Effect Of Ag On Microstructure And Properties Of Mg-11Gd-2Y-0.5Zr Alloy

Magnesium alloy is the lightest commercial engineering structure metal material in the world.It is widely used in automobile,military industry and aerospace fields.However,magnesium alloys also have the disadvantages of poor mechanical properties and weak creep resistance at high temperature,which makes the application of magnesium alloys greatly limited.Therefore,the development of high strength heat resistant magnesium alloy is an important direction for the development of magnesium alloy,and the addition of rare earth elements in magnesium alloy is an important way to strengthen the properties of magnesium alloy.Ag has good solid solution strengthening,second phase strengthening and fine crystal strengthening in magnesium,and proper amount of Ag is added with rare earth elements,which can significantly improve the mechanical properties of magnesium alloys.In this paper,Mg-11Gd-2Y-0.5Zr alloy was used as the matrix alloy and Ag element was added.On the basis of heat treatment process optimization,the influence of adding different Ag contents（0,0.5,1.0,1.5,2.0,wt.%）on the microstructure and properties of the alloy was investigated.The optimization results of heat treatment process show that after the as-cast Mg-11Gd-2Y-1.5Ag-0.5Zr alloy was treated with 525℃for 8 h,most of the second phase in the alloy was solid solution in the matrix,a small amount of insoluble second phase was evenly distributed,and the grain size is obviously refined.When the alloy was aged 10 h at 225℃,the grain size reached the minimum and the precipitated phase was evenly distributed.The optimum heat treatment process of the alloy is 525℃solid solution 8 h and 225℃aging 10 h.The microstructure of the as-cast Mg-11Gd-2Y-0.5Zr alloy consists ofα-Mg、Mg₅Gd and Mg₂₄Y₅phases.The addition of Ag creates new phase Ag₂Gd.As the amount of Ag addition increases,the number of the second phase of the as-cast alloy continues to increase and refine,and the grain size decreases gradually to the continuous network distribution along the grain boundary.But when the addition of Ag is more than 1.5%,the second phase begins to aggregate.The addition of proper amount of Ag can increase the yield strength and tensile strength of as-cast alloy at room temperature.When the addition amount of Ag is 1.5%,the tensile strength of the alloy reaches the maximum value of 226 MPa.The as-cast microstructure and mechanical properties of the alloy were optimized when the Ag content was 1.5%.The second phase of the aged Mg-11Gd-2Y-x Ag-0.5Zr alloy is the same as that of the as-cast alloy.With the increase of Ag addition,the second phase of the aging alloy increases,and the grain size decreases first and then increases,which is the smallest and the second phase distribution is more uniform when the addition amount of Ag is 1.5%.Adding appropriate amount of Ag can significantly increase the room temperature and high temperature strength of aging alloy.With the increase of Ag content,the tensile strength and yield strength of aged alloy at room temperature and high temperature first increased and then decreased,reaching the maximum value when the Ag content was1.5%.The maximum tensile strength at room temperature and 300℃was up to 355.5MPa and 256.3 MPa,respectively.And the performance below 250℃is high and very stable,and has good heat resistance,even at 300℃high temperature there is still a relatively satisfactory high temperature strength.The aged microstructure and mechanical properties of the alloy were optimized when the Ag content was 1.5%.The aged Mg-11Gd-2Y-1.5Ag-0.5Zr alloy has good creep resistance at high temperature under（225℃、250℃、275℃）/（110 MPa、130 MPa、150 MPa）creep conditions,and still has good creep resistance under 275℃/110 MPa creep conditions.In the scope of this study,the creep mechanism changes from dislocation slip mechanism to dislocation climbing mechanism with increasing temperature.Under the condition of 275℃/130～150 MPa high temperature and high stress creep,the power law creep law fails and the creep mechanism is complicated.