| Magnesium alloys have become a new favorite of " new structural materials in the 21st century" because of their excellent characteristics,which have replaced a large number of steel and aluminium parts due to their high specific strength and excellent damping,and this trend will become more and more obvious.Magnesium alloys,as the earliest lightweight alloys,have been successfully used in 3C,aviation,navigation,automobile,etc,people pay more and more attention to them,in the future,they will inevitably develop into one of the most potential structural and functional materials.However,due to the lattice properties of magnesium,magnesium alloys have low strength and elongation at room temperature.Among them,AZ91 magnesium alloy has been successfully applied in the shell of electrical products,small-sized thin or special-shaped brackets,but the poor elongation of AZ91 alloy limits its wider application.So,alloying and heat treatment were used to improve the structure and properties of AZ91 alloy to achieve the purpose of high strength and high plasticity,and the grain refinement process and mechanism of the alloy were studied to provide reference value for the development of magnesium alloy grain refinement technology.Therefore,as-cast AZ91-X(Sn,RE(Ce/La))alloys with fine structure and good properties were prepared by alloying,further improvement of microstructure and comprehensive mechanical properties of the alloys by reasonable heat treatment process.The structure of the alloys was characterized by metallographic microscope,XRD and SEM.The mechanical properties of the alloys were tested by an electronic universal material tensile testing machine.The following conclusions were drawn:(1)The grain size of as-cast AZ91 alloy was refined by adding Sn element.The average grain size decreased from 105 to 45 ?m,and the coarse Mg17Al12 phases changed from continuous distribution to fine discontinuous distribution.There was a concomitant relationship between Mg2Sn and Mg1/Al12 phases,and a habit plane between them.Mg2Sn phases were heterogeneous nucleation point of Mg17Al12 phases.When Sn content was 0.8wt.%,the tensile strength and elongation of the alloy reached 206 MPa and 8.15%,compared with as-cast AZ91 alloy.which were increased by 40.14%and 67.35%,respectively.(2)The grain boundaries of AZ91-Sn alloys were clearly visible after solution treatment(400?×10 h,water quenching),almost all Mg17Al12 and Mg2Sn phases were solid-soluble in ?-Mg matrix.After heat treatment,the fine spherical Mg2Sn phases were dispersed in the matrix and the Mn17Al12 phases were semi-continuous at the grain boundary of AZ91-0.8Sn alloy,the mechanical properties of the alloy were obviously improved.The mechanical properties of the alloy reached the peak value after ageing at 200? for 12 h,the tensile strength was 240 MPa and the elongation was 7.21%.The improvement of mechanical properties was attributed to the combination of solution strengthening and dispersion strengthening.Optimum heat treatment scheme:solid solution condition:400?×10 h,water quenching,aging condition:200?×12 h.(3)The addition of RE promoted the refinement of the AZ91 alloy.A small amount of RE can improve the properties of the alloys.When RE content was more than 1.0wt.%,the mechanical properties of the alloys decrease because of the growth and coarsening of long strip Al11RE3 phases.After the addition of Sn element into AZ91-RE alloy,when the alloy melts solidified,the RE element inhibited the diffusion of Sn element,resulting in the formation of no Mg2Sn phase,and the Sn and RE formed long strip hard brittle phase RESn3,which ultimately reduced the mechanical properties of the alloy.(4)The addition of Sn element was better than that of RE(Ce/La)in improving the structure and mechanical properties of AZ91 alloy. |
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