Study On The Strengthening-toughening Mechanisms And Corrosion Behaviors Of Low-alloyed Mg-Zn-Ca Series Alloys

Magnesium(Mg)alloys have low density and high specific strength,making them the preferred lightweight materials in fields of transportation,electronics,aerospace and so on.In recent years,dilute Mg alloys have become a research hotspot due to their low resource dependence and ease of processing.However,dilute Mg alloys lack effective strengthening phases,and the grains are prone to coarsening,making them difficult to achieve high strength.Furthermore,the matrix of dilute Mg alloys contains a limited number of solute atoms,leading to a lower concentration of easily passivated element within the corrosion product film.Consequently,it is difficult to form a dense surface film on the dilute Mg alloy,resulting in inferior corrosion resistance.Moreover,the strength-corrosion trade-off is well common in Mg alloys.Although increasing secondary phases can result in better strengthening effects,it can also lead to more micro-galvanic corrosion sites and deteriorate corrosion resistance.Therefore,the pressing challenge at hand is how to achieve synergistic enhancement in both the strength and corrosion resistance of the dilute Mg alloys.Due to the characteristics of weak texture and excellent room-temperature formability,the Mg-Zn-Ca(ZX)low-alloyed alloys demonstrate outstanding stamping capabilities and thus possess significant potential for application in automotive and aerospace industries.Moreover,the ZX series alloys also show good application potential in orthopedic implant materials,as Zn and Ca elements are essential elements for the human body.However,there is a strength-formability trade-off in Mg alloys.Improving the strength of ZX alloy without affecting its ductility is a crucial issue that requires attention.In addition,coarse Ca-containing phases are usually formed in ZX alloys,which could induce micro-galvanic corrosion,thus impairing corrosion resistance.Due to the absence of passivation elements,the ZX alloy struggles to form a dense corrosion product film,thereby being unable to impede the rapid expansion of pitting corrosion.Alloying serves as a crucial method to regulate the composition and quantity of second phases,along with the component of surface oxide films,thereby playing a significant role in improving the corrosion resistance of Mg alloys.The Al and Sn elements are typical easily passivated elements as well as strengthening and toughening elements.The Al has excellent solid-solution strengthening effect and second phase refinement effect,and Sn has good precipitation strengthening effect.The Sn is also a biocompatible element.Therefore,it is anticipated that alloying with Al/Sn could achieve a synergistic improvement in the strength and corrosion resistance of ZX alloys.Aimed at the above issues,we employ pre-deformation before aging to promote solute atom segregation at twin boundaries,achieving a synergistic enhancement in the strength and plasticity of ZX series alloys.The Mg-2Zn-0.5Ca(wt.%,ZX20)alloy we designed shows excellent bake hardenability.To weaken the pitting corrosion of coarse intermetallic,through Al alloying,we regulate the composition of the second phases to reduce the potential difference between the Ca-containing phases and the matrix,which mitigates the micro-galvanic corrosion,achieving synergistic improvement in the strength and corrosion resistance of the cast ZX20 alloy.To enhance the protectiveness of the oxide film,a dense oxide film doped with SnO2 is formed after adding Sn in the rolled ZX20 alloy,which suppresses the rapid expansion of pitting corrosion,and the Sn-bearing alloy shows a synergistic improvement in the strength and corrosion resistance.The main conclusions are as follows:(1)The synergistic enhancement mechanism of strength and plasticity in dilute ZX alloy based on solute segregation is revealed.After pre-deformation and aging treatment,the flow stress of the ZX20 alloy is increased by～60 MPa and the elongation to failure is increased by～5%.The significant bake hardenability is attributed to the co-segregation of Zn and Ca atoms at the twin boundaries and dislocations.Furthermore,the pinning effect of twin boundary segregation can effectively retard the twin boundary migration and suppress the crack propagation,leading to an enhancement of plasticity.(2)The influence of Al addition on the microstructure and mechanical properties of cast Mg-2Zn-xAl-0.5Ca(x=0,0.5,1,2 wt.%)alloy is studied.With the increase of Al content,the grain size first decreases and then increases,and the second phase changes from Ca2Mg6Zn3 to Al-rich Ca2Mg6Zn3 and Al2Ca phases.When the Al content ranges from 0 to 1 wt.%,continuous Ca2Mg6Zn3 distributes along grain boundaries;however,when the Al content reaches 2 wt.%,the second phases at the grain boundaries exhibit a discontinuous distribution.With the Al content increases,the strength of the alloy first increases and then decreases.After adding 0.5 wt.%Al,the yield strength of the alloy increases to～101 MPa compared to the ZX20 alloy(～86 MPa).(3)The influence of Al addition on the corrosion behavior of the cast Mg-2Zn-xAl0.5Ca(x=0,0.5,1,2 wt.%)alloys is revealed.The corrosion resistance of the alloy first increases and then decreases as the Al content increases.The alloy with 0.5 wt.%Al shows the lowest hydrogen evolution corrosion rate in 3.5 wt.%NaCl solution(from 9.5±2.2 mm/y down to 4±1.2 mm/y).The grain refinement strengthens the corrosion barrier effect of the network-like Ca2Mg6Zn3 phase at grain boundaries,confining corrosion to smaller regions.The potential difference between the Ca2Mg6Zn3 phase enriched with Al and the Mg matrix is lower,weakening the micro-galvanic corrosion.The addition of Al promotes the formation of Al2O3 in the oxide film,making the film more compact and protective.(4)The influence of Sn addition on the microstructure and mechanical properties of rolled Mg-2Zn-xSn-0.5Ca(x=0,0.5,1,2 wt.%)alloy is studied.With rising Sn content,the Ca2Mg6Zn3 phase gradually decreases and the coarse blocky MgSnCa phase gradually increases.When the Sn content reaches 2 wt.%,the area fraction of the second phases significantly increases,and the texture type transforms from weak texture extended along transverse direction(TD)to strong basal texture.The yield strength of the alloy is increased from 170 MPa to 210 MPa after adding 1 wt.%Sn.(5)The influence of Sn addition on the corrosion behavior of rolled Mg-2Zn-xSn0.5Ca(x=0,0.5,1,2 wt.%)alloy is revealed.As the Sn content increases,the corrosion resistance of the alloy first increases and then decreases.After adding 1 wt.%Sn,the hydrogen evolution corrosion rate of the alloy in 3.5 wt.%NaCl solution is decreased from 5.7±0.25 mm/y to 2.1±0.10 mm/y.Although the potential difference between the MgSnCa phase and α-Mg is higher than that of the Ca2Mg6Zn3 phase,the Sn addition can generate compact and stable oxide film containing SnO2 and more ZnO on the alloy surface,thus inhibiting the propagation of localized corrosion induced by nobler MgSnCa phase.When the Sn content is 0-1 wt.%,the beneficial effect of Sn on the oxide film dominates the corrosion behavior;however,when the Sn content reaches 2 wt.%,the micro-galvanic corrosion of the MgSnCa phase dominates the corrosion behavior,deteriorating the corrosion resistance.