Microstructure Control, Mechanical Properties, Corrosion Behavior And Thermal Stability Of Low-alloyed Mg-Al-Mn-Ca-Ce Alloys

Magnesium(Mg)alloys have been widely utilized in the fields of national defense,aerospace,transportation,biomedical and electronic communications due to their low density,good castability,biocompatibility,and high specific strength.Lowalloyed Mg alloys have attracted extensive attentions in researchers at home and abroad,because of their excellent low production cost and excellent workability.However,conventional low-alloyed Mg alloys are less effective in second-phase strengthening and solid solution strengthening,resulting in inferior mechanical properties.In recent years,researchers have found that low-alloyed Mg alloys can be effectively strengthened by forming high-density G.P.zones,nanoclusters,and solute atom segregation at dislocation lines/twin boundaries during aging treatment.However,due to the inadequate second-phase particles and solute atom segregation at grain boundaries,significant grain coarsening occurs during the solid-solution process,reducing the effectiveness of grain boundary strengthening in low-alloy systems.Therefore,it is of great significance to develop stable fine-grained microstructures for the preparation of low-alloyed Mg alloys with high-strength.Furthermore,Mg exhibits highly active chemical properties and a low corrosion potential,leading to relatively poor corrosion resistance.The addition of rare earth elements can effectively enhance the corrosion resistance of Mg alloys and simultaneously weaken texture strength of alloys,thus improving their ductility.However,few studies have been performed on the effects of modest amount of rare earth elements on the age hardening and corrosion behavior of low-alloyed Mg alloys.Moreover,excessive rare earth elements are easy to combine with other elements to form coarse second phases,which are difficult to be redissolved in the alloy,negatively impacting the age-hardening and plastic deformation of alloys.Therefore,investigating the influence of rare earth element content on the corrosion resistance and age-hardening of low-alloyed Mg alloys is of importance in the development of high-strength and corrosion resistant Mg alloys.In conclusion,this study is based on the design strategy of rare earth element microalloying,which focuses on the Mg-0.6Al-0.5Mn-0.2Ca(wt.%,AMX100)alloy.The effect of Ce contents on the microstructure,mechanical properties,and corrosion resistance of the alloy was investigated.By controlling the grain structure and second phase constituents,a corrosion resistant Mg-0.6Al-0.5Mn-0.2Ca-0.3Ce(wt.%,AMX100-0.3Ce)alloy sheet with age hardening capability was prepared.The evolution of the microstructure of the AMX100-0.3Ce alloy during heat treatment has been investigated,revealing the influence of heat treatment temperature,secondphase,and texture on the recrystallization and grain growth behavior.By adjusting the volume fraction of the second phase through multiple-pass controlled rolling,the AMX100-0.3Ce alloy sheet with high thermal stability was obtained.The main conclusions are as follows:(1)The effects of Ce content(0.1 wt.% and 0.3 wt.%)and heat treatment temperatures on the microstructure of the AMX100 alloy have been studied.It is observed that the number density and volume fraction of second phases in the alloy are positively correlated with Ce content.After single-stage lowtemperature annealing process(350 °C/5 min),the texture intensity of the AMX100 alloy gradually decreased with increasing Ce content,and the average grain size of the three alloys was ~6 μm.After two-stage annealing(350 °C/5 min and 480 °C/5 min),both AMX100 and AMX100-0.1Ce alloys show homogenous microstructure with average grain size of ~11 μm,while the abnormal grain growth occurs in the AMX100-0.3Ce alloy.(2)The effects of Ce content and heat treatment temperature on the age hardening and mechanical properties of the AMX100 alloy have been investigated.After aging treatment at 200 °C for 2 h,dense Al-Ca G.P.zone are formed in the matrix of the two-stage annealed alloy,resulting in an increase in yield strength by ~25-45 MPa.Due to the addition of Ce element,insoluble Al-Mn-Ce and Al-Ce phases are formed in the alloy,reducing the Al content in the matrix.Thus,the strength improvement of the AMX100-0.3Ce alloy is minimal.For single-stage annealed alloys,the solid solution atoms in the alloy matrix were inadequate to form a high number density of G.P.zones,resulting in a slight increase in yield strength by about ~1-8 MPa.(3)The effect of Ce micro-alloying on the microstructure and corrosion behavior of AMX100 alloy have been elucidated.After adding 0.3 wt.% Ce to the AMX100 alloy,the Al8Mn5 phase transforms into the Al8Mn4 Ce phase.The corrosion potential of the Al8Mn4 Ce phase is lower than that of the Al8Mn5 phase,reducing the adverse effect of micro-galvanic corrosion on corrosion resistance.The corrosion film of the AMX100-0.3Ce alloy contains Ce O2,which improves the stability and compactness of the corrosion product film,effectively reducing the corrosion rate.(4)The influences of thermo-mechanical processing and heat treatment on the corrosion behavior of AMX100 and AMX100-0.3Ce alloys has been analyzed.Through rolling,solid-solution and aging treatment,the AMX100-0.3Ce alloy achieves fine-grained structure with a uniform distribution of second phases,which prevents pitting corrosion and improves the corrosion resistance.In additional,the small and uniformly distributed Al-Ca G.P.zones in the matrix of the alloy do not cause severe micro-galvanic corrosion.After aging treatment,the strength of the AMX100-0.3Ce alloy is improved without deteriorating its the corrosion resistance.(5)The mechanisms for the high-temperature thermal stability of the AMX100-0.3Ce alloy have been revealed.Increasing the rolling pass from 4 passed to 13 passes,the thermal stability of the alloy is significantly improved.After annealing at 480 ℃ for 5-1200 min,the 13-pass rolled AMX100-0.3Ce alloy consistently maintains uniform fine-grained structure with an average grain size of ~13 μm.Furthermore,the yield strength of the alloy remains almost unchanged,stabilizing at ~150 MPa.In contrast,the alloy subjected to 4-pass rolling exhibits abnormal grain growth after annealing at 480 ℃ for 5 min,with its yield strength is decreased by ~30 MPa after 1200 minutes.The increase in rolling passes promotes the continuous dynamic precipitation of thermally stable Al-Mn phases in the matrix.The phases are distributed more uniformly,effectively pinning grain boundaries and inhibiting grain coarsening.