The Effect Of Gd On The Microstructure And Properties Of ZMT813 Alloy

Magnesium alloy is one of the lightest metal structural materials in industrial applications.It has the characteristics of high specific strength,high specific rigidity,and good shock resistance.It is widely used in the automotive industry,aerospace,and military fields.The new Mg-Zn-Mn-Sn wrought magnesium alloy has excellent processing properties and mechanical properties at room temperature.The alloy strength is remarkably high after hot deformation and aging treatment,but its strength is still a certain gap compared with some high rare earth magnesium alloys.As a rare earth element,Gd can refine grains in magnesium alloys,and can also form a second phase,thereby affecting the mechanical properties of the alloy.In order to further improve the mechanical properties of the alloy under the premise of controlling the cost,this paper uses the Mg-8Zn-1Mn-3Sn(ZMT813)alloy as the base alloy and adds a small amount of rare earth element Gd to it.Using optical microscope,X-ray diffractometer,scanning electron microscope and energy spectrum analysis,differential thermal analysis,electron backscatter diffraction analysis and mechanical performance testing and other experimental analysis methods,the effect of the Gd content on the microstructure and mechanical properties of ZMT813 alloy in different states was studied.The research results of this paper show that:(1)The as-cast ZMT813 alloy is mainly composed of?-Mgmatrix,?-Mn,Mg₇Zn₃phase,and Mg₂Sn phase.After adding the Gd element,the dendrites of the alloy are refined,and a new MgSnGd phase appears in the alloy,which increases with the increase of the Gd content.The formation of MgSnGd phase consumes Sn element,resulting in a decrease in the content of Mg₂Sn phase.With the increase of Gd content,the reticulated second phase in the alloy microstructure is gradually broken,and the Mg₇Zn₃ distributed along the grain boundary is gradually refined.After homogenization treatment(the process is optimized to 330?/16 h+420?/4 h),the Mg₇Zn₃ phase cannot be completely dissolved into the matrix due to the solid solubility limit of Zn in the magnesium alloy(Maximum 6.2 wt.%),and the MgSnGd phase also remains on the substrate due to its high thermal stability.(2)Hot extrusion crushes the second phase in the alloy to form extrusion streamlines along the extrusion direction.As the Gd content increases,the extrusion streamlines in the extruded alloy gradually become denser,the volume fraction of the MgSnGd phase gradually increases,and the size of it also increases.The average grain size of the alloy first decreases and then increases as the Gd content increases,ZMT813-0.2Gd alloy has the finest grains.The addition of Gd will gradually strengthen the basal surface texture of the alloy,making it difficult to start the basal surface slip,thereby increasing the strength of the alloy.Among the extruded alloys,ZMT813-0.2Gd alloy has the best comprehensive mechanical properties,and its yield strength,tensile strength and elongation was 262 MPa,377 MPa and 14.1%,respectively.However,when the Gd content is increased to 0.5 wt.%,the increase of the grain size,the decrease of the Mg₂Sn strengthening phase,and the coarsening of the MgSnGd phase lead to a decrease in the strength of the alloy.(3)The alloy has been treated with solution treatment and aging treatment(optimized process is 420?/2 h+180?/12 h),the MgSnGd phase in the alloy can delay the recovery process and retain some dislocations,increasing the heterogeneous nucleation points of the?1'phase(MgZn₂)during the aging treatment,thereby enhancing the aging strengthening effect and accelerating the aging response.However,the increase of Gd content will also lead to the decrease of the precipitation of Mg₂Sn in the alloy and weaken the effect of aging strengthening.Therefore,the addition of Gd element needs to consider the two effects.Experimental studies have found that when the Gd content is 0.2 wt.%,the MgSnGd phase indirectly promotes the precipitation of the?1'phase well,and the content of the Mg₂Sn precipitated phase in the alloy does not decrease too much.Therefore,ZMT813-0.2Gd among the aged alloy has the best comprehensive mechanical properties,and its yield strength,tensile strength and elongation was 348 MPa,389 MPa and 2.5%,respectively.