Achieving enhanced ductility in a dilute magnesium alloy through severe plastic deformation
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27/4/04
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Achieving Enhanced Ductility in a Dilute Magnesium Alloy through Severe Plastic Deformation KIYOSHI MATSUBARA, YUICHI MIYAHARA, ZENJI HORITA, and TERENCE G. LANGDON Experiments were conducted to evaluate the utility of a new processing procedure developed for Mg-based alloys in which samples are subjected to a two-step processing route of extrusion followed by equal-channel angular pressing (designated as EX-ECAP). The experiments were conducted using a Mg-0.6 wt pct Zr alloy and, for comparison purposes, samples of pure Mg. It is shown that the potential for successfully using ECAP increases in both materials when adopting the EX-ECAP procedure. For the Mg-Zr alloy, the use of EX-ECAP produces a grain size of ⬃1.4 m when the pressing is undertaken at 573 K. By contrast, using EX-ECAP with pure Mg at 573 K produces a grain size of ⬃26 m. Tensile testing of the Mg-Zr alloy at 523 and 573 K after processing by EX-ECAP revealed the occurrence of significantly enhanced ductilities with maximum elongations of ⬃300 to 400 pct.
I.
INTRODUCTION
THE low density and good machinability of magnesium alloys makes them attractive for a wide range of structural applications in transportation and, especially, in the automotive field.[1,2] These alloys also exhibit excellent damping capacities, so that they are especially attractive for use in applications where it is necessary to dampen external vibrations, as in helicopters and satellites. These beneficial features have led to the prediction by Friedrich and Schumann[3] of a “new age of magnesium.” Despite the inherent potential advantages of magnesiumbased alloys, their hexagonal crystal structure provides only a limited number of active slip systems, giving low levels of ductility and, consequently, difficulties in forming complex components. It is well established in recent work that the ductilities of metallic alloys may be significantly enhanced, often to the superplastic range, by processing the alloys through the introduction of severe plastic deformation (SPD).[4] Conventional methods of SPD processing include equal-channel angular pressing (ECAP),[5,6,7] where a sample is pressed repetitively through a die confined within a channel bent through an angle at, or close to, 90 deg, and high-pressure torsion (HPT),[8,9,10] where a disk is subjected to a high pressure and concurrent torsional straining. The procedures of ECAP and HPT are both effective in producing substantial grain refinement in fcc metals such as pure aluminum,[11,12] pure nickel,[9,10] and a range of aluminum alloys,[13–16] but, in practice, ECAP appears to be the more useful technique because it utilizes fairly large samples, it can be scaled up relatively easily to produce large KIYOSHI MATSUBARA and YUICHI MIYAHARA, Graduate Students, and ZENJI HORITA, Professor, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan. TERENCE G. LANGDON, Professor, is with the Departments of Aerospace & Mechanical En
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