The Effects Of Zn And Sc On Microstructure And Mechanical Properties Of Mg-Sn Alloys

Magnesium alloys have attracted increasing interest in the applications of aerospace,automotive and electronic products due to their low density,high specific stiffness and specific strength,and good damping performance.However,the general poor mechanical properties,especially the low strength and ductility limit the broad applications of magnesium alloys.Strengthening the magnesium alloys through adding alloying elements and plastic deformation processing have become the main approaches.In recent years,the Mg-Sn based alloys have attracted particular attentions.The Mg₂Sn intermetallic compounds in Mg-Sn based alloys has good thermal stability and can improve the mechanical properties at elevated temperature,however,the mechanical properties of Mg-Sn based alloys at room temperature are still lower than traditional AZ and ZK based magnesium alloys.The addition of Zn element to magnesium alloy can significantly improve its mechanical properties due to the solid solution strengthening,while rare earth elements can refine the microstructure of magnesium alloy and improve its mechanical properties at room temperature by forming secondary phases with good thermal stability.In this thesis,based on Mg-4.5Sn alloy,Mg-4.5Sn-xZn?x=0,1,3,5 wt.%?alloy and Mg-4.5Sn-5Zn-xSc?x=0.1,0.3,0.4 wt.%?alloy were melted in argon-protected medium frequency induction furnace,and then hot-rolled and annealed.The effects of alloying elements,Zn and Sc,on the microstructures and mechanical properties of as-cast and as-rolled Mg-4.5Sn alloys were studied by X-ray diffraction,X-ray fluorescence,optical microscope,scanning electron microscope,electron microprobe,electron backscattered diffraction,transmission electron microscope and mechanical testing machine,respectively,to reveal the strengthening mechanisms.The work aims to develop the economically available magnesium alloys with high strength and heat resist properties.The microstructures and mechanical properties of Mg-4.5Sn-xZn alloy were studied.It was found that adding different content of Zn?1-5 wt.%?could inhibit the growth of dendrite and refine the grain size of the alloy.With the increase of Zn content,the secondary phases transform from??-Mg+Mg₂Sn?eutectic phase to??-Mg+MgZn?eutectic phase and Mg₂Sn phase,and Mg₂Sn coarsens greatly.In addition,due to grain refinement and solution strengthening,the yield strength?YS?of as-cast alloys increases with the increase of Zn content,however,the coarsening behavior of Mg₂Sn results in the rapid expansion of primary cracks during tensile test,therefore,the elongation?EL?and ultimate tensile strength?UTS?of the alloys decrease sharply.After solid solution heat treatment,almost all of the secondary phases dissolve in?-Mg,and precipitates dynamically form during hot rolling and disperse in the?-Mg.With the increase of Zn content,the number and morphology of particles in the alloy do not change significantly.Compared with as-cast alloys,the mechanical properties of as-rolled alloys have been significantly improved.The strengthening mechanisms include work hardening,grain refinement strengthening and precipitates strengthening.With the increase of Zn content,the effect of solution strengthening in the alloy is enhanced.Therefore,as-rolled Mg-4.5Sn-5Zn alloy exhibits an optimum combination of mechanical properties.The YS,UTS and EL are 175.4 MPa,284.4 MPa and 12.8%,respectively.The microstructures and mechanical properties of Mg-4.5Sn-5Zn-xSc alloy were studied.It was found that the grain size of as-cast alloy is decreased from 205 to 95?m by adding Sc?0.1-0.4 wt.%?.With the increase of Sc content,the secondary phase changes from coarse quasi-continuous to finely dispersed morphology.However,the addition of excessive Sc leads to agglomeration of Sc-rich phase.Compared with Mg-4.5Sn-5Zn alloy,as-cast Mg-4.5Sn-5Zn-0.3Sc alloy exhibits an optimum combination of mechanical properties due to grain refinement strengthening and precipitates strengthening.The YS,UTS and EL are 71.6 MPa,214.3 MPa and 11.3%,respectively,which are increased by 13%,28%and 69%,respectively.In addition,the agglomeration of the secondary phases in Mg-4.5Sn-5Zn-0.4Sc alloy results in a sharp decrease in mechanical properties.After solid solution heat treatment,the Sc-rich phases with good thermal stability are not dissolved in the?-Mg,and are elongated and broken during hot rolling.As the Sc-rich phase consumes a large amount of Sn and Zn elements,the dynamic precipitation of particles in as-rolled alloys is weakened.However,proper addition of Sc elements weakens the basal texture and deflects the basal texture along the transverse direction,which on the one hand improves the elongation of the alloys and on the other hand weakens the asymmetry of tension and compression.The as-rolled Mg-4.5Sn-5Zn-0.3Sc alloy exhibits the best mechanical properties.The YS,UTS and EL are 164.2 MPa,293.0 MPa and 21.5%,respectively,and the increase of elongation is 68%.The increase of elongation is due to the weakening of basal texture.The mechanical properties of Mg-4.5Sn-5Zn-0.4Sc alloy are significantly reduced due to the agglomeration behavior of the Sc-rich phase.