Creep behavior of TiBw/Ti In-situ composite fabricated by reactive hot pressing
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I. INTRODUCTION RECENT years have seen a surge of interest in developing titanium matrix composites. These composites exhibit an obvious improvement in strength and modulus compared to unreinforced titanium alloys. A number of investigations have indicated that many reinforcements are unstable in titanium alloys, and different types of reaction products can form at the interfaces of reinforcements and matrices depending on the composition of the reinforcing phase.[1–7] The properties of the composites tend to deteriorate as the thickness of this interfacial reaction zone increases. In the past, alternative reinforcing phases have been tried.[8,9,10] Among these, TiC and TiB are particularly attractive because they are completely compatible with titanium matrices. In recent years, novel processing routes, such as exothermic dispersion (XD*), rapid solidification processing (RSP), *XD is a trademark of Martin Marietta Corporation, Bethesda, MD.
combustion assisted casting (CAC), and reactive hot pressing (RHP), have evolved in which the reinforcements are grown in situ.[11–18] Such in-situ composites have good interfacial bonding between the ceramic reinforcement and titanium matrices, leading to higher mechanical properties in the insitu Ti-based composites.[11] The use of discontinuously reinforced titanium matrix composites for high-temperature applications requires a comprehensive understanding of the mechanisms affecting their creep behavior. However, the number of published creep studies on these composites is limited.[17,20–22] The results of Zhu et al.[20] on TiBw/Ti-6Al-4V composite indicate higher creep strength in the composite compared with the unreinforced matrix. Furthermore, the work shows a change in stress exponent from n ⫽ 2.3 under low stress to n ⫽ 7.2 under high stress. Recently, Raganath and Mishra[21] reported that the values of the stress exponent of Ti-based composites are exceptionally high (n ⫽ 6 to 7) due to the change in creep mechanism from lattice to pipe diffusion at 823 K. They further showed that the creep data cannot merge even after introducing a threshold stress 0 into the powerlaw creep equation, where 0 represents a lower limiting Z.Y. MA, Graduate Student, and S.C. TJONG, Associate Professor, are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong. S.X. LI, Professor, is with the State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China. Manuscript submitted March 22, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
stress below which creep cannot occur. To interpret the creep data, a kinetic strengthening term involving the volume fraction of reinforcement was incorporated into the constitutive equation of power-law creep.[21] More recently, Tsang et al.[22] have investigated the creep behavior of titanium matrix composites reinforced by various volume fractions of TiB whiskers. They reported that the stress exponent for c
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