Effect of flux addition on the microstructure and hardness of TiC-reinforced ferrous surface composite layers fabricated
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I. INTRODUCTION
GENERALLY , metal-matrix composites reinforced with ceramics have excellent properties compared to unreinforced metals because they show high strength, high elastic modulus, and improved resistance to wear and oxidation. In particular, because TiC-reinforced ferrous or nickel-base composites are very hard and have thermodynamically stable TiC particles inside the matrix, whose structure can be modified by subsequent heat treatment,[1,2] they can be applied to structures requiring resistance to abrasion and corrosion and high-temperature properties. Ferrous composites reinforced with titanium carbides are typically fabricated by a powder-metallurgy route through the process of mixing TiC powders with steel powders, densification, and sintering and are commercialized in such brand names as Ferrotic, TiCAlloy, and Ferrotitanit.[3,4,5] In this fabrication method, homogeneous mixing of TiC and steel powders is hard to achieve, and the surface of both powders is prone to contamination during mixing. Another way to fabricate TiC-reinforced composites is by casting using liquid Fe-Ti-C alloys,[6,7] FeTi and Fe-C alloys,[8,9] or Fe-Ti alloys and graphite[10] to obtain thermally stable carbides from the liquid by in situ precipitation of TiC particles. However, this method demands a complex process to achieve a uniform TiC particle dispersion. Other fabrication techniques such as liquid-phase sintering,[11] self-sustaining high-temperature synthesis,[12] plasma spraying,[13] and surface alloying using a laser[14–18] have also been used. Recently, new attempts have been made SEONG-HUN CHOO, Postdoctoral Research Associate, and SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, and SOON-JU KWON, Professor, Department of Materials Science and Engineering, are with Pohang University of Science and Technology, Pohang, 790-784 Korea. Manuscript submitted January 21, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
by direct irradiation using a high-energy electron beam in order to achieve surface hardening or surface alloying.[19,20] When a material surface is irradiated by a high-energy electron beam of several MeV, electron energy is transferred to the material surface by collisions with the electrons of the material. The energy from these electrons is transferred nearly instantaneously to the lattice as heat[21] that is high enough to melt materials having high melting temperatures. Upon irradiation of a metal surface on which ceramic powders are evenly deposited, the metal surface is melted, while the ceramic powders are either partially or completely melted and then precipitated again during solidification, thereby fabricating a surface composite. This electron-beam-irradiation method rarely forms pores or cracks because of high thermal efficiency and homogeneous heating and cooling. It can be continuously performed due to its application in air, and thus can treat a very large area conveniently at one time, which makes it advantageous for the fabrication of large-sized structures or parts and f
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