-dependent deformation and environmental effects. An attempt is made to assess the limitations of currently available titanium aluminide composites with respect to application needs and to suggest avenues for improvements in key properties. INTRODUCTION

Titanium aluminide composites [1,2], continuously reinforced with silicon carbide (SiC) fibers, have recently received considerable attention due to their potential to replace titanium and nickel-base alloys in aerospace systems, such as advanced turbine engines [3] and hypersonic vehicles [4] where specific strength and stiffness at high temperatures are critical. Figure 1 is a schematic of a possible application of titanium aluminide composite technology in an advanced turbine engine. Rotating components in compressor sections that utilize advanced composites can have significantly reduced weight, allowing important improvements in design that are not possible using conventional nickel-base superalloys [5]. This weight savings translates into higher thrust-to-weight ratios and lower specific fuel consumption in the engine. Other rotating, as well as static, applications of titanium aluminide composites have lately been the subject of increased attention. These include shafts, blades, vanes, stators, actuators, struts, and nozzles. The majority of these applications will utilize unidirectionally reinforced composite architectures. If these composites are to see application in advanced engines, however, progress must be made in several areas, including the improvement of off-axis tensile and creep properties, low cycle fatigue resistance, environmental and burn resistance for those exposed to gas flow paths, and life-management and damage tolerant design methods. Hypersonic vehicle applications require composites with good specific strength and stiffness at elevated temperature, combined with improved transverse properties. As shown in Figure 2, these composites will be used primarily as hatchannel stiffened pane