Effect Of Alloying And Heat Treatment On Microstructure And Mechanical Properties Of Mg-Sn Alloy

Magnesium alloys are widely used in engineering structural materials due to their high specific strength,high damping properties,low density,etc.The most representative one is rare-earth magnesium alloy system.Rare-earth magnesium alloys has high strength,excellent high-temperature service performance and good processing/forming ability,but the high cost and scarcity of rare-earth elements limit the further promotion on its application fields.Therefore,the researches of rare-earth free magnesium alloy are expected to break through this bottleneck.Alloying,heat treatment and thermoplastic deformation processing can effectively improve the mechanical properties of magnesium alloys.In this paper,the Mg-Sn-Al/Zn-Ca/Cu alloys were selected,and the alloy samples with different compositions were prepared by ordinary casting method and rapid hot extrusion processing.The effects of alloying elements and deformation processing on the microstructure were investigated.The law of microstructure evolution is used to study the heat treatment process of as-cast alloys.The microstructure of extruded alloys is studied by TEM and HRTEM and the internal microstructure and formation mechanism of the second phase are explored.The main conclusions are as follows:The main constituent phases of the Mg-4Sn-xAl-0.5Ca(x=1,2,3)(wt.%)alloy are a-Mg,Mg2Sn and CaMgSn phases.As the amount of A1 addition increases,the grain size gradually decreases and the number of the second phase increases,and the tensile strength,elongation and hardness of the as-cast alloy increase as well.The Mg-4Sn-xAl-0.5Ca alloy is subjected to solution treatment and aging treatment at different temperature,and second phases solid-dissolved in the matrix will be re-precipitated,which have refined size and uniform dispersion.According to the age hardening curve,the higher the amount of A1 addition,the higher the peak value of the aging hardness.As the aging temperature increases,the peak of the hardness appears earlier but the hardness decreases slightly.The main constituent phases of the Mg-4Sn-xZn-0.5Ca(x=1,2,3)(wt.%)alloy are substantially the same as that of the Mg-4Sn-xAl-0.5Ca alloy.As the amount of Zn addiyion increases,the grain size decreases gradually and the number of second phases increases.The tensile strength and hardness of the alloy increase with the increase of Zn addition.The Mg-4Sn-xZn-0.5Ca alloy was subjected to solution treatment and aging treatment at different temperature.It is found that the effect of size refinement and dispersed distribution about the re-precipitation of the second phase in Mg-4Sn-xZn-0.5Ca is better than Mg-4Sn-xAl-0.5Ca alloy.The aging curve first increases and then decreases.The higher the Zn addition,the higher the aging hardness peak.The addition of Cu element can refine the grain size of the as-cast alloy and promote the precipitation of Mg2Sn phases during solidification.Appropriate addition of Cu can increase the tensile strength of the as-cast alloy,but the excessive addition of Cu leads to element segregation,which causes the coarse second phase and the layered eutectic structure to be dispersed at the grain boundary,resulting in a decrease in the mechanical properties of the alloy.After hot extrusion processing,the microstructure of the alloy is significantly improved.The microstructure of the as-extruded Mg-6Sn-lAl-xCu(x=0.5,1,2)(wt.%)alloy consists of dense and uniform recrystallized grains.The recrystallized grains are refined as the amount of Cu added increases.The mechanical properties of the as-extruded alloy are significantly improved compared to the as-cast alloy.The Mg-4Sn-1Zn/Al-xCa(x=0.1,0.3,0.5)(wt.%)alloy consists of uniform ?-Mg equiaxed grains and CaMgSn phases.The formation of CaMgSn phase is affected by the segregation of Sn or Ca elements to some extent.With the increasing amount of Ca addition,the volume fraction of CaMgSn phases is significantly increased,and the phase size is remarkably refined.In the optimal comprehensive performance of Mg-4Sn-1Zn-xCa extruded alloy,the tensile strength can reach 261.5±2.9MPa,the yield strength is 139.1±3.1MPa,and the elongation after fracture is 13.2%.The optimum tensile properties of the extruded Mg-4Sn-lAl-xCa alloy can reach 273.0±2.1MPa,the yield strength is 153.3±2.6MPa,and the elongation after fracture is 11.2%.The compressive fracture strength of the two alloys is relatively close.In extruded alloys,the CaMgSn phase plays an important role in performance enhancement.The mechanism of the second phase strengthening is mainly that the second phase particles hinder the dislocation motion during the plastic deformation process,so that it cannot be further advanced and expanded.The higher the volume fraction of the second phase,the better the effect of the dispersion distribution and the greater the strengthening effect in the alloy.TEM method demonstrates that CaMgSn ternary phase mainly consists of Mg2Sn and Mg2Ca phases internally,while Mg2Sn nanoparticles are wrapped in Mg2Ca.The orientation relationship(OR)among Mg,Mg2Sn and Mg2Ca inside CaMgSn phase is[1 2 10]Mg ||[001]Mg2Sn ||[4 5 1 3]Mg2Ca;[0001]Mg2Sn ||[16 9 11]Mg2Ca.At the same time,the addition of Ca can refine the size of second phases to some extent.Isolated Mg2Sn nanophases are found in the high density dislocation regions.The OR between separate nano-Mg2Sn and Mg matrix is[11 0 2]Mg ||[001]Mg2Sn.The formation mechanism of CaMgSn phase is inseparable from its unique internal microstructure.During phase transformation,the Ca atoms dissolved in high-density dislocation regions formed re-precipitated Mg2Ca phases,while isolated Mg2Sn nanophases will act as the nucleation sites of CaMgSn phases.The dynamic precipitation of CaMgSn phase can be regarded as the formation of Mg2Ca phase with Mg2Sn nanoparticles as cores,which is induced by the activation and aggregation of dislocations.