| Magnesium(Mg)alloy,as the lightest metal structure material,has a good application prospect in the field of automobile and aerospace.Recently,low-cost Mg-Ca-(Al)-(Mn)alloys have attracted extensive attention because of their thermally stable Laves phase,Guinier-Preston(G.P.)metastable phase and excellent mechanical properties.However,as compared with aluminum alloys,Mg alloys still show absolute low-strength and poor plasticity at ambient temperature,which restricts their commercial application.In order to further improve the mechanical properties of low-alloyed Mg-Ca-(Al)-(Mn)alloys,this paper investigated the microstructure and mechanical properties of high-strength Mg-0.3Ca(X03,wt.%),Mg-0.7Al-0.3Ca(AX0703,wt.%)and Mg-0.7Al-0.3Ca-0.4Mn(AXM070304,wt.%)extrusion alloys.The effects of low alloying of Ca,Al and Mn on the microstructure and mechanical properties of Mg alloys was revealed.The formation mechanism of ultrafine grained magnesium alloys was studied.The strengthening and toughening mechanisms of X03,AX0703 and AXM070304 alloys were studied by scanning electron microscope(SEM),transmission electron microscope(TEM),electron backscatter diffraction(EBSD)and three-dimensional atomic probe(3DAP).The strain hardening behavior and plastic instability of X03 and AXM070304 alloys during tensile deformation at room temperature were studied by TEM and digital image correlation(DIC).With the increasing of extrusion temperature,the tensile yield strength(TYS)of X03 alloy was decreased and elongation to failure(EL)was increased significantly.The as-extruded X03-170℃alloy with extrusion temperature of 170℃has an ultra-high TYS of 411 MPa;the TYS and EL of as-extruded X03-200℃alloy with extrusion temperature of 200℃are 361 MPa and 10.6%,respectively,exhibiting comprehensive tensile mechanical properties with high strength and excellent plasticity.The high TYS of the as-extruded X03-170℃and X03-200℃alloys is mainly attributed to the grain boundary strengthening of the ultrafine grains segregated at the grain boundaries of solute Ca atoms.With the increasing of heat treatment time at 300℃for the as-extruded X03-200℃alloy,the growth of recrystallized grain size and the weakening of solute grain boundary segregation,which weakens the grain boundary strengthening effect of solute Ca atomic grain boundary segregation,resulting in a decrease in the TYS of the alloy.With the decreasing of extrusion temperature or the increasing of heat treatment time,the average particles diameter and the average volume fraction of the Mg2Ca phase are increased,while the inter-particle spacing is decreased,which weakened the precipitation strengthening effect of Mg2Ca phase and further reduced the alloy’s TYS.The X03 alloy with an average recrystallized grain size smaller than 1.6μm has obvious plastic instability behavior;while the alloy with an average recrystallized grain size larger than 2.0μm has obvious strain hardening behavior.The plastic instability behavior of the ultra-fine grain of X03 alloy with complete dynamic recrystallization is due to the segregation of solute Ca atoms at the ultrafine grain recrystallized grain boundary,which stabilizes the grain boundary dislocation emission source and increases the critical stress for grain boundary dislocation nucleation.When the tensile stress reaches the peak value,the density of movable dislocation increases sharply,which leads to an increase in the plastic deformation strain rate of the alloy,and then leading to the tensile stress decrease.The TYS of as-extruded AX0703-200~oC and Mg-1.0Al-150~oC alloys are 376MPa and 272 MPa,respectively.The dynamically recrystallized grain size and the second phase number density of the two as-extruded alloys are similar,and the texture strength of the former is weaker than that of the latter,but the TYS of the as-extruded AX0703-200~oC alloy is 104 MPa higher than that of the as-extruded Mg-1.0Al-150~oC alloy.The higher TYS of the as-extruded AX0703-200~oC alloy is mainly due to the stronger grain boundary strengthening of the co-segregated ultrafine grains of the solute Al and Ca atoms.Under the same annealing heat treatment conditions,the TYS of the as-annealed AX0703 alloy is obviously higher than that of as-annealed Mg-1.0Al alloy,which is mainly due to the stronger grain boundary strengthening effect of the co-segregation of solute Al and Ca atoms at the grain boundaries.The results show that the as-extruded AX0703-200~oC alloy has slower rate of recrystallized grain growth under the same heat treatment conditions as Mg-1.0Al-150~oC alloy,which is mainly due to the co-segregation of solute Al and Ca atoms at the grain boundaries.The as-annealed AX0703 alloy with recrystallized grain size greater than 2.0μm has excellent EL(EL is generally greater than 25%),and its strong plasticity mechanism mainly includes:the refinement of grain size and the increase of strain hardening rate;the reduction of coarse eutectic particles;texture weakening;the addition of alloying Ca element and the co-segregation of solute Al and Ca atoms at the grain boundaries.The TYS of the as-extruded AXM0703xx(x=0,0.2,0.4 and 0.7,wt.%)alloys increases while the EL decreases with increasing Mn content.When the Mn content is 0.4 wt.%,the as-extruded AXM070304 alloy has the best strength and plasticity.The TYS,UTS and EL of the as-extruded AXM070304-200~oC alloy are 412 MPa,418 MPa and 14.8%,respectively,exhibiting comprehensive mechanical properties with ultra-high strength and excellent plasticity.The main strengthening mechanism is grain boundary strengthening with co-segregation of solute Al and Ca atoms at the grain boundaries and precipitation strengthening with high number density planar Al2Ca phase and monolayer G.P.zone,high number density Al2Ca particle phase andβ-Mn particle phase.The formation mechanism of ultrafine grains in the as-extruded AXM070304-200~oC alloy is to promote recrystallized grains nucleation and inhibit recrystallized grains growth.The reason for promoting nucleation is that with increasing of deformation intensity,the strain energy increases,and then forming a large number of LAGBs.High volume fraction LAGBs and high density dislocations promote dynamic recrystallized grains nucleation.The growth of recrystallized grains was inhibited mainly by the co-segregation of solute Al and Ca atoms at the grain boundaries and the pinning of nanosized Al2Ca andβ-Mn particle phases at the grain boundaries.The as-extruded AXM070304-200~oC alloy still exist weak plastic instability behavior.The mechanism of the plastic instability in the dynamic recrystallized(DRXed)region is due to the fact that the segregation of solute Al and Ca atoms at the grain boundaries requires a high energy barrier for dislocation emission.When the tensile stress reaches the peak value,the movable dislocation density increases suddenly,and then the strain rate of plastic deformation increases,leading to the tensile stress decrease.However,the dislocation density in the un DRXed region does not react and recover during the tensile plastic deformation,the dislocation density is basically constant,and the dislocation density still maintains a high density,which can alleviate the degree of plastic instability. |
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