Research On Microstructure,Properties And Superplasticity Of Mg-2.76Li-3Al-2.6Zn-0.39Y Alloy

In recent years,in order to reduce energy consumption,lightweight alloys have developed rapidly.Magnesium-lithium alloys are especially used in lightweight fields such as automobiles,aircraft,high-speed rail and 3C products.In addition to light weight,these products also require high strength indicators.It is significant to obtain high strength,excellent plasticity and ultra-light Mg-Li alloys by alloying,large plastic deformation.In this paper,Mg-2.76Li-3Al-2.6Zn-0.39Y alloy was used as a research target.Fine grained sheets were obtained by casting,homogenization,multidirectional forging and hot rolling.The microstructure evolution was analyzed by OM,TEM,XRD;room temperature mechanical properties and high temperature superplastic behavior of the alloy were studied.The main research contents and results are as follows:(1)The grain refinement of Mg-2.76Li-3Al-2.6Zn-0.39Y alloy was obvious by cooling multidirectional forging process.meanwhile,the MDF showed obvious bimodal structure in each pass,and the minimum grain size obtained at six passes was 7.9 ?m.Through room temperature mechanical properties tests,the UTS,YS and elongation of the alloy were the maximum at six passes of the MDF,which were 256±4 MPa,189±5 MPa and 38.5%respectively,and the maximum Vickers hardness value was 73 ± 3 HV,which was also obtained at six passes of the MDF.(2)The minimum average grain size of the hot-rolled Mg-2.76Li-3Al-2.6Zn-0.39Y alloy was 6.8 ?m.At this time,the UTS,YS and elongation of the alloy were 275 ± 8 MPa,207± 7 MPa and 29%.After annealing,the UTS,YS and elongation of the alloy were 223± 6 MPa,126 ± 5 MPa and 37%,respectively.Taking hot-rolled tensile samples as research objects,the contribution value of different strengthening methods to yield strength was estimated.(3)After high temperature tensile tests,Mg-2.76Li-3Al-2.6Zn-0.39Y alloy obtained the maximum elongation which was 223.0%at 633 K and 1.67×10-4 s-1 deformation conditions.The maximum stress sensitivity index m was 0.42.The grip section,gauge section and void behavior of high temperature specimens were studied.It was found that the soft deformation mechanism of the alloy was DRX,and the cavity growth mechanism was mainly controlled by plastic deformation.(4)In this paper,the modified Johnson-Cook,modified Zerilli-Armstrong and Arrhenius constitutive model were created.Among them,the calculated calue of the Arrhenius model was in good agreement with the measured value.The modified Johnson-Cook model and the modified Zerilli-Armstrong constitutive model had the best prediction effect at the reference strain rate,but the modified Johnson-Cook model had the better prediction effect.(5)The superplastic deformation mechanism of Mg-2.76Li-3Al-2.6Zn-0.39Y alloy was studied.The calculated stress index n was 3,the range of deformation activation energy Q was 130,86?158.13 kJ/mol,the lattice size index p was 0,and the constitutive equation was constructed.