In situ formation of three-dimensional TiC reinforcements in Ti-TiC composites
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INTRODUCTION
APPLICATIONS requiring subspace and SU.l~ersonic flight will require new concepts in materials to obtain optimum performance under desired higher operating temperatures and stresses. One such application is the design and fabrication of a new skin material for the proposed high-speed space vehicle. Traditional materials, such as aluminum, titanium alloys, stainless steel, and in some exotic applications, Nb- and V-strengthened steels, do not have the necessary combined hightemperature strength, modulus, and strength-to-weight ratios that are desired. Carbon-based materials (C materials) are superb candidates for development. Monocarbides of group IVa, Va, and Via transition metals have extremely high hardness and excellent elevated temperature strengths because of their ordered structure. Titanium carbide has the highest specific strength and specific modulus among the transition metal carbides. Polycrystalline TiC and substoichiometric TiC have been studied by other investigators, t~-6] However, applications of TiC are limited by its lack of ductility, even at elevated temperatures. Ti-TiC composite materials have been investigated in an attempt to improve mechanical properties through the presence of a ductile metal matrix, tT] This system combines the advantages of high strength and high modulus of titanium carbide with the reasonable ductility of the metallic matrix to form a ceramic reinforced composite material which is relatively light in weight yet maintains a high specific strength and modulus even at high temperatures. Selected solutes such as AI, B, N, V, and Zr affect the structure and properties by changing the degree of long-range order of TiC or by solid solution strengthening the Ti matrix. [s-13j One purpose of this investigation is to determine the effect of AI additions on the properties of Ti-TiC composites. Aluminum can be present in Ti in solid solution up to 6 wt pct. [121This element is a well-known solute element in alpha Ti, since it greatly Y. LIN, Graduate Student, R.H. ZEE, Associate Professor, and B.A. CHIN, Professor, are with the Department of Materials Engineering, Auburn University, Auburn, AL 36849-5351. Manuscript submitted June 13, 1990. METALLURGICAL TRANSACTIONS A
stabilizes the alpha phase while increasing the strength and lowering the density of titanium. Solubility of A1 in substoichiometric TiC up to 3 wt pct has also been reported, while in near-stoichiometric compositions, the solubility is negligible, t13] No effect of A1 on the mechanical properties of TiC has been reported in the literature. In the present study, 6 wt pct of A1 was added to Ti-TiC composites, and the resulting microstructures and mechanical properties were measured. Only results from material containing between 40 and 50 vol pct of TiC are reported in this paper. The Ti-TiC composite studied here was produced by mixing graphite powder and titanium during melting, thus forming TiC particles as a second phase in a Ti matrix. The reinforcement TiC, in this case, was formed inside the m
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