| As the lightest metal structure material,magnesium alloy is widely used in many fields,such as aerospace,automobile,medical and electronic products.Mg-Gd-Y-Zr system magnesium alloy has excellent aging strengthening properties and good heat resistance.It has become one of the most potential commercial alloy systems.The alkaline earth element Ca can reduce the flammability,improve the oxidation resistance and corrosion resistance of the molten magnesium alloy,and expand its application scope.Moreover,the price of Ca is low and the density is small,and the addition of Ca to the magnesium alloy can still retain the characteristics of lightweight magnesium alloy.Therefore,it will be an important research direction to add Ca element to rare earth magnesium alloy and develop new magnesium alloy containing Ca on the basis of ensuring its mechanical properties.In this paper,the microstructure and properties of Mg-11Gd-3Y-x Ca-Zr(x=0,0.2,0.4,0.6,0.8 wt%)magnesium alloy were studied by means of optical microscope(OM),scanning electron microscope(SEM)with EDS,X-ray diffractometer(XRD)and transmission electron microscope(TEM).The results show that:The microstructure of as cast Mg-11Gd-3Y-0.5Zr alloy is mainly composed of?-Mg matrix,Mg5Gd and Mg24Y5phases.With the addition of Ca,the grains are significantly refined,and with the increase of Ca content,the formation of new phase Mg2Ca promotes the precipitation of Mg5Gd and Mg24Y5phases.The tensile strength of the alloy increases with the increase of Ca content.When the Ca content is 0.8%,he tensile strength reaches the maximum value of 217 MPa,the elongation decreases with the increase of Ca.The heat treatment process of Mg-11Gd-3Y-0.8Ca-0.5Zr as cast alloy was optimized,and the optimal heat treatment process was obtained as follows:solution process 485?×16 h+505?×16 h.The aging process is 225?×12 h.The phases of aging Mg-11Gd-3Y-0.5Zr alloy is?-Mg,Mg5Gd and Mg24Y5.With the increase of Ca element,new phase Mg2Ca is produced,and the second phase is significantly increased,the grain size is smaller.Compared with the structure of as cast alloy,the coarse second phase transformation is uniform,small and dispersive,and the grain boundary is clearer.At the same tensile temperature,the tensile strength of the alloy reaches the peak value at 0.2%Ca,and reaches 327.43 MPa at room temperature.In the range of 25??250?,the alloy is insensitive to the temperature change,and its properties are relatively stable;at 300?,the tensile strength remains 254.48 MPa,and it has good heat resistance.However,the percentage elongation decreased with the increase of Ca content.Among them,the alloy with 0.2%Ca not only has the highest room temperature and high temperature strength,but also has high elongation and good comprehensive properties.In the scope of this study,the addition of micro amount of Ca(0.2%?0.4%)can increase the self corrosion potential,reduce the corrosion current density and improve the corrosion resistance of Mg-11Gd-3Y-0.5Zr alloy,but the addition of excessive Ca(0.6%?0.8%)will deteriorate the corrosion resistance of the alloy.The main corrosion products of Mg-11Gd-3Y-x Ca-0.5Zr alloy are Mg(OH)2.The aging Mg-11Gd-3Y-0.2Ca-0.5Zr magnesium alloy has excellent creep resistance at high temperature.Even under the harsh condition of 250?/130 MPa,the creep rate of the alloy is only 1.6545×10-8,and the creep strain after 100 h creep is only 0.995%.Creep is mainly controlled by grain boundary slip and dislocation slip.With the increase of creep temperature and stress,the creep mechanism changes from grain boundary slip to dislocation slip. |
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