Effect Of Solid Solution And Rolling On The Microstructure And Mechanical Properties Of Mg-3Li-8Gd-2Y-1.5Ag Alloy

Mg-Li alloy is the lightest metal structure material.Its low density,high specific strength and specific elastic modulus,and excellent electromagnetic shielding properties make it widely used in aerospace,automobile,electronics,and national defense.However,the low strength of Mg-Li alloy limits its application in the engineering field.Alloying,heat treatment,and rolling deformation are of great significance to improve the strength of Mg-Li alloy.In this thesis,Mg-3Li-8Gd-2Y-1.5Ag alloy was chosen as the research subject,solution heat treatment and rolling deformation were carried on it to improve its mechanical properties.The microstructure of this alloy was investigated by XRD,OM,SEM,and TEM.The mechanical properties of this alloy were investigated by tensile testing and microhardness testing.Combined with the microstructure evolution,the essential reasons for strengthening the mechanical properties of the alloy were analyzed.It is found that the microstructure of as-cast Mg-3Li-8Gd-2Y-1.5Ag alloy is mainly composed of?-Mg matrix,coarse Mg₂₄(Gd,Y)₅ phases continuously distributed along the grain boundaries,a small number of granular Mg₅(Gd,Y)phases,and square Mg-Gd-Y phases distributed in grains.Prolonging the solution time promotes the dissolution of Mg₂₄(Gd,Y)₅phases,but the grain growth of the matrix also happens.The optimized solution treatment process is 520 ?/4 h.After solution treatment at 520 ?/4 h,the hardness of the alloy is the highest,reaching 84.2 HV.The grain growth of the alloy is the smallest with the average grain size only coarsing from 24.4?m to 25.6?m.Both the strength and elongation of the alloy increase after the solution treatment.The ultimate tensile strength of the alloy increases from 191 MPa to 202 MPa,the elongation increases from 4.4%to 5.2%.Combined with the microstructure evolution,the reasons for the increase of strength and elongation are that the dissolution of Mg₂₄(Gd,Y)₅ phase improves the solid-solution strengthening effect and reduces the possibility of crack formation around the phase.There are some ? phases appear at grain boundaries and twin boundaries after solution treatment and hot-rolling with 30%reduction.There are a few twins,irregular globular?₁ phases and needle-like?'phases appear in grains.The Mg₂₄(Gd,Y)₅ phases distributed along the grain boundaries elongate and break.When the rolling reduction increases to 70%,the number of twins increases and there are some stacking faults appear in them.?phases disappear,and the number density of?₁phases and?'phases becomes higher.Mg₂₄(Gd,Y)₅ phases are further broken.The tensile results show that the increase in rolling reduction can achieve a synergistic improvement in the strength and plasticity of Mg-3Li-8Gd-2Y-1.5Ag alloy.The comprehensive properties of the alloy with 70%reduction reach the optimal state with the yield strength,ultimate tensile strength,and elongation of 130 MPa,243 MPa,and 9.2%,respectively.The reasons for the increase of strength are attributed to the second phase strengthening caused by the broken Mg₂₄(Gd,Y)₅ phases,?₁ phases,and?'phases,the twin boundary strengthening and the stacking fault strengthening.The reasons for the increase of elongation are attributed to the disappearance of?phases at the grain boundaries and the breaking of Mg₂₄(Gd,Y)₅ phases,which decrease the possibility of crack formation.