Investigation On The Thermal Deformation And Precipitation In Mg-Al-Sn High Strength Wrought Alloy

Recently,magnesium alloys receive great attention as one of the important lightweight alloys.However,the widespread use of Mg alloys has been limited by its insufficient strength.Wrought Mg alloys offer better mechanical properties in comparison to cast Mg alloys.Effect of Sn content on the microstructures and mechanical properties of AZ80 alloy were systematically investigated by X-ray diffraction?XRD?,optical microscopy?OM?,scanning electron micrpscopy?SEM?,transmission electron microscopy?TEM?,electron back scanning electron microscopy?EBSD?,calphad,thermal simulation and tensile properties testing.With addition of Sn in AZ80 alloy,Mg₂ Sn phases formed.The phases in AZ80-xSn alloy were ?-Mg,Mg₁₇Al₁₂ phase,Mg₂ Sn phase and Al-Mn phase.The solidification temperature and the formation temperature of Mg₁₇Al₁₂ phase decreased with addition of Sn.Mg₁₇Al₁₂ phases and Mg₂ Sn phases distributed at the grain boundaries as eutectic phases.Hot compression deformation was performed to analyze the effect of Sn on dynamic recrystallization behavior and texture evolution.The results indicated that Sn could promote dissolution of Mg₁₇Al₁₂ phases during hot compression.Addition of Sn promoted the offset of dynamic recrystallization during hot compression,results in a lower ultimate compression stress.Mg₁₇Al₁₂ phases effectively induced particle-stimulated nucleation during hot compression and decreases basal texture.Mg₁₇Al₁₂ phases also pinned up grain boundaries and decrease sizes of recrystallized grains.Therefore,stronger texture and larger recrystallized grains were found in AZ80 alloy containing Sn.After extruding at 350 oC,a large number of Mg₁₇Al₁₂ phases formed in AZT800 alloy.As-extruded AZT800 alloy had better mechanical properties and lower asymmetry.With the addition of Sn?>1 wt.%?,sizes of as-extruded grain increased,compression strength decreased and asymmetry increased.To analyze effect of Sn on microstructure and mechanical properties in the as-aged alloy,extruded temperature was increased to 400 oC to ensure large solid solution atoms were in the matrix after extrusion.With the addition of Sn,dislocation density in as-extruded samples decreased.Low density of dislocation was beneficial to second phases precipitate uniformly during T5 treatment.Precipitation sequences changed with the addition of Sn.During aging treatment,Mg₁₇Al₁₂ phases precipitated first when Sn content < 1 wt.%.Mg₂ Sn phases formed preferentially when Sn content > 1 wt.%.DPMg₁₇Al₁₂ phases were short and coarse which were hindered by Mg₂ Sn phases.Density of CPMg₁₇Al₁₂ phases increased in Sn content alloys.AZT801 and AZT802 alloy have better mechanical properties.Mg₁₇Al₁₂ phase and Mg₂ Sn phase promoted growth of each other.Mg₁₇Al₁₂ phase,Mg₂ Sn phase and Mg matrix have the habit plane.[011]Mg₂Sn//[???11]_Mg17Al12,?01????_Mg2Sn//?01????_Mg17Al12;[2??????0]_Mg//[011]_Mg2Sn,?01???????_Mg//?01????_Mg2Sn;[2??????0]_Mg//[???11]_Mg17Al12,?0002?_Mg//?101?_Mg17Al12?Temperature of extrusion affected number of second phases and grain size.Texture of as-extruded AZT802 alloy was insensitivity to temperature of extrusion.After extrusion at 380 oC,grain sizes were small and homogeneous.AZT802 alloy extruded at 380 oC have good mechanical properties as extruded and aged state.Aging temperature affected density of second phases and morphology.DP phases were insensitivity to aging temperature.Density of CP phases decreased and sizes of CP phases increased with increase of aging temperature.Mechanical properties of as-aged alloys increased with decrease of aging temperature but were limited by DP phases.Zn in this system of alloy promoted formation of second phases during extrusion and aging treatment.Addition of Zn decreased solidus temperature.But when Zn content > 2 wt.%,second phases became coarse.With addition of Zn,density of CP phases and the area ratio of DP phases increased.AZT812 alloy had appropriate solidus temperature and outstanding mechanical properties.The yield strength and ultimate tensile strength of as-aged AZT812 alloy were 296 MPa and 408 MPa.The elongation of this alloy was 7%.