Study On Composition Optimization,microstructure And Properties Of High-Strength And Heat-resistance Mg-Gd Alloys

As the lightest metallic structural material in practical engineering applications,magnesium alloys are widely used in prospects persuing weight-reduction,such as aerospace,weaponry,transportation and electronic products.In particular,the Mg-Gd-Y series alloys have excellent mechanical properties at room temperature and elevated temperature,so they are ideal materials for light-weight structures.However,in the actual smelting and casting process,it is found that the yttrium oxide inclusions are easily formed in the Mg-Gd-Y alloys due to the active nature of the Y element.Such inclusions are often distributed in the casting parts in the form of oxide film fragments,which may severely damage the mechanical properties of the cast parts,decrease the yield of castings and makes it difficult to control the Y content between different batch.The Y inclusions issue greatly limits the application of Mg-Gd-Y alloys.Therefore,to reduce the Y content in the Mg-Gd-Y series alloys is of great significance for reducing the yttrium oxide inclusions and develop new low-Y content or Y-free magnesium rare earth alloys with high strength and heat resistance.Firstly,based on the Mg-9Gd-3Y-0.5Zr-0.5Zn alloy,under the condition that the total mass fraction of rare earth is 12 wt.%,the Mg-x Gd-(12-x)Y-0.5Zn-0.5Zr(x=9,10,11,12)alloys were prepared by replacing a part of the Y element with Gd element which is less active and easier to control.The effects of Y content on the microstructure,mechanical properties at room temperature and elevated temperature of the Mg-x Gd-(12-x)Y-0.5Zn-0.5Zr alloys were studied.It is found that the microstructure of the as-cast Mg-x Gd-(12-x)Y-0.5Zn-0.5Zr alloys consisted of?-Mg matrix and(Mg,Zn)₅(Gd,Y)secondary phases distributed along the grain boundaries.The grain size of the Mg-x Gd-(12-x)Y-0.5Zn-0.5Zr alloys gradually increased with the decrease of Y content.Especially,the average grain size of the as-cast Mg-12Gd-0.5Zn-0.5Zr alloy was much coarser than that of the Y-containing alloys,which was about 112?m.With the decrease of Y content,the mechanical performance at room temperature of the peak-aged alloys did not decrease significantly,but the 9Gd-3Y and 12Gd alloys exhibited relatively lower elongation.At high temperature,with the decrease of Y content,the number of second phase particles at the grain boundary decreased,the pinning effect on the grain boundary weakened,and the yield strength and tensile strength of the alloy decreased obviously.At 300?,the yield strength and tensile strength of 9Gd-3Y alloy were 193 MPa and 248 MPa,respectively,while those of 12Gd alloy were only 120 MPa and 158 MPA,respectively.The fracture surfaces of the room temperature tensile tests were characterized by cleavage surfaces and tearing edges,indicating that the fracture mode of the alloys at room temperature was quasi-cleavage fracture.Compared with room temperature tensile tests,there were a few dimples and necking phenomenon on the fracture surface at 150?,which exhibited a mixed fracture mode of quasi-cleavage and toughness.The number of dimples on the fracture surface decreased with the decrease of Y content.With the further increase of tensile temperature to300?,the dimples of 9Gd-3Y alloy became more and deeper,which showed the characteristic of ductile fracture,while the fracture mode of 10Gd-2Y,11Gd-1Y and 12Gd alloy was intergranular fracture.Simply reducing Y content and increasing Gd content is dentrimental to the high-temperature performance of the alloys,so further composition optimization is needed.Based on the study on Mg-x Gd-(12-x)Y-0.5Zn-0.5Zr alloys,in order to develop a high-strength heat-resistant magnesium alloy completely free of Y elements,Er element was used to replace the Y element completely,and different content of Zn element was added.The influence of Zn content on microstructure,mechanical properties and tensile fracture behavior of Mg-9Gd-3Er-x Zn(x=0,0.5,1,2)alloys was studied.The microstructure of the as-cast alloys mainly consisted of?-Mg matrix and(Mg,Zn)₃(Gd,Er)eutectic phase.With the increase of Zn content,the grain size of the as-cast alloy was refined slightly.After solution treatment,a lamellar LPSO structure appeared in the?-Mg matrix,and its volume fraction increased with Zn content.It is found that the addition of Zn can effectively improve the mechanical properties and heat resistance of the peak-aged Mg-9Gd-3Er-x Zn alloys.When the content of Zn is 0.5 wt.%,the alloy showed better mechanical properties due to the strengthening of precipitated phase and the solid solution strengthening of Zn.At room temperature,the yield strength was 216 MPa,the tensile strength was 295 MPa and the elongation was 7.1%.At elevated temperature,the activity of dislocation was enhanced,and the strengthening effect of LPSO structure was more effective.1Zn alloy showed good heat resistance at 150?,the yield strength was 149 MPa,the tensile strength was 236 MPa and the elongation was 19.2%.