Microstructure,mechanical Properties And Age-hardening Behaviour Of Mg-7Sn Alloy

As global energy consumption increases,the lightweight of vehicles could significantly reduce energy consumption and carbon emissions.Mg alloys,with their low density and high specific strength,have great potential for weight reduction in automotive materials.Mg-Sn alloys are low-cost alloy systems with good thermal stability and high precipitation strengthening that have received a lot of attention from researchers.Currently,the poor age-hardening behavior and low room-temperature plasticity of Mg-Sn alloys limit their development and application.This paper investigates the microstructure evolution,toughness mechanism,and age-hardening behavior of Mg-7Sn（wt.%）alloy by hot extrusion and stress aging.Firstly,the microstructure and mechanical properties of Mg-7Sn alloys are investigated at the same extrusion ratio with different extrusion temperatures.The E160,E220 and E230alloys are prepared by extrusion at 160℃,220℃and 230℃with an extrusion ratio of 9:1,respectively.The basal texture forms after extrusion deformation,and the basal texture intensities weaken with increasing extrusion temperature,which is mainly attributed to the dynamic recrystallization（DRX）and activation of non-basal slip.The extruded E160 alloy exhibits high tensile strength at room temperature(σ_UTS=277.2 MPa)due to grain refinement and precipitation strengthening arising from a high volume fraction of nanoscale Mg₂Sn phases.As the extrusion temperature increases,the DRX of extruded alloy is promoted.The grain size of the alloy is refined and the plasticity of the alloy is further improved.The improved plasticity of alloy is related to grain refinement and activation of the non-basal slip systems.The contributions of grain boundary strengthening,precipitation strengthening and texture strengthening to the yield strength of the alloy are also quantitatively calculated and it is found that grain boundary strengthening is the main strengthening mechanism,followed by precipitation strengthening of the nanoscale Mg₂Sn phase.Secondly,the microstructure and mechanical properties of Mg-7Sn alloys are investigated at the same extrusion temperature with different extrusion ratios.The 9E230 and17E230 alloys are prepared by extrusion at 230℃with the extrusion ratio of 9:1 and 17:1,respectively.The microstructure of the extruded alloys consists ofα-Mg and nanoscale Mg₂Sn phase,and the grain size is refined with the increase of the extrusion ratio.The average grain sizes of the extruded 9E230 and 17E230 alloys are 2.1μm and 1.86μm,respectively.Both two alloys appear basal texture after extrusion deformation and the texture intensities of the alloy increase with the increase of the extrusion ratio.Theσ_TYS,σ_UTSandε_TELof the extruded17E230 alloy are 189.25 MPa,259.97 MPa and 18.85%,respectively,which increases by3.82%,0.75%and 80.72%as compared with 9E230 alloy with an extrusion ratio of 9:1,respectively.The good ductility of the extruded 17E230 alloy is mainly attributed to grain refinement and activation of the non-basal slip system.The comparison of different strengthening mechanisms shows the precipitation strengthening is dominant in the incremental yield strength of the extruded alloy with increasing extrusion ratio.Based on the above research,it is found that low extrusion temperature and large extrusion ratio could increase the internal distortion energy of Mg-Sn alloy and the dislocation density,promote the nucleation of the precipitation phase,and finally affect the morphology,number,distribution,and orientation of the precipitation phase.Therefore,this project provides a method that uses the compressive stress-assisted aging to optimize the above properties of the precipitated phase of Mg-Sn alloy to improve the precipitation behavior of the alloy.As a result,the age-hardening behavior of the solid solution Mg-7Sn alloy is investigated at 220℃with 40 MPa compressive stress.The phase transition occurs from the metastableβ’（Mg₃Sn）phase to theβ（Mg₂Sn）phase during stress aging.Theβ’with L1₂structure has an orientation relationship with the Mg matrix of 11 0_β’//112 0_αand1 1 0_β’//[0001]_α.The precipitated sequence in the stress-aged alloy is S.S.S.（Supersaturated solid solution）→GP→β’（Mg₃Sn）→β（Mg₂Sn）.The stress-aged alloy exhibits a high peak-aged hardness（～71 HV）and a short time to reach the peak hardness（12 h）as compared with the aged alloy without stress.A long-time aging platform with 71 HV for 96 h is observed during stress aging.The formation of the platform is related to a balance of hardening and softening arising from microstructure evolution during stress aging.Stress aging provides a promising heat treatment way to improve the aging hardening response of Mg alloys.