Dynamic deformation and fracture behavior of ultrafine-grained aluminum alloy fabricates by equal-channel angular pressi
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I. INTRODUCTION
MANY investigations on very fine grains in materials have been actively conducted to simultaneously improve strength, ductility, toughness, and superplasticity.[1–11] Thermomechanical processing,[12] mechanical alloying,[13] rapid solidification,[14] and crystallization of amorphous alloys[15] are the main methods to achieve grain refinement. Since sufficient bulk production is not easy to obtain from these methods and a considerable number of pores often exist inside products, their application to mass production has been limited. In order to overcome the disadvantages, various methods such as equal-channel angular pressing,[2,5–8] equal-channel angular rolling,[3] severe torsional straining,[1] and accumulative roll-bonding,[4] in which grain refinement is achieved by applying severe plastic deformation to materials without heat treatment or composition change, have been recently developed. Among them, the equal-channel angular pressing is very effective in obtaining ultrafine grains by controlling the amount of strain and the processing path. However, most studies on improving the mechanical properties of ultrafine-grained materials fabricated by the equalchannel angular pressing have been mainly evaluated under static or quasi-static loading, and few studies have been YANG GON KIM, Research Assistant, BYOUNGCHUL HWANG, Research Associate, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] WOO GYEOM KIM, Research Assistant, and DONG HYUK SHIN, Professor, are with the Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, Korea. Manuscript submitted January 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
conducted on the deformation and fracture behavior occurring under dynamic loading. Recently, Wei et al.[16] evaluated the quasi-static and dynamic compressive properties of ultrafine-grained tantalum using a compression Kolsky bar and found a slight softening phenomenon due to adiabatic heating under dynamic loading. Gray et al.[17] also investigated the dynamic compressive properties of ultrafine-grained materials with varying strain rates and temperatures. In these cases, the temperature and strain-rate sensitivity of the ultrafine-grained materials were significantly higher than those of conventionally annealed polycrystalline materials. Since the ultrafine-grained materials practically undergo high-speed deformation processing such as forging and superplastic forming to be applied to aerospace, automotive, defense, and precision machining parts, the dynamic deformation and fracture behavior should be taken into critical consideration in terms of selection, development, and designing of materials. In general, resistance to fracture under dynamic loading is lower than that under quasi-static loading,[18–21] and, thus, the dynamic deformation and fracture behavior should be closely examined and evaluated before materials are appli
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