Microstructural evolution and superplasticity of Al-5.8Mg-0.23Mn alloys processed by reciprocating extrusion
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I. INTRODUCTION
IN the past three decades, many advanced techniques have been developed to refine the microstructures of alloys and thus to improve their properties. These techniques fall into three main categories: rapid solidification plus powder metallurgy processing, mechanical alloying plus powder metallurgical processing, and thermomechanical processing.[1,2,3] However, the first two techniques involve complicated processes that greatly increase the cost of manufacturing the final products, and have limited practical applications. The third method is more economical, but produces sheet form products and hence cannot be applied to manufacturing thick parts. Recently, intense strain methods have been developed to produce bulk fine-grained products without a complicated process. These methods are equal channel angular pressing,[4–7] torsion straining,[7,8] and cyclic deformation.[9,10,11] All of these methods are performed at temperatures well below the recrystallization temperature, so repeated deformation creates a high accumulated plastic strain in the materials. Increasing the plastic deformation yielded very fine micron or submicron grains and a larger proportion of high-angle boundaries. Based on such a philosophy of accumulating plastic strain, all of these methods are usually applied to low-strength alloys for cold or warm working because high-strength alloys require very high force to overcome deformation and friction, and samples tend to break during deformation. Small work pieces are preferred for the same reason. To overcome these drawbacks, the proposed method, “Reciprocating Extrusion,” was developed not only to directly process conventional billets of either low or high strength, but also to refine the SHIH-WEI LEE, Postgraduate, and JIEN-WEI YEH, Professor, are with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted July 18, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructure and improve the mechanical properties,[12–19] since the method operates at a high processing temperature near the recrystallization temperature and with a high plastic strain in each extrusion pass to induce dynamic recrystallization. The Al-Mg alloys such as 5086, 5083, and 5456 alloys have been widely applied in structural applications for the bodies of dump trucks, large tanks, pressure vessels, boats, and buildings. However, the microstructure of Al-Mg alloys could be refined since shear bands develop under high strain. [9,20,21] Furthermore, fine-grained Al-Mg sheets and bulk alloys have an improved superplasticity and formability at reasonable operating temperatures.[22–25] To extend their usage in the thick gage and to exploit superplastic forming technology for the components of commercial products, this study investigated refinement in both the grain structure and the second-phase particles of Al-5.8Mg-0.23Mn alloys by reciprocating extrusion, and assessed the improvement in mechanical proper
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