| As the lightest commercial metallic materials, magnesium alloys have someadvantages such as high specific strength, high specific stiffness and formability. It hasbeen widely used in area space, automobile and electronic communications. However,its low strength and poor ductility have restrained the further applications. Due to somelong period ordered structures in the Mg-RE-Zn alloys, they were proved to havecomprehensive excellent properties. As a perfect grain refiner, Zr element has beenadded to many magnesium alloys with the microstructure refined and mechanicalproperties improved. However, Zr elements react with RE elements in the as-cast alloysand reduce the yield of the RE elements. In addition, during solidification, Zr elementsare prone to sinking and will reduce the metallurgical quality of the materials. Incontrast, Mn addition will purify the ingot and effectively refine the as-extruded alloyswith the formability of the alloys improved. In the current paper, Mn element other thanZr element is added into Mg-RE-Zn alloys to investigate the effects of Mn contents anddeformation process on the microstructure and mechanical properties of this alloy.Various amount of xMn elements (x=0,0.4,0.8,1.2,1.6,2.0, wt.%) was added toMg-10Gd-6Y-1.6Zn alloys. We investigate the effect of Mn elements and deformationprocess on the microstructures and mechanical properties of the alloys under differentconditions, using optical microscopic (OM), scanning electron microscopic (SEM)equipped with energy dispersive microscopic, transmission electron microscopy (TEM),micro-hardness and tensile testing. Combined with the macroscopic and microscopicfracture analysis, the strengthening and toughening mechanism of this alloy is revealed.The results shows that Mn elements have no refining effect on as-cast alloys andthe microstructure of as-cast alloys are the typical dendritic structures. The as-castalloys consist of α-Mg matrix and network dendrites compound. The dendritescompounds are Mg12(Gd,Y)Zn and Mg24(Gd,Y,Zn)5phases when the cooling rate issmall. With Mn contents increased, the dendrite spaces have no obvious difference, butthe relative amounts of the Mg12(Gd,Y)Zn and Mg24(Gd,Y,Zn)5phases changed. Afterhot-extrusion, the alloys fully recrystallized and change into equiaxed grains. As the Mncontent improves, the average grains size was decreased and the alloy with0.8(wt.%)Mn addition exhibits the minimum grain size. The main phase in the as-extrusion alloysis Mg12(Gd,Y)Zn which has two morphologies. One is gray cloud-like phase that is distributed along the grain boundaries while the other is plate-like phase distributedwithin the grains. When0.8(wt.%)Mn element was added to the alloy, the secondphases are mainly distributed along the grain boundaries with some twisted phasesexisting in local area. This microstructure features attributed to the excellent mechanicalproperties. After T5treatment, strength greatly increases with the elongation decreased,but the changing laws of the mechanical properties are identical with the as-extrusionalloys. The optimal solution treatment is heated at530℃for16h. The T6alloys showsrelatively poor strength and elongation which may result from the growth of the grains.Under the rapid cooling rate, a metaphase Mg3Y2Zn3precipitates from the alloys.The rapid cooling rate ensures the refined as-cast and as-extrusion microstructure aswell as the excellent mechanical properties. Forging pre-deformation ensure an evensmaller recrystallized grains and higher mechanical properties. The alloys with higherstrength but obviously-decreased elongation were obtained through T5treatment. Thestrengthening mechanisms of these alloys are caused by grain refinement, precipitationand second phase. Additionally, their toughening is due to the uniform plasticdeformation and grain rotation resulting from the refined grains. |
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