Effect Of Sm On The Microstructure And Properties Of ZMT613 Alloy

With low density,high specific strength,excellent damping and shock absorption,magnesium alloy is one of the most promising metal materials.Deformed magnesium alloy as an important part of magnesium alloy,the development of new high-performance deformed magnesium alloy is still the focus of magnesium alloy research.In this paper,ZMT613 deformed magnesium alloy as the base alloy,using the alloying method to add a small amount of rare earth elements Sm,using metallographic microscopy,scanning electron microscopy and energy spectrum analysis,differential thermal analysis,X-ray diffraction analysis,hardness testing and room temperature tensile mechanical properties testing and other testing and analysis means,to study the effect of rare earth element Sm on the microstructure and room temperature mechanical properties of ZMT613magnesium alloy in the as-cast and extruded states.On this basis,the effects of different heat treatment processes（solution treatment,single-stage aging and double-stage aging）on the microstructure and room temperature tensile mechanical properties of the extruded ZMT613-x Sm（x=0,0.2,0.5,0.8,1.2,）alloy are systematically investigated,and the main results are as follows:The as-cast ZMT613 alloy is mainly composed ofα-Mg,Mg Zn₂,Mg₇Zn₃,singlet Mn and Mg₂Sn phases.After the addition of the rare earth element Sm,a ternary rare earth phase Mg Sn Sm is formed in the alloy.As this ternary phase is formed first during solidification,a large amount of Sn element is consumed,which makes the content of the later formed Mg₂Sn phase decrease.With the increase of Sm element content,the coarseα-Mg dendrite gradually refines,and the Mg Sn Sm phase appears in the crystal and near the grain boundaries,and with the further increase of Sm content,the Mg Sn Sm phase also gradually increases and tends to coarsen.The dynamic recrystallization of ZMT613 alloy is sufficient and no extrusion flow lines generate after the hot extrusion treatment.After the addition of Sm element,obvious extrusion flow lines are formed along the extrusion direction,while the grain size of the alloy is further refined,and the average grain size is reduced to 1μm;with the further increase of Sm element,the number and size of the second phase in the alloy are significantly increased.When Sm is added by 0.8 wt.%,the mechanical properties of the alloy in the extruded condition are the optimum,with tensile strength,yield strength and elongation of 335 MPa,221 MPa and 10.9%,respectively.ZMT613-0.5Sm alloy is selected as the test alloy.After solid solution treatment at430°C for 4h,the alloy shows more uniform grain size,sufficient dissolution of the second phase and the best solid solution effect.The single-stage aging temperature of the alloy is 180°C.The hardness of the alloy rises rapidly from 53.8 HB to 87.0 HB within0～3h,and slows down within 3～12h,and reaches a peak hardness of 94.9 HB at 14h,after which the alloy over-ages and the hardness gradually decreases and stabilizes.This determines the best single-stage aging process is 180°C×12h.The single-stage aging study of ZMT613-x Sm alloy shows that after the single-stage aging treatment,the grain size of ZMT613 alloy does not change significantly compared with the solid solution state,but more fine and diffuse Mg Zn₂ phase precipitates in the alloy,which makes the mechanical properties of the alloy improve greatly,and its tensile strength,yield strength and elongation are 350 MPa,302 MPa and 7.8%,respectively.Compared with the extruded state,the tensile strength and yield strength are 9.4%and36.7%higher.The grain refinement of the alloy after the addition of Sm element is due to the high melting point of Mg Sn Sm phase which can nail the grain boundaries and hinder the grain growth during the single-stage aging.When the Sm addition is 0.5wt.%,the fine diffuse second phase in the alloy increases significantly,as the Sm content continues to increase（≥0.8wt.%）,the precipitated phase in the alloy decreases significantly and the Mg Sn Sm phase has coarsening phenomenon.The overall mechanical properties of the single-stage aged ZMT613-0.5Sm alloy is the best,with the tensile strength,yield strength and elongation of 401 MPa,332 MPa and 6.2%,which are14.7%and 9.8%higher than those of the single-stage aged ZMT613 alloy,respectively.The double-stage aged study showes that the grain size of the alloy is similar to that of the single-stage aged after the double-stage aged treatment;the grain size decreases with the increase of Sm content.When a small amount of rare earth element Sm is added（0.3-0.5wt.%）,When a small amount of rare earth element Sm（0.3-0.5wt.%）is added,the fine diffuse Mg Sn Sm phase forms in the alloy,which increases the precipitation density of the Mg Zn₂ phase during aging.When the Sm content is too high（>0.8wt.%）,the coarse Mg Sn Sm phase increases and the cutting effect is enhanced.Due to the pre-aging during double-stage aging,the Mg Zn₂ phase precipitated by final aging is more numerous and more diffuse and fine.The tensile strength,yield strength and elongation of ZMT613 alloy in the two-stage aged treatment are 386 MPa,333 MPa and 7.6%,respectively.The mechanical properties of the alloy in the aged state are further improved by the addition of Sm elements,among which the best mechanical properties are obtained for ZMT613-0.5Sm alloy,whose tensile strength,yield strength and elongation in the two-stage aged state are 417 MPa,317 MPa and 3.6%,respectively and its tensile strength and yield strength increase by 30.3%and 43.5%,respectively,compared with the extruded ZMT613 alloy.