Effect of surface carburization on dynamic deformation and fracture of tungsten heavy alloys
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I. INTRODUCTION
WHEN edge parts of a penetrator are effectively fallen off during high-speed impact on an armor plate, its penetration performance can be improved. Called “self-sharpening” effect, this phenomenon is actively pursued in many recent studies on tungsten heavy alloys.[1–7] It is known that the self-sharpening arises from the formation of adiabatic shear bands during dynamic deformation of the penetrator, since the deformation is highly localized and the localization is further accelerated at the same area by thermal softening due to temperature rise.[8] Adiabatic shear bands formed in tungsten heavy alloys have a form of “deformed band” that is characterized by highly localized shear strain compared to its surrounding region. They do not show any clear interfaces in their edges, unlike the other type of adiabatic shear band, e.g., “transformed band” formed generally in aluminum or titanium alloys. However, there is a confusion in the definition between deformed band and “a narrow region of intense deformation.” Since the former is a result of dynamic deformation that accompanies thermal softening process, the shear bands in tungsten heavy alloys can be classified as adiabatic shear bands. In general, the formation of adiabatic shear bands mainly depends on thermal-mechanical properties of the material. When the heat generated from deformation is not readily SUG-WOO JUNG, Research Assistant, and SUK-JOONG L. KANG, Professor, are with the Center for Interface Science and Engineering of Materials, Korea Advanced Institute of Science and Technology, Daejeon, 305-701 Korea. DONG-KUK KIM, Research Assistant, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. JOON-WOONG NOH, Senior Research Scientist, is with the Agency for Defense Development, Daejeon, 300-600 Korea. Manuscript submitted August 31, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
dissipated, thermal softening in the deformed region is accelerated, leading to localized deformation; thus, materials with low thermal conductivity are more prone to the formation of shear bands.[9] Because of high thermal conductivity of tungsten, accounting for over 90 wt pct of tungsten heavy alloys, adiabatic shear banding rarely occurs. From the aspect of mechanical properties, the higher the yield strength and hardness are, the faster the plastic deformation proceeds, favorably affecting the localized deformation, which directly causes the shear band formation.[10] However, high hardness easily brings embrittlement and causes easy fragmentation during penetration. In order to enhance the penetration performance, therefore, it is desirable for the penetrator to have sufficiently high hardness on the surface and an appropriate toughness level inside to be able to maintain the structure upon high-speed impact. Carburization results in a considerable increase in surface hardness, which plays an important role in the self-sharpening. However, fragmentation may occu
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