Basic Research On Effect Ce Addition On Microstructure And Mechanical Properties Of Mg-Al And Mg-Ca Based Alloys

Magnesium alloy,with high specific strength,low density,and excellent shock absorption performance,has a wide application prospect in the aerospace field.However,the low strength and poor plasticity of magnesium alloy are still the main reasons hindering its industrial application.In this paper,Mg-Al and Mg-Ca alloys with excellent properties are developed through the micro-alloying design of Ce and optimization of extrusion process,including high-strength Mg-Al-Ce,Mg-Ca-Ce,and Mg-Ca-Ce-Mn Alloys.Based on microstructure analysis and mechanical property test,the following main results were obtained:In order to improve the mechanical properties of Mg-Al alloys,ultrafine-grained Mg-3.0Al(wt.%)binary alloys were developed by extrusion at low temperature(?220?),yield strength(YS)?285 MPa,Tensile strength(UTS)-328 MPa,elongation(EL)?11%;fine dynamic recrystallized grains and high-density dislocations are the main reasons for the excellent mechanical properties of the extruded Mg-3Al alloy.However,the Mg-3Al binary alloy exhibits low thermal stability.After annealing at 250? for 2 hours,the YS of the alloy is significantly reduced to?158 MPa,the UTS is reduced to?233 MPa,and the EL is increased to?33%.After annealing treatment,the alloy recovered significantly,and the density of dislocations was greatly reduced with little change in grain size,resulting in a reduction in its strength.In contrast,the Mg-0.1Al-0.2Ce(wt.%)ternary alloy obtained by adding a small amount of Ce element after extrusion deformation at high temperature(?300?)still shows super high strength(YS?370 MPa,UTS?372 MPa,EL?5.2%),indicating the high temperature thermal stability of the microstructure of this alloy is better.The experimental results of the three-dimensional atom probe show that during the extrusion process of Mg-0.1Al-0.2Ce alloy,dynamic segregation of elements along dislocation lines occurs,they are mainly clusters of Al-Ce elements.These clusters can improve the recovery rate of dislocations at high temperatures,thereby ensuring that the density of dislocations continues to multiply during the early stages of extrusion,which is conducive to the high-density nucleation of dynamic recrystallized grains.In addition,Al-Ce segregation at the grain boundaries greatly increases the resistance of grain boundary migration and facilitates the formation of fine recrystallized grains in the extruded alloy.Ultrafine grains,high-density dislocations and associated element segregation are the main reasons for the high strength of this Mg alloy.First-principles simulation results also confirm that the co-segmentation of Al-Ce elements along the dislocation has the lowest segregation energy.Finally,Ce element can also greatly improve the mechanical properties of Mg-Ca based alloys.For example,the average grain size of Mg-1.0Ca(wt.%)binary alloy after extrusion at 300? is?1.2 ?m,and the YS of the alloy is?310 MPa.In contrast,the average grain size of the Mg-0.2Ce-0.3Ca ternary alloy obtained by micro-alloying of Ce after 300? extrusion can be refined to?100 nm,which is significantly finer than that of Mg-Ca binary alloy,the YS and UTS of the alloy have been increased to?372 MPa and?377 MPa,respectively.The average grain size of Mg-0.3Ca-0.2Ce-0.5Mn(wt.%)quaternary alloy developed based on the Ce and Mn multiple micro-alloying strategy can be further refined to 60?100 nm after extrusion at 300?.Therefore the YS of the alloy is further increased to?403 MPa and the UTS?411 MPa.The co-segmentation of Ca-Ce element and the dispersion distribution of nano-Mn particles may be the main reasons for the formation of ultrafine grains and the enhancement of strength in magnesium alloys.