Microstructure and mechanical behavior of the NiAl-TiC In situ composite
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THE intermetallic compound NiAl is being considered as a potential alternative to superalloys for advanced engines. This is because B2-NiAl has a high melting point (1638 7C), low density (5.90 g/cm3), high Young’s modules (240 GPa), and excellent oxidation resistance at high temperature. Additionally, the thermal conductivity of NiAl is 4 to 8 times greater than that of Ni-base superalloys.[1] To date, the use of NiAl has been limited due to the two major drawbacks: (1) poor ductility and toughness at ambient temperatures and (2) low strength and creep resistance at elevated temperatures. Thus, it is necessary to simultaneously improve both the toughness and creep resistance of these materials to make them competitive with currently used alloys. One approach to achieve these goals would be to reinforce the NiAl matrix with either continuous or discontinuous second phases, including fibers, whiskers, and particles. Discontinuously reinforced composites are less problematic to fabricate than long fiber composites. Furthermore, they behave isotropically and are not as sensitive as long fiber composite to mismatch in coefficient of thermal expansion between matrix and reinforcements.[2] Therefore, extensive research and development have focused on discontinuously reinforced NiAl matrix composites.[2–8] The TiB2 particles have been introduced into NiAl matrices via XD* processing[2–5] and the hot-press-aided exo*XD is a trademark of Martin Marietta Corporation, Bethesda, MD.
thermic synthesis (HPES) technique.[6,7] Since the reinforcements are produced in situ by these processes, the com-
Z.P. XING and Y.F. HAN, Professor, are with the Institute of Aeronautical Materials, Beijing 100095, People’s Republic of China. J.T. GUO, Professor, and L.G. YU, Engineer, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China. Manuscript submitted July 24, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
posites are characterized by clean, contaminant-free interfaces. With large volume fractions (20 to 30 vol pct) of TiB2 particles in the NiAl matrix, the high-temperature strength of NiAl can be increased two- to threefold.[2,3,4,6] However, the introduction of TiB2 particles has little effect on the room-temperature fracture toughness.[8] Very little information is available on the tensile behavior of such composites. Furthermore, no systematic research on the behavior of other reinforcements, such as TiC, in NiAl matrix has been reported. In this article, the fabrication of nominally Ni-50 at. pct Al matrix composites containing 0 and 20 vol pct TiC particles by HPES technique is reported. The microstructure, interfaces, and mechanical properties of the composites in several conditions are studied in detail. II.
EXPERIMENTAL PROCEDURES
The reagents used in this research consisted of elemental powders of Ni (98 pct, ≤1 mm), Al (98 pct, ≤13 mm), Ti (99 pct, ≤75 mm), and carbon black (99.9 pct, ≤10 mm). Chemical analysis showed that the major impurities of Ni powder w
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