Development of Very High Strength and Ductile Dilute Magnesium Alloys by Dispersion of Quasicrystal Phase
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I.
INTRODUCTION
REDUCING weight of automobiles is a high priority for enormous savings in fuel consumption and global warming. It is therefore essential to use more of magnesium alloys, which are the lightest of all structural metals, and thus important to develop magnesium alloys with better mechanical properties. Common techniques for strengthening alloys are grain refinement, precipitation, and dispersion of hard particles. Magnesium alloys have a high slope of Hall–Petch plot, therefore grain size refinement is very effective in strengthening.[1] Grain refinement occurs during wrought processing, but which also leads to generation of a basal texture, resulting in anisotropy of deformation in tension and compression. Fine-grained alloys show better ductility and less anisotropy. Grain refinement during wrought processing can be enhanced by particle stimulated nucleation (PSN) during dynamic recrystallization by dispersion of hard intermetallic phase particles.[2] An alloy system of great interest is the ternary Mg-Zn-RE, where RE is Y or a rare earth element. Mg-rich alloys in this system contain a ternary intermetallic phase forming on grain boundaries which ALOK SINGH, Chief Researcher, is with the Microstructure Design Group, Structure Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. Contact e-mail: [email protected] YOSHIAKI OSAWA, Chief Researcher, is with the Structure Materials Unit, National Institute for Materials Science. HIDETOSHI SOMEKAWA, Senior Researcher, is with the Toughness Design Group, Structure Materials Unit, National Institute for Materials Science. TOSHIJI MUKAI, Professor, is with the Department of Mechanical Engineering, Kobe University, 1-1 Rokkadai, Kobe, Japan. CATHERINE J. PARRISH, Engineer, is with the Materials and Processes, Boeing Research and Technology, The Boeing Company, Huntington Beach, CA. DONALD S. SHIH, Technical Fellow, is with the Boeing Research and Technology, The Boeing Company, St. Louis, MO. Manuscript submitted June 27, 2013. Article published online October 16, 2013 3232—VOLUME 45A, JULY 2014
strengthen the grain boundaries, and strengthened by solid solution and precipitation of Mg-Zn phases.[3] The Mg-Zn-Y alloy system contains three ternary phases, each of which makes a two phase field with a-Mg.[4] One of them is a superstructure of magnesium, known as long-period superstructure-ordered phase (LPSO), whose dispersion strengthens the alloy. Very high strengths have been reported in combination with fine grain sizes, but with limited ductility.[5–7] Another ternary phase Mg3Zn6Y has a quasicrystalline structure with an icosahedral symmetry, or i-phase.[8,9] Good strengths with moderate ductility have been reported by dispersion of this phase by wrought processes.[10–18] Quasicrystal phases are known to have properties such as very high hardness and low surface energy.[19] They also show matching or epitaxial interfaces with crystalline phases, including magnesium matrix, on all facets, because of their high crys
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