The mechanism of formation of TiB 2 particulates prepared by In Situ reaction in molten aluminum
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I.
INTRODUCTION
PARTICULATE-REINFORCED light metals have great potential for structure materials due to their high specific strength and modulus. Attempts have been made to enhance the material properties via a ceramic second phase of higher strength and higher stiffness. The reinforcing phases and the metal matrix are combined by various processing techniques such as powder metallurgy, preform infiltration, spray deposition, casting technologies, rheocasting, and compocasting.[1–5] These artificial composites have resulted in the realization of strength and modulus goals. However, the composites have rarely been economically viable. Additionally, these processes also have problems such as interfacial reactions leading to undesirable products, thermodynamic and mechanical incompatibility, and internal stresses resulting from the mismatch of thermal expansion coefficients of various phases. Recent studies[6–12] have shown that in situ fabrication for producing aluminum matrix composites can produce a new class of naturally stable composites for advanced structural and wear application. More recently, we used a novel technique to generate fine particulates in the parent matrix. This new technique has a number of advantages: (1) the interface is very clean and continuous owing to the fact that the reinforcements are formed by an in situ reaction in the parent matrix, and (2) the reaction is typically accomplished at a lower temperature (760 7C to 900 7C) and during very short time, showing that the process is economical. The technique, based on our present investigations, is promising for mass production of B. YANG, Postdoctoral Candidate, is with the Materials Physics and Advanced Composites Department, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China; also Associate Professor, Department of Materials Science and Engineering, Nanchang University, Nanchang 330029, People’s Republic of China. Y.Q. WANG, Professor, is with the Materials Physics and Advanced Composites Department, Institute of Metal Research, Chinese Academy of Sciences. B.L. ZHOU, Professor, is with the Materials Physics and Advanced Composites Department, Institute of Metal Research, and International Center for Materials Physics, Chinese Academy of Sciences. Manuscript submitted May 28, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
aluminum matrix composite by virtue of its low production cost, high production rate, and strong possibility of producing near-net shape parts. Titanium diboride, which has great potential not only as a refractory material, but also as an electronic material because of its high melting point, hardness, electrical conductivity, and thermal conductivity, has been widely used as a reinforcement phase. So far, no one has studied the mechanism of formation of TiB2 particulates prepared by in situ reaction in molten aluminum. The aim of this study is to elucidate the mechanism of formation of TiB2 particulates prepared by this method in order to reveal the characteristic
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