Creep behavior of in situ dual-scale particles-TiB whisker and TiC particulate-reinforced titanium composites
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S.C. Tjong and X.M. Meng Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, Peoples Republic of China (Received 22 April 2002; accepted 6 June 2002)
A titanium composite reinforced by in situ dual-scale particle, high-aspect-ratio TiB whiskers and fine TiC particulates was fabricated by a reactive hot pressing technique from a B4C–Ti system. The composite was subjected to creep investigations in compression at 873–923 K. This composite exhibited a stress exponent of 4.5–4.6 and a creep activation energy of 298 kJ/mol. By comparison, unreinforced Ti exhibited a stress exponent of 5.2–5.3 and a creep activation energy of 259 kJ/mol. No change in the stress exponent with varying creep rates was observed in both composite and unreinforced Ti under the investigated creep rates. The creep resistance of the composite was more than one order of magnitude higher than that of the unreinforced Ti. The load transfer mechanism accounted for this result. The creep of both composite and unreinforced Ti was controlled by lattice diffusion in the titanium matrix.
I. INTRODUCTION
Recent years have seen an increased interest in developing titanium matrix composites with discontinuous reinforcements for high-temperature structural applications. Among the reinforcements, TiB and TiC are particularly attractive since they are completely compatible with a titanium matrix. In recent years, novel processing techniques, such as exothermic dispersion, rapid solidification processing, combustion-assisted synthesis, and reactive hot pressing (RHP), have been developed in which the reinforcements are grown in situ in the titanium matrix.1–10 Such composites have good interfacial bonding between the in situ reinforcement and the titanium matrix, and a crystallographic orientation relationship has been observed between them.11 Thus, the adverse effect of the surface layer associated with the added reinforcements is avoided. Therefore, the in situ titanium matrix composites are expected to exhibit superior mechanical properties.1,10 A fundamental knowledge of the mechanisms influencing creep behavior of the titanium matrix composites is required for their use in high-temperature applications. Unfortunately, the creep studies on these composites are limited, and no consistent experimental results and mechanistic explanations have been obtained so far.12–18
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 9, Sep 2002
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First, the addition of ceramic reinforcements into the titanium matrix generally results in significant creep strengthening.7,12–18 However, the strengthening mechanisms are not yet well understood, though microstructural strengthening13,15 and increase in the modulus of composite12,13 have been suggested as possible strengthening mechanisms. Secondly, a change in the stress exponent from n ⳱ 2.3 under low creep rates to n ⳱ 7.2 under high creep rates has been report
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