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NTRODUCTION

METAL matrix composites (MMCs) reinforced with ceramic phase have become of great interest because of the combined effects of metallic and ceramic materials relative to the corresponding monolithic alloys. Thus, various fabrication methods have been developed, such as powder metallurgy, stir casting, pressure infiltration method, etc. Furthermore, in recent years there has been a growing interest in the development of technologies for the in situ production of MMCs, such as Lanxide’s PRIMEX* process, Martin *PRIMEX is a trademark of Lanxide.

Marietta’s XD** process, self-propagating high-temperature **XD is a trademark of Martin Marietta.

synthesis (SHS) and reactive-gas injection.[1–10] Many kinds of ceramic phases have been added as a reinforcement in Al matrix composites: particles, whiskers and fibers of the oxides (Al2O3, MgO, TiO2, etc.), carbides (SiC, TiC, B4C, etc.), nitrides (AlN, BN, Si3N4, etc.), and borides (TiB2, etc.). Boron nitride (BN) is an interesting material because of its unique combination of properties, such as its low density, high melting point, high thermal conductivity, and high electrical resistivity.[11,12] There are many reports that the BN/Al system shows much better wettability than several other ceramic/metal systems.[13–18]

KON BAE LEE, Professor, is with the School of Metallurgical and Materials Engineering, Kookmin University, Seoul 136-702, Korea. JAE PYOUNG AHN, Postdoctor, is with the Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720. HOON KWON, Professor, School of Metallurgical and Materials Engineering, Kookmin University, is jointly appointed at the Center for Advanced Aerospace Materials, POSTECH, Pohang 790-784, Korea. Manuscript submitted February 28, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A

Fujii et al.,[13] in particular, reported that the equilibrium contact angle was 0 deg at 1000 ⬚C. Thus, they suggested that BN might be an optimum material to be reinforced into an Al matrix, from the standpoint of wetting. However, since most of the work on the BN/Al system has been focused on wettability and on the an interfacial reaction between BN and Al, and not on its real bulk composites, its mechanical properties are rarely, if ever, known. Therefore, an Al composite reinforced with BN was fabricated by the pressureless infiltration method in this study in order to investigate its characteristics. The PRIMEX process is an innovative technique for fabricating MMCs by the spontaneous infiltration of molten Al alloy containing Mg into a ceramic filler or preform under a nitrogen atmosphere, without the aid of vacuum or externally applied pressure.[5–10] For the pressureless infiltration in this process, both nitrogen and Mg are needed. Previous studies have investigated that the spontaneous infiltration of molten Al alloy, in providing the matrix of Al composites in this process, is influenced by Mg content, additional alloying elements, nitrogen concentration in the gas atmosphere, infiltration temperature and time, etc.[5–10