Research On Deformation Microstructure And Mechanical Properties Of Mg-Al-Ca-Sm Alloys

As a new emerging metallic material in the 21st century,magnesium alloys have the advantages of high specific strength,good damping performance and low density.They have broad application prospects in automobile,aerospace and other fields.However,its hexagonal close-packed(HCP)crystal structure results in poor room temperature formability,limiting its wide application.For magnesium alloys,alloying is one of the simple and effective methods to achieve grain refinement,precipitation strengthening and texture weakening.Recent studies have found that mechanical properties of Mg-Al-Ca alloys can be optimized by adjusting the Ca/Al ratio to generate different types of Ca containing phases.It has been reported that the co-addition of alkaline earth metals and rare earth elements can effectively improve the room temperature and high temperature mechanical properties of Mg-Al alloys through solid solution strengthening,precipitation strengthening,grain refinement and texture weakening.Therefore,the addition of rare earth elements to Mg-Al-Ca alloys is expected to further improve the formability of magnesium alloys.Therefore,based on the above research,four groups of low-alloyed Mg-1Al-0.3Ca-x Sm(x=0,0.2,0.7,1.0wt%)alloys with different Sm content under the condition of fixed Ca/Al ratio were designed in this paper.The effects of Sm element on the microstructure and mechanical properties of as-cast,extruded and rolled alloys were studied to provide a theoretical basis for the development of high-performance magnesium alloys.The main conclusions are as follows:(1)As the Sm element content increases from 0 wt.%to 1.0 wt.%,grains of the as-cast Mg-1Al-0.3Ca alloy transform from dendrites to equiaxed grains,with the grain size decreases.Granular and fishbone Al₂Ca phases are the main types of second phases in as-cast Mg-1Al-0.3Ca alloy.After the addition of Sm element,the amount of Al₂Ca phase decreases,while the type,size and volume fraction of the Al-Sm phase increase.At the same time,the lamellar Al₂Sm phase and the needle-like Al₁₁Sm₃ phase become dominant.(2)The extruded Mg-1Al-0.3Ca-x Sm(x=0,0.2,0.7,1.0 wt%)alloys mainly contain Al₂Ca and Al₂Sm phases.The degree of texture weakening for the extruded Mg-1Al-0.3Ca-x Sm(x=0,0.2,0.7,1.0 wt%)alloys is positively correlated with the Sm element content.The nano-scale rod-shaped and polygonal Al₂Ca phase distributes within grains and at grain boundaries,while the micro-scale Al₂Ca and Al₂Sm particle phases are mainly distributed along grain boundaries.The ducticity of alloys containing Sm is not improved because part of larger Al₂Sm particles distribute along ED and form the second phase band.The Al₂Sm phases with high thermal stability formed at grain boundaries can effectively hinder the grain boundary sliding and improve the high temperature mechanical properties of the Mg-1Al-0.3Ca-x Sm alloys.(3)After rolling deformation,the original large-sized second phases distributing along the ED in the extruded alloy are refined,leading to the increased strength.By comparing the difference in texture strength of rolled Mg-1Al-0.3Ca and Mg-1Al-0.3Ca-0.7Sm alloys before and after annealing,it is concluded that Sm element has no effect on the completely static recrystallized structure of Mg-1Al-0.3Ca-x Sm alloy.The rolled Mg-1Al-0.3Ca-0.7Sm alloy after annealing at 300?for 5 min has the best mechanical properties(YS:203.9 MPa,UTS:251.5 MPa,EL:15.4%),because some dislocations are retained and some grains are completely recrystallized.