Control of surface carburization and improvement of dynamic fracture behavior in tungsten heavy alloys
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THE penetration performance of a penetrator can be improved when its edge parts effectively fall off during high-speed impact with an armor plate.[1] Called the “selfsharpening” effect, this phenomenon has been studied intensively in recent years.[2–8] The self-sharpening has now been found to result from a highly localized deformation and thermal softening of the material, which can induce the formation of adiabatic shear bands during the penetration.[9] Studies on self-sharpening in tungsten heavy alloys were carried out primarily to enhance the formation of the adiabatic shear band through, for example, changing matrix elements, refining tungsten grains, and adjusting porosities.[4,5,8] Recently, as a new concept, Jung et al.[7] reported that surface carburization treatment of heavy alloys might be effective in promoting the cleavage fracture of tungsten grains during high-speed impact, enhancing the self-sharpening, and improving the penetration performance. In their surface carburization process, however, it was difficult to suppress the carbon diffusion beyond the carburization layer; a drastic reduction of impact toughness resulted with an increase in surface hardness by the carbide formation at the surface. To improve the surface hardness with a minimal reduction in impact toughness, it is, therefore, necessary to suppress the carbon diffusion into the sample interior. In the present investigation, we developed a new surface SUG-WOO JUNG, Graduate Student, and SUK-JOONG L. KANG, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea. Contact e-mail: [email protected] SUNGHAK LEE, Professor, is with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea. EUN-PYO KIM, Senior researcher, JOON-WOONG NOH, Division Chief, and WOON-HYUNG BAEK, Director, are with the Technical and Research Center - 5, Agency for Defense Development, Taejon, 300-600, Korea. Manuscript submitted May 30, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
carburization process that can fabricate tungsten heavy alloys with a high surface hardness and, at the same time, with a high impact toughness. To study the effect of the new technique on dynamic fracture behavior, the dynamic torsional test and penetration performance test were conducted. Dynamic shear stress/shear strain curves were obtained using a torsional Kolsky bar, and the test data were analyzed in terms of microstructure, dynamic deformation, and fracture behavior. High-speed impact tests were also conducted to analyze the fracture behavior during actual penetration.
II. EXPERIMENTALS Samples of 93W-4.9Ni-2.1Fe (wt pct) tungsten heavy alloys were prepared using conventional processing steps: sintering, heat treatment, swaging, and aging. Proportioned W (average particle size of 2.5 m, purity of 99.9 pct, TaeguTec. Ltd., Taegu, Korea), Ni (3.5 m, 99.7 pct, SigmaAldrich Co., Milwaukee, WI), and Fe (3.5
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