Deformation Behaviors And Strengthening Mechanisms Of High-strength Mg-Sn-Zn-Al Alloys

As the lightest metal structure materials,Mg alloys have broadapplication prospects in the fields of automobile,aerospace,electronic communications and national defense,known as“the most development potential green engineering material in the 21st century.”Mg-Sn-based alloys have exhibited great potential in developing wrought Mg alloys with high strength and high ductility owing to their good creep resistance and excellent precipitation strengthening.After the addition of suitable Zn and Al elements in Mg-Sn based alloys,not only can improve the precipitation kinetics of Mg₂Sn,but also refine the Mg₂Sn precipitates and make them uniformly distributed to produce excellent Mg-Sn-Zn-Al alloy.In this paper,Mg-8Sn-2Zn-2wt.%Al?TZA822?alloys wereprepared by the conventional permanent mold casting method.The influence of hot extrusion and initial microstructure?grain size and second phases volume fraction?on the microstructure,texture and mechanical of TZA822 alloys was systematically investigated.Furthermore,the hot deformation behavior of extruded TZA822 alloys was investigated by means of compression test at various deformation conditions.Meanwhile,based on the dynamic material modeling and continuum criterion,the processing maps?at strain 0.1⁰.7?of the studied alloys constructed to optimize hot working parameters.In addition,combined the evolution of microstructure under different deformation conditions,the DRX mechanisms of the TZA822 alloys were determined.The results showed that as-cast TZA822 alloy exhibited a typicaldendritic microstructure,and it was mainly composed of?-Mg matrix,Mg₂Sn particles with dispersed and massive distribution.The average secondary arm spacing was 30.1?m.After hot extrusion,the alloy exhibited a typical bimodal structure,and it was mainly composed of dynamic recrystallization and second phases broken form extrusion strips,the average grain size was 4.7?m.The extruded alloy exhibited a typical basal fiber texture.Compared with the as-cast alloy,the TYS of the extruded TZA822 alloy increased from 116 MPa to 182 MPa,the UTS increased from 158 to 226 MPa,and the EL reached about 17.9%.The superior strength and ductility of the extruded TZA822 alloy was the result of the joint action of grain boundary strengthening,precipitating strengthening and texture strengthening.The strengthening effect of extruded TZA822 alloy in terms ofmicro and nano-Size Mg₂Sn precipitates was investigated.The results revealed that TZA822-HT showed finer grain size,more homogenous microstructure and weaker basal texture than TZA822-AC one.The morphology of nano-size Mg₂Sn precipitates exhibited a significant change in basal plane from rod-like to spherical due to the decrement in the fraction of micro-size particles before extrusion.Meanwhile,the spherical Mg₂Sn precipitates provided a much stronger strengthening effect than did the rod-like ones,which was attributed to uniform dispersion and refinement of spherical precipitates to effectively block basal dislocation slip.As a consequence,the extruded TZA822-HT alloy showed a higher TYS of 245MPa,UTS of 320 MPa and EL of 26.5%,a lower degree of yield asymmetry?R=0.96?as well as a higher work-hardening rate.The hot deformation behavior of extruded TZA822 alloys was investigated.The results revealed that the flow stress was sensitive to the deformation conditions,which increased with decreasing temperature and increasing strain rate.The constitution equation of the extruded TZA822alloy was developed,with average Q and n being 189.5 kJ/mol and 6.45,suggesting that the dominant deformation mechanism was likely to be cross-slip of dislocations.Moreover,the optimize hot working parameters of the extruded TZA822 alloy were determined to be 350 ^oC/0.01 s^-11 and350 ^oC/10 s^-1,respectively.The flow instability region was confirmed as200-250 ^oC/0.1-10 s^-1,and the instability mechanism was twinning and flow localization bands.In addition,detailed EBSD and TEM characterization suggested that continuous DRX?CDRX?showing progressive subgrain rotation was the operating nucleation mechanism at350 ^oC/0.01 s^-1,while discontinuous DRX?DDRX?characterized by grain boundary bulging and CDRX could occur simultaneously at 350 ^oC/10 s^-1.