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MICROSTRUCTURAL CHARACTERIZATION OF A1203/GAMMA TITANIUM ALUMINIDE COMPOSITES# G.DAS * AND S. KRISHNAMURTHY **

* Metcut-Materials Research Group, Wright-Patterson AFB, OH. Now at Pratt & Whitney, P.O. Box 109600, West Palm Beach, Fl 33410-9600 ** Metcut-Materials Research Group, Wright-Patterson AFB, OH. Now at Universal Energy System, 4401 Dayton-Xenia Road, Dayton, Ohio 45432 ABSTRACT Alumina (A1203) fibers were incorporated into gamma titanium aluminide(TiAI) based powders by hot isostatic pressing (HIP'ing). The microstructure of as-HIP'd and heat treated composite specimens were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM studies reveal the presence of an amorphous reaction zone at the fiber/matrix interface. Numerous dislocations, dipoles and loops as well as twins are observed in A1203 fibers. In addition, it is determined that the fiber/matrix interface stability is significantly affected by the matrix microstructure. INTRODUCTION There is considerable interest in the development of fiber-reinforced gamma titanium aluminide-based composites because of their potential for high stiffness and strength at elevated temperature. Studies involving silicon carbide (SiC) fibers in gamma titanium aluminide matrices showed that SiC fibers readily react with gamma matrices (1,2). In addition, the coefficient of thermal expansion (CTE) of gamma matrices is nearly twice that of SiC fibers. This may lead to the development of residual stresses during cooling of the composite. However, based on CTE match and chemical compatibility, the A1203 fibers are considered to be a better candidate as reinforcement for gamma titanium aluminide matrices (2, 3). This study is a continuation of earlier work (2) and aims at characterizing the nature of the reaction zone and the fiber/matrix interface stability in A1203/gamma composites as well as the microstructure of alumina fibers and gamma matrices. MATERIALS AND PROCEDURES Three gamma titanium aluminide alloy powders were used as starting matrices. The chemical composition of these alloy powders is shown in Table I. Two of the powders were produced by the rapid solidification rate (RSR) process while the third was made by the plasma rotating electrode process (PREP). According to chemical analysis, the two RSR powders correspond to two-phase gamma + alpha-2 microstructure while the PREP powder corresponds to singlephase gamma. Typical features of the as-received powders are shown in Figure 1. One of the RSR powders (Ti-48AI-lTa) contains shattered particles with a high level of oxygen and carbon interstitials. The other RSR powder (Ti-48AI-3V) consists of spherical particles with somewhat high oxygen content. The PREP powder (Ti-48AI-2Nb-0.3Ta) contains coarser spherical particles with a lower interstitial content. The A1203 fiber (Saphikon) was in the form of single crystal monofilament with the Vc'axis parallel to the fiber axis. The fiber was characterized by a diameter of -175 pm and had a smooth surface. # Work performed a