Correlation of microstructure with wear and fracture properties of two-layered VC/Ti-6Al-4V surface composites fabricate
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I. INTRODUCTION
RESISTANCE to wear, oxidation, and corrosion of materials largely depends on the composition and microstructure of the material surface, and properties required for the surface may be different from those of bulk substrates in many instances. For enhancement of surface properties, researches on direct irradiation of high-energy beams, such as pulsed-laser beam or electron beam, have been carried out to harden the surface or to make surface composites.[1–4] Because fast cooling on the material surface can be obtained from energy-irradiation methods and input energy hardly affects the substrate, these methods can achieve required surface properties while maintaining substrate properties.[5,6] In the fabrication of surface composites using a high-energy beam, the surface region is melted and then solidified, and thus, it hardly has an interfacial-bonding problem. Quasistable phases with excellent properties can also be obtained by overcoming the equilibrium solubility limit caused by a fast cooling rate during solidification.[5,6,7] Therefore, many studies have focused on the desired surface properties and microstructures by forming carbides, borides, and nitrides on the surface of ferrous or titanium alloys using laser or electronbeam irradiation.[8,9,10] Because electron-beam irradiation in general should be JUN CHEOL OH and CHANG SUP LEE, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: shlee@postech. ac.kr SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology. Manuscript submitted November 21, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
conducted in a vacuum, studies on this method have been less active than those on the laser-beam irradiation method, which allows atmospheric irradiation. Recently, many researches have been conducted on the fabrication of surface-hardened materials or surface composites using a highenergy electron accelerator (several MeV energy range), which accommodates atmospheric irradiation.[11,12,13] When ceramic powders are deposited evenly on a metal surface and then irradiated by an electron beam, ceramic/metal surface composites can be fabricated as ceramic powders, substrates are melted, and ceramic elements are penetrated into the substrate and precipitated. Studies on surface composites have mainly focused on improvement of wear properties, microstructural modification, and their high-temperature application. In the case of titanium alloys, however, the thickness of surface composites that can be fabricated by an one-time electron-beam irradiation is relatively thin (about 1 mm), and their microstructures are varied with thickness, thereby limiting the enhancement of mechanical properties. It is, thus, highly required to form thicker surface-composite layers with homogeneous microstr
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