Study On Microstructure,properties And Deformation Behavior Of Mg-Zn-Ca-Zr Alloy Fabricated By Hot Extrusion-Shearing Process

At present,the use of large plastic deformation process to refine the alloy grain and control texture,and then improve the alloy microstructure and mechanical properties has become the focus of magnesium alloy research.In this paper,the ultra-fine grained Mg-3Zn-x Ca-0.6Zr(x=0,0.6,1.2,1.8wt.%)magnesium alloy with high strength and toughness was prepared by a new extrusion-shear(ES)process which combines initial forward extrusion with equal channel angular pressing(ECAP).OM,SEM,EDS,XRD,EBSD and TEM were used to study the effects of Ca addition,homogenization treatment and extrusion shearing process on the microstructure,phase composition and mechanical properties of ultra-fine grained magnesium alloy,as well as the evolution mechanism and deformation behavior of twins and dislocations of ultra-fine grained magnesium alloy under room temperature compression.The effects of grain size,second phase and texture evolution on the strength and toughness of ultra-fine grained magnesium alloy during extrusion and shearing were discussed.The mechanism and strain evolution law of dynamic recrystallization(DRX)of the alloy were revealed.The results show that after adding Ca element to the as-cast Mg-3Zn-0.6Zr alloy,two kinds of Ca₂Mg₆Zn₃ phases with different morphologies with good thermal stability are formed in the as-cast Ca₂Mg₆Zn₃ alloy,and the microstructure of the alloy is mainly composed of?-Mg and Ca₂Mg₆Zn₃?Mg Zn phases.After homogenization treatment,Ca₂Mg₆Zn₃ phase still exists in Mg-Zn-Ca-Zr alloy.After extrusion shearing,the grains in the forming area are finer and more uniform than those in the extrusion area,and the average grain sizes of Mg-3Zn-0.6Ca-0.6Zr and Mg-3Zn-1.2Ca-0.6Zr alloys in the forming area are0.99?m and 0.96?m,respectively.The extrusion shearing process makes the Ca₂Mg₆Zn₃phase broken and refined into nano-sized particles,which are pinned to the dynamic recrystallization grain boundary,causing strong particle stimulate nucleation(PSN),and significantly refine the alloy grains.Moreover,large number of Mg Zn phases in Ca-containing alloys were dynamically precipitated at DRX grain boundaries and within grains,which in turn inhibits the grain growth of DRX.In addition,in the early stage of the ES process,the Ca₂Mg₆Zn₃ phase at the grain boundary promotes the accumulation of dislocations,and part of the grain boundary changes to low-angle grain boundary(LAGBs)and high-angle grain boundary(HAGBs).After the die corner shearing,the accumulated LAGBs gradually transformed into HAGBs due to continuous dynamic recrystallization(CDRX).Through the novel ES process and the addition of Ca element,the basal plane texture was significantly weakened and the DRX grain was refined.With the increase in Ca content,the strength and toughness of ES alloy increased obviously,and the yield asymmetry decreased gradually.When the content of Ca was 1.2 wt%,ES alloy exhibited the best combination of strength and toughness(UTS=342 MPa,?=20.9%)due to nano-scale Ca₂Mg₆Zn₃ phase,grain refinement and texture weakening.The experimental results of ultra-fine grained(UFG)Mg-3Zn-1.2Ca-0.6Zr alloy compression at room temperature show that the compression deformation is mainly divided into three stages,and the work hardening rate decreases at first,then increases and then decreases.In particular,compression along the ED direction activates the coordinated deformation of{10(?)2}tensile twins in UFG magnesium alloy at room temperature,and twin and dislocation slip coordinated deformation make the compressive yield strength of UFG magnesium alloy reach 206 MPa.With the increase of compression strain,the interaction between twin and dislocation slip promotes deformation,and the twin boundary fraction and dislocation density increase continuously.{0001}basal texture strength increases at first and then decreases,resulting in basal poles separation.When the twin nucleation is saturated,the twins have almost no contribution to grain refinement and plastic deformation,and the dislocation mechanism is dominant in the later stage of alloy deformation.As dislocation density gradually increases,the formation of dislocation cells reduces the work hardening rate.