Basic Research On The Microstructure And Mechanical Properties Of Mg-Al Alloy Sheets Tuned By Multi-Alloying Strategy

As the lightest structural metallic material,magnesium alloy exhibits great potentials to be used in the fields of automobile,aerospace and 3C products.However,great challenge is still existed in the application of Mg alloys due to the poor mechanical properties.The addition of rare earth elements can effectively increase the absolute strength of magnesium alloys.However,the magnesium alloys containing a large amount of rare earth elements are costly,which will greatly limit their widely applications in industry.Therefore,how to fabricate magnesium alloys with high-strength and highplasticity with low cost has become a bottleneck problem in this field.In this paper,the multi-alloying strategy(including the Al,Zn,Sn,Ca,Mn,Ce elemnts)is used to tune the microstructure and mechanical properties of the as-extruded and subsequently rolled MgAl based alloys.The following main results are obtained:After rolling of the as-extruded Mg-Al-Zn-Mn alloy,the second phase dynamically precipitates and spherizes with increase of the rolling reduction.The grain grows during the high-temperature rolling,and the average grain size of the as-extruded Mg alloy,the 34%and 60%rolled samples are estimated to be 7.2?m,10.7 ?m,10.6 ?m,respectively.On the other hand,the high density compression twinning can effectively separate the Mg matrix and a similar grain refinement effect would be induced.Moreover,the dislocations would significantly multipy during the rolling process.Consequently,the tensile strength of the alloy is gradually increased.The 60%rolled Mg alloy exbibits the optimal mechanical property,with the ultimate tensile strength(UTS)of 363 MPa and the elongation of?7.2%.After rolling reduction of 60%at temperature of 250?,the average grain size of Mg-3Al1Sn(wt.%)sheet is gradually refined from 10.69 ?m in the as-extruded sample to 3.52 ?m,and the UTS is gradually increased from 238 MPa to 256 MPa,and the elongation is increased from 21.6%to 29.2%.The average grain size of Mg-6Al-1Sn sheet is refined from 10.76 ?m in the as-extruded state to 3.98 ?m,and the UTS is gradually increased from 275 MPa to 294 MPa,and the elongation is increased from 23.8%to 26.8%.A small amount of Mg17Al12 phase and Mg2Sn phase precipitate in Mg3Al-1Sn alloy.With the increase of Al content,more Mgi7Al12 phase and Mg2Sn phase with smaller size are precipitated in Mg-6Al-1Sn alloy,which results in an improvement of strength of Mg-6Al-1Sn alloy as compared with Mg-3Al-1Sn alloy.And the LDH value increases as the grain size increases under the same rolling temperature.The highest LDH values of the Mg-3Al-1Sn alloy is 3.24 mm obtained after rolling reduction of 62%under temperature of 350?,and the highest LDH values of the Mg-6Al-1Sn alloy is 3.43 mm obtained after rolling reduction of 78%under temperature of 350?.The grain strucutures of Mg-Al-Ca alloys are largely refined due to dynamic recrystallization after rolling,and the refinement mechanism is closely related to the compositions of the alloy.A large amount of Al2Ca nanophases precipitates in Mg1.8Al-1.8Ca alloy after rolling.These phases strongly limit the growth of recrystallized grains,resulting in the formation of ultra-fine grains.In regard to the Mg-0.3Al-0.6Ca0.3Zn-0.2Ce-0.1Mn alloy,segregation of Ca and Zn solute elements along grain boundary occurs after rolling,which would hinder the migrations of grain boundaries and lead to the formation of ultra-fine grains.For the Mg-1.8Al-1.8Ca alloy sheet,the average grain size is gradually refined from 2.42 ?m to 1.42 ?m and the yield strength is gradually increased from 241 MPa to 298 MPa after 78%rolling at temperature of 300?.Grain refinement,precipitation of a large number of fine Al2Ca phases at grain boundaries,and accumulation of high density dislocations inside the grains are all beneficial to the improvement of strength.For the Mg-1.8Al-1.8Ca alloy,the average grain size of the alloy sheet is gradually refined from 3.7 ?m to 1.1 ?m,and the yield strength is gradually increased from 193 MPa to 290 MPa after 78%rolling at temperature of 250?,while the elongation is decreased from 23.6%to 11.6%.Grain refinement,segregation of Ca and Zn solute elements at the grain boundaries,and accumulation of high density dislocations inside the grains can contribute to the improvement of strength.