Study On The Deformation Behavior,Microstructure Evolution And Mechanical Properties Of Mg-3Sn-2Al-1Zn-(5Li/0.6Nd)alloy

As the lightest structural metallic material,magnesium alloy has wide application potential in diverse fields such as telecommunication,automobiles,medical machinery,aviation and aerospace,and rail transit,for its high specific strength and specific stiffness,good damping capacity,and recyclability as an ideal light-weight and recyclable material.Due to the strong metallic properties of industrial pure magnesium,it is easy to be corroded and oxidized at room temperature,and Mg has hexagonal close-packed（HCP）structure,so it has poor plasticity.The critical resolved shear stress（CRSS）of pure magnesium single crystal is only4.8～4.9MPa;obviously,it cannot be directly used as a structural material,and its mechanical properties need to be improved by strengthening means such as alloying and heat treatment.In practical applications,alloying is the most basic and effective method for strengthening pure magnesium.Other strengthening methods are basically based on alloying.Therefore,designing reasonable alloy components is an important step for strengthening magnesium alloys.The study found that Al element is one of the most effective alloying elements at present and has an important effect on the properties of magnesium alloys.And a small amount of Zn element is an effective grain refiner forα-Mg,but in order to prevent the phenomenon of loose pores,the content of Zn in the magnesium alloy should not be too high and should not exceed 1%.In addition,in the AT-based magnesium alloy,there is generally a finely dispersed binary Mg₂Sn phase precipitation.Since Mg₂Sn is a phase with an A₂B structure,its melting temperature is as high as 770°C,which can play a better dispersion strengthening effect in magnesium alloys.Meanwhile,the rare earth elements have a good interaction with the magnesium matrix,which can promote the weakening of texture in magnesium alloys;and the addition of Li element can make the magnesium alloy have extremely low density and room-temperature plasticity,which is a very lightweight structural material with great potential.By combining the existing research and the phase diagrams of Mg and alloy elements,three kinds of magnesium alloys are designed in this paper,which are:Mg-3Sn-2Al-1Zn alloy,Mg-3Sn-2Al-1Zn-5Li alloy and Mg-3Sn-2Al-1Zn-0.6Nd alloy;the mechanical properties and thermal deformation properties of these three alloys were systematically studied.Perform solution and aging treatments on the three alloys,and study the effect of heat treatment on the mechanical properties of different alloys at room temperature;through comparative studies,the effects of the addition of two elements Li/Nd on the mechanical properties of TAZ321 alloy can also be determined.The results show that the addition of Li element can improve the dendrite morphology of the as-cast TAZ321 alloy,and the grains of the as-cast TAZ321-5Li alloy are equiaxed;but the Nd element can promote the growth of dendrites,and the as-cast TAZ321-0.6Nd alloy has a more dense dendrite morphology.It was also found that the solution treatment can improve the dendrite segregation of the three alloys,at the same time,the precipitation phase decreases in them;however,a large number of precipitated phases appear after the aging treatment.In summary,compared to the other two alloys,Li-containing alloy has more precipitated phases in different states.And in the three alloys in different states,EDS results show that the precipitated phase is mainly Mg₂Sn phase.The results of the mechanical property test show that the as-cast TAZ321-5Li alloy has the best comprehensive mechanical properties.Its yield strength,tensile strength,and elongation are 111.7 MPa,211.5 MPa,and 17.2%,respectively.After solid-solution,the yield strength（99.0 MPa）and tensile strength（203.2 MPa）of the TAZ321-5Li alloy decreased,but the elongation increased slightly（18.1%）.For TAZ321 and TAZ321-0.6Nd alloys,the solution treatment has a certain strengthening effect,but the elongation has decreased.For the TAZ321alloy,the aging treatment has a strengthening effect;but for the TAZ321-5Li and TAZ321-0.6Nd alloys,the elongation of them are significantly reduced due to the occurrence of a large number of precipitated phases,which may cause stress concentration.Based on thermal compression experiments,constitutive equations and heat processing maps of TAZ321 alloy,TAZ321-5Li alloy and TAZ321-0.6Nd alloy can be constructed through the Arrhenius model,then the optimal hot working process parameters for the three alloys are determined.310-350℃/10^-1-1 s^-1 can be considered as the optimal hot working process parameters for TAZ321 alloy,the degree of dynamic recrystallization of the alloy is relatively complete under this condition and its dynamic softening mechanism is continuous dynamic recrystallization.Moreover,the instability domain is mainly located in the low temperature and high strain rate region(200-300°C/10^-2-1 s^-1),and the main causes of instability are deformation twinning and microcracks.For TAZ321-5Li alloy,the optimal hot working process parameters are 320-350℃/10^-3-0.02 s^-1,the dominant dynamic softening mechanism is dynamic recovery,but there is continuous dynamic recrystallization behavior,too.The instability domain is mainly located in the high strain rate region;the reasons for the instability are mainly flowing deformation and microcracks.Furthermore,330-350℃/0.6-1s^-1 can also be considered as the optimal hot working process parameters for TAZ321-0.6Nd alloy,the dominant dynamic softening mechanism is still continuous dynamic recrystallization,and the instability domain is also mainly located in the low temperature and high strain rate region(200-300℃/10^-1-1 s^-1);the main causes of instability are deformation twinning,etc.;however,its dynamic recrystallization degree is better than that of TAZ321alloy at low temperature.