Effects Of Sn Addition On The Microstructure And Mechanical Properties Of AZ80 Wrought Magnesium Alloy

Effects of Sn addition on the microstructures and mechanical properties of AZ80 alloy were systematically investigated by means of optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM)equipped with energy dispersive spectronmetry(EDS),transmission electron microscopy(TEM)and tensile properties testing.This paper mainly focus on the microstructural evolution,morphology characteristics of second phases,room temperature tensile properties of AZ80-xSn(x=0,1.0,2.0,4.0,wt.%)alloys.The research results indicate that the microstructure of as-cast AZ80 alloy consists of a-Mg and Mg17Al12 phases,with Sn added in AZ80 alloy,Mg2Sn phase precipitates in the Mg matrix.The volume of Mg17Al12 and Mg2Sn phases increase with the increasing Sn addition in as-cast and as-extruded AZ80-xSn alloys.During the extrusion process,Sn element can promote the dynamic recrystallization process,and the average grain size of extruded alloys are 10.30,13.03,14.02 and 10.24 ?m,which shows that the solute Sn element can benefit the grain growth when Sn addition is lower than 2 wt.%.With more Sn element addition,more second phases precipitate in the alloy,which suppress the growth of grain.After T6 treatment,when the content of Sn below 2 wt.%,the volume fraction of DP phase increase with more Sn addition,however,when Sn addition is greater than 2 wt.%,the volume fraction of DP phase decrease with the increment of Sn addition.After aging treatment,the DP phase in AZ80-1Sn alloy and the CP phase in AZ80-2Sn alloy are optimized.The microstructure of DP-Mgi7Ali2 phase in AZ80-1Sn is typical lamellar structure which is the straight and long lamellae,and the CP-Mg17Al12 phase in AZ80-2Sn has a much higher density and more homogeneous distribution.The two kinds of optimized microstructure both can improve the strength of alloys.After Sn element added,the strength of extruded and aged AZ80 alloys are remarkably improved.When the content of Sn element is 2 wt.%,the alloy possesses the best mechanical properties.The effects of extrusion temperature and T5 aging treatment on the microstructure and mechanical properties of AZ80-2Sn alloy have been investigated.The extrusion temperature has a great deal of effects on the microstructure of AZ80-2Sn alloy.Fine equiaxed grains are obtained in alloys extruded at 380? and 400?.However,in the alloy extruded at 350?,the grain size is nonuniform with a small number of large-size grains surrounded by regions of fine equiaxed grains.The average grain size of the three alloys are 8.8,9.5 and 12.9 ?m respectively.The average grain size increases as the extrusion temperature increasing.Compared with the second phases in alloys extruded at higher working temperature,which are precipitated in the form of particles and well dispersed along the grain boundary,some bulk bar-shaped precipitations which distribute along the extrusion direction are observed in the alloy extruded at 350?.The texture in alloys extruded at 350,380 and 400? is a typical extrusion fiber texture with(0002)planes parallel to the extrasion direction,and when extrusion temperature is 380?,AZ80-2Sn alloy has the maximum texture intensity.In aging treatment,Mg2Sn particles are uniformly distribute on the ?-Mg matrix,and Mg17Al12 phases precipitates as continuous and discontinuous phase.It is noted that the volume fraction of discontinuous Mgi7Ali2 phase of alloy extruded at 400? significantly increases.The strength of extruded AZ80-2Sn alloys can be significantly improved by extrusion and T5 aging treatment.The AZ80-2Sn alloy,which is extruded at 380? and aged at 175? for 3h,has the best comprehensive mechanical properties:.The ultimate tensile strength(UTS),yield strength(YS)and elongation are 391.6MPa?279.5MPa?9.8%respectively.For AZ80-2Sn alloy,the optimized solute treatment is 420?/1h.As-solute alloy was give aging treatment at different temperature.As the results showed,different aging temperature has different influence in the microstructure,mechanical properties and fracture mechanism of AZ80-2Sn alloy.After aging treatment,DP-Mg17Al12 phase precipitates in three kinds of morphology:(?)finely lamellar and oval-shaped ellipsoid;(?)coarsely lamellar and oval-shaped ellipsoid;(?)only oval-shaped ellipsoid.In the alloy aged under 150?,there are only ? and ? groups are observed,while in higher aging temperature,all three types of second phases appear.And the lamellar spacing of DP phase enlarges with increasing the aging temperature.Meanwhile,the second phases including both Mg17Al12 rod phases and Mg2Sn particles formed at 150? are finer and denser than that formed at 473?.The mechanical properties are affected by the properties of DP-zone and CP-zone.When aging at 423K,the precipitates in CP-zone are fine and density.In this situation,the mechanical property of CP-zone is stronger than that of DP-zone.Therefore fracture mainly displays distorted DP-Mgi7Ali2 phases.And treating at 473K,coarsening CP-Mg17Al12 phases weaken the mechanical property of CP-zone and fracture happens in this area.From the results of tensile properties test,the optimized aging treatment is 150?/35 h,and the ultimate tensile strength(UTS)and yield strength(YS)are 293.3 MPa and 421.8 MPa respectively...