Microstructure And Mechanical Properties Of Mg-Gd-(RY/Sm)-Zn-Z Alloy

Magnesium alloys,as the lightest and green structural metal material,have important application value in many fields such as automobile,aerospace,military industry,communication.However,magnesium alloys still show low strength and poor heat resistance,which restricts their further applications.Mg-Gd alloys have become a research hotspot in recent years for their high strength and good heat resistance.Ordinarily,the high-strength Mg-Gd alloys usually have high Gd content,which cannot meet the design concept of light weight and low cost of magnesium alloys.In present work,Mg-9Gd-based alloys with medium Gd content was used to prepare high-strength and heat-resistant wrought magnesium alloy by introducing a second rare earth element,combining extrusion deformation and heat treatment process.First,the microstructure and mechanical properties of the as-cast,solid-solution and extruded Mg-9Gd-3RY-1Zn-0.5Zr alloys containing the rare-earth element RY are investigated.The dominant intermetallic phases in the as-cast GRYZK9310 alloy are Mg₅RE phase,Mg₃RE,and fine14H-type LPSO phase.After being solution-treated,the alloy mainly contained of a number of lamellar 14H-LPSO phases and irregular Mg₅RE particles.After extrusion,microstructures consist of un-recrystallized regions and DRXed regions.The Mg₅RE particles were disintegrated distribute at extrusion stringers.Besides,a large number of nanoscales Mg₅RE particles and fine 14H-type LPSO phases were dynamically precipitated in the DXRed regions.The as-extruded alloy exhibits excellent mechanical properties at both RT and high temperatures.Its yield strength at room temperature is 387 MPa and its tensile strength is 421 MPa.Its yield strength and tensile strength at 250°C are 319 MPa and 326 MPa,respectively.In addition,the extruded alloy has a weak tensile-compression asymmetry in the ED direction,and the tensile-compression asymmetry ratio is 0.95.The dominant strengthening mechanisms were revealed as dispersion strengthening and fine grain strengthening.The grain refinement and weaker texture contribute to the weaker yield asymmetry.Secondly,the Mg-9Gd-xSm-1Zn-0.5Zr(x=0,1.5,3,4.0)alloy containing light rare-earth element Sm was studied.The results show that the addition of Sm significantly refines the grain size of the as-cast alloy and the second phase mainly is the Mg₃Gd phase in the as-cast GS90 alloy,while the main second phase existing in the as-cast Mg-9Gd-xSm-1Zn-0.5Zr(x=1.5,3.0,4.0)alloy is Mg₃(Gd,Sm)phase,both of which have a porous network structure.After extrusion,the Mg-9Gd-3Sm-1Zn-0.5Zr alloy has the best mechanical properties.The yield strength at room temperature is 369 MPa,the tensile strength is 394 MPa,and the elongation is8%.With the increase of extrusion temperature,the yield strength and tensile strength of GS93 alloy decreased,while the elongation increased.Besides,the proportion of non-recrystallizedb regions decreased,and the size of recrystallized grains increased.In addition,compared with the as-extruded GS93 alloy,the number of broken second phases distributed along the extrusion zone in the solution-extruded GS93 alloy is reduced,the strength does not change significantly,and the elongation is improved.After aging heat treatment of solution-extruded GS93 alloy,the yield strength and tensile strength of the alloy at room temperature have been significantly improved and the yield strength of GS93 alloy can reach 442 MPa and the tensile strength can reach451 MPa.