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Effects of Zn Content and Heat Treatment on Microstructures and Mechanical Properties of Mg-Zn-Mn Wrought Magnesium Alloys

Zhang Dingfei1,2,Zhao Xiabing1,Shi Guoliang1,Qi Fugang1(1.Chongqing University,Chongqing 400045,China)(2.National Research Center for Magnesium Alloys,Chongqing 400044,China)  
The roles of Zn content and thermomechanical treatment in affecting microstructures and mechanical properties of Mg-x wt%Zn-1wt%Mn(x=4,5,6,7,8,9) wrought Mg alloys were investigated.The microstructure during extrusion was extremely refined by dynamic recrystallization(DRC).As Zn content increasing,the DRC grains tended to grow up;at the same time,more second phase streamlines would be present which restricted their further growth.During solution treatment,the DRC grains would rapidly grow up;however,more Zn content could obstruct the grain boundary expanding which resulted in finer grains.The microstructures of the aged specimens were examined by transmission electron microscopy(TEM).The age-hardening resulted mainly from two metastable phases,known as β1' and β2',the interfaces between which and the matrix were coherent or semicoherent.β1' phases formed as rods with their long axis parallel to the [0001]α direction of the α-Mg matrix could act as a more enormous impediment to the motion of dislocations than β2' formed as plates on(0001)α.Based on X-ray diffraction,both phases might be the Laves phase MgZn2.Compared with the one-step aging,β1' and β2' in two-step aged alloys were much finer and more dispersed,since nano-sized and coherent G.P.zones formed during the pre-aging stage provided more effective nuclei for the two phases during the second stage.Higher Zn content was apt to cause overaging,so large Mg-Zn blocks were formed by consuming β1' and β2' after aging at 180 oC for 16 h.Mn was present as rod-shaped precipitates in the aged specimens,which could act as nuclei for β1' and β2' and drive them into coarser ones.The mechanical properties of as-extruded samples were not so sensitive to the changes of Zn content.The strength of as-aged samples rose as a parabolic curve with the increasing of Zn content,while Zn content exceeded its solid solution limit in Mg(6.2 wt%),strength rising became slow,and the elongation quickly deteriorated,so a Mg-Zn-Mn alloy with 6wt% Zn possessed the best mechanical properties.
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