Microstructure And Mechanical Properties Of Microalloyed Mg-Sn Series Alloys

Owing to the low density,high specific strength and specific stiffness and satisfactory damping capacity of magnesium(Mg)alloys,they have attracted worldwide attentation,particularly in the automotive,aircraft and consumption electrical products.Meanwhile,among various Mg alloy systems,Mg-Sn based alloy have been stood out,due to their cost advantage,excellent creep resistance,loose extrusion conditions and so on.Unfortunately,a high Sn content would inevitably increase the costs and Mg-Sn binary alloy still exhibit sluggish aging kinetics and low ductility and strength,which inevitably greatly restrict their further application.Our research is revolved on the Mg-1.0 wt.%Sn alloy,while the comprehensive mechanical properties are improved through microalloying(Zn;Ca;Al;Ce),compound micalloying(Zn and Ca;Ce and Gd)and microalloying in combination with pre-twinning deformation.The details of research content and relevant results obtained are as follows:(1)The addition of 0.5 wt.%Zn improved both the yield strength and the ductility of the Mg-1.0Sn alloy because of the combined effects of solid-solution hardening and solid-solution softening.The significantly increased yield strength after 0.5 wt.%Ca addition,owing to grain refinement and precipitation strengthening,whereas the ductility improvement was attributed to the refined grains and weakened ED-tilted texture.After Zn and Ca compound addition,the better ductility was mainly ascribed to the increased activation of prismatic slip,enhanced of grain boundary cohesion and improved intergranular strain propagation capacity.Besides,Individual addition of Ca as well as the combined addition of Zn and Ca both strongly affected the strain hardening behaviour of the Mg-1.0Sn alloy,owing to more basal slip activation and grain refinement.(2)Compared with the AZ31 alloy,the ratio between tension and compression in Mg-1.0Sn-0.5Ca-0.5Zn(TZX100)was greatly improved,owing to refined grains,weakened and scattered texture amd a certain amount of Ca Mg Sn phase(strip-like distributed along ED).The excellent three-point bending performance of TZX100 alloy at room temperature was dependent on grain refinement,improvement in microstructural homogeneity and weakened ED splited texture(The improvement of thickness-direction strain accommodative ability in the outer tensile region of bended sheet).(3)With the increase of Ca content in Mg-1.0Sn-0.5Zn alloy,the microstructure changed seriously,includes grain refinement,secondary phase transformation(from intial Mg₂Sn to Ca Mg Sn,Mg₂Ca and even Ca₂Mg₆Zn₃ phase)and texture modification(from strong basal texture to weakened,ED splited texture).The main factors,which induced mechanical properties variation,contained grain refinement,precipitation strengthen and weakened texture.As for the Zn addition in Mg-Sn-Ca alloy,it would accelerate the formation of Ca Mg Sn and Ca₂Mg₆Zn₃ phase,which contributed to the improvement in microstructural homogeneity and accompanied texture tailoring,weakening through particle stimulated nucleation(PSN)effect.As a result,the n-value and formability of Mg-Sn-Ca alloy increased with Zn content increasing,meanwhile the value of YS/UTS_avewas decreased gradually.(4)In Mg-Sn-Ca alloy,Ce addition resulted in grain refinement and secondary phase transformation(from initial Ca Mg Sn to(Ca,Ce)Mg Sn and Mg₁₂Ce phase).The addition of Ce gave rise to texture weakening(i.e.decrease of maximum intensity and increase of tilting degree)of as-extruded Mg-Sn-Ca based alloys,closely associated with the increased Ce contained secondary phase,which aggravated the restriction on DRXed grain growth during extrusion.Actual experimental data(EBSD measurements)in conjunction with VPSC indicated that the addition of 0.2 wt%Ce could bring about vigorously activated basal slip and homogeneous distributed tensile strain during the tensile process,and consequently contributed to the ductile improvement.As for Al addition in Mg-Sn-Ca alloy,it promoted the formation of more Ca Mg Sn and Mg₂Ca phase.The segeration of Al,Ca in grain boundary and PSN effect induced by Ca Mg Sn phase common led to the texture weakening and scattering(from an ED-spilted texture to a symmetrical and elliptic texture).Owing to the appearance of additional TD texture component,Al addition accelerated the activation of basal slip,improved the ductility when tension along TD,and ultimately alleviated the mechanical anistorpy.(5)The addition of Ce into Mg-Sn-Zn alloy could bring out obvious grain refinement and the newly formation of Ce Mg Sn and Ce Mg Zn₂phase,which hinder the grain growh and the annihilation of{10-12}tensile twin during annealing of pre-twining deformation.Due to the appearance of{10-12}tensile twins,pre-twining deformation could modify the texture from strong basal texture to complete ED-spilted texture(the basal plane of grain is nearly perpendicular to the ED),which could be retained even in the subsequent annealing process.The strength and ductility of as-extruded TZE100 alloy are improved simultaneously via the combination of Ce addition(retention of refined grains)and pre-twining deformation(effective texture tailoring).In Mg-Sn-Zn alloy,the addition of Gd,Ce gave rise to the emerging of Gd Mg Sn and Ce Mg Sn phase,which eventually refined grain and weakened texture via PSN effect.The improvement of mechanical properties is more significant in compound Gd,Ce addition than counterpart of their separate addition.The ascension of strength was ascribed to the grain refinement and precipitate strengthening.The lift of ductility was pertain to texture weakening(more activation of basal slip),grain refinement and improved microstructural homogeneity(alleviation of stress concentration)...