Microstructure And Properties Of Low Alloyed Mg-Zn-Re-Ca-Mn Alloys

As the lightest metal structural material in engineering applications,the magnesium alloy has great application potential in aerospace,weapon equipment,automobile,electronic product and other fields,and is known as"the green engineering material in the 21st century".In this paper,a small amount of rare earth(Sm/Y)was added into Mg-Zn-Mn-Ca alloy to develop a new type of low alloying deformed magnesium alloy with high performance by hot extrusion and annealing.X-ray diffraction(XRD),scanning electron microscope(SEM),electron backscattering diffraction(EBSD)and transmission electron microscope(TEM)were used to characterize the microstructure of the alloys in different states and their effects on mechanical properties.The effect of element segregation on grain growth was mainly explored.The main results are as follows.The as-extruded Mg-2Zn-xSm-0.8Mn-0.6Ca(x=0,1)alloys are mainly composed of?-Mg phase,Ca₂Mg₆Zn₃ phase and nanoscale Mn particles.In the Mg-2Zn-1Sm-0.8Mn-0.6Ca alloy,the secondary phase containing Sm is not formed due to the solid solution of Sm in the matrix.The average grain size of the two extruded alloys is 2.4?m according to EBSD analysis,and both the two alloys have a weak basal texture.During annealing,the growth rate of grain size of Mg-2Zn-1Sm-0.8Mn-0.6Ca alloy is significantly slower than that of Mg-2Zn-0.8Mn-0.6Ca alloy,and the difference of grain size increases with the increase of annealing temperature,which leads to the increasing difference of yield strength between the two alloys.The yield strength of Mg-2Zn-0.8Mn-0.6Ca and Mg-2Zn-1Sm-0.8Mn-0.6Ca as extruded alloys decreases from 223 MPa and 255 MPa to 195MPa and 241 MPa,respectively.As a result,the difference of yield strength between the two alloys increases from 32 MPa to 46 MPa.From the perspective of geometrically necessary dislocations,the driving force of grain size growth is almost the same in both alloys,that is,the grain growth is mainly related to the retarding effect.It is found that the co-segregation of Zn,Ca and Sm at grain boundaries exists in the Mg-2Zn-1Sm-0.8Mn-0.6Ca alloy,which is more conducive to dragging the grain growth.The content of rare earth in the extruded Mg-2RE-0.8Mn-0.6Ca-0.5Zn(RE=Sm,Y)alloy increases,and Mg₄₁Sm₅ and Mg₂₄Y₅ phases appear in the alloy containing Sm and Y,respectively.The extruded Mg-2RE-0.8Mn-0.6Ca-0.5Zn(RE=Sm,Y)alloy shows excellent mechanical properties,especially the Mg-2Y-0.8Mn-0.6Ca-0.5Zn alloy.The strengthening factors of extruded Mg-2RE-0.8Mn-0.6Ca-0.5Zn alloys mainly include:a)micron grain size,b)residual dislocation,c)uniformly distributed nano precipitation,d)solute co-segregation at grain boundaries.In addition,for the Mg-2Y-0.8Mn-0.6Ca-0.5Zn alloy with relatively low recrystallization degree,the texture strengthening also contributes to the strength.It is found that it is difficult to fundamentally improve the mechanical properties of the alloy by alloying only,and it may be better to change the deformation process to form fine grain structure and increase the dislocation density.