A secondary-cooling-segment electromagnetic stirring(S-EMS)experiment was performed at 150 A and 4 Hz to evaluate the effect of S-EMS on solidification characterization near the white band.The upper and lower parts of...A secondary-cooling-segment electromagnetic stirring(S-EMS)experiment was performed at 150 A and 4 Hz to evaluate the effect of S-EMS on solidification characterization near the white band.The upper and lower parts of the white band exhibited average secondary dendritic arm spacing of 205.4 and 214.4μm,respectively.The S-EMS operation resulted in large Lorentz forces and cooling intensity,which could produce additional dendritic arms with low carbon concentrations,leading to local negative segregation.Moreover,a 3D flow-temperature-magnetic coupling numerical model was established.The results revealed that the magnetic induction intensity and Lorentz force were symmetrically distributed along rollers S1 and S2.The average velocity magnitude increased by approximately 42.52%,58.69%,and 64.11%for liquid fractions of 0.7,0.8,and 0.9,respectively.During the S-EMS operation,the Lorentz force may alter the velocity of the solidification front and promote the dissipation of superheat.Additionally,the influence of S-EMS on grain nucleation and growth was investigated using Gibbs free energy theory and component undercooling.Furthermore,a formation model for the white band was established,and the mechanism of white band formation was elucidated according to the changes in the solute-enriched layer,solute precipitation,and diffusion.展开更多
The mechanics-corrosion and strength-ductility tradeoffs of magnesium(Mg)alloys have limited their applications in fields such as orthopedic implants.Herein,a fine-grain structure consisting of weak anodic nano-lamell...The mechanics-corrosion and strength-ductility tradeoffs of magnesium(Mg)alloys have limited their applications in fields such as orthopedic implants.Herein,a fine-grain structure consisting of weak anodic nano-lamellar solute-enriched stacking faults(SESFs)with the average thickness of 8 nm and spacing of 16 nm is constructed in an as-extruded Mg96.9Y1.2Ho1.2Zn0.6Zr0.1(at.%)alloy,obtaining a high yield strength(YS)of 370 MPa,an excellent elongation(EL)of 17%,and a low corrosion rate of 0.30 mm y−1(close to that of high-pure Mg)in a uniform corrosion mode.Through scanning Kelvin probe force microscopy(SKPFM),one-dimensional nanostructured SESFs are identified as the weak anode(∼24 mV)for the first time.The excellent corrosion resistance is mainly related to the weak anodic nature of SESFs and their nano-lamellar structure,leading to the more uniform potential distribution to weaken galvanic corrosion and the release of abundant Y^(3+)/Ho^(3+)from SESFs to form a more protective film with an outer Ca_(10)(PO_(4))_(6)(OH)_(2)/Y_(2)O_(3)/Ho_(2)O_(3) layer(thickness percentage of this layer:72.45%).For comparison,the as-cast alloy containing block 18R long period stacking ordered(LPSO)phase and the heat-treated alloy with fine lamellar 18R-LPSO phase(thickness:80 nm,spacing:120 nm)are also studied,and the characteristics of SESFs and 18R-LPSO phase,such as the weak anode nature of the former and the cathode nature of the latter(37-90 mV),are distinguished under the same alloy composition.Ultimately,we put forward the idea of designing Mg alloys with high mechanical and anti-corrosion properties by constructing"homogeneous potential strengthening microstructure",such as the weak anode nano-lamellar SESFs structure.展开更多
基金supported by the National Natural Science Foundation of China(No.51774031)the Project funded by China Postdoctoral Science Foundation(No.2023M730230).
文摘A secondary-cooling-segment electromagnetic stirring(S-EMS)experiment was performed at 150 A and 4 Hz to evaluate the effect of S-EMS on solidification characterization near the white band.The upper and lower parts of the white band exhibited average secondary dendritic arm spacing of 205.4 and 214.4μm,respectively.The S-EMS operation resulted in large Lorentz forces and cooling intensity,which could produce additional dendritic arms with low carbon concentrations,leading to local negative segregation.Moreover,a 3D flow-temperature-magnetic coupling numerical model was established.The results revealed that the magnetic induction intensity and Lorentz force were symmetrically distributed along rollers S1 and S2.The average velocity magnitude increased by approximately 42.52%,58.69%,and 64.11%for liquid fractions of 0.7,0.8,and 0.9,respectively.During the S-EMS operation,the Lorentz force may alter the velocity of the solidification front and promote the dissipation of superheat.Additionally,the influence of S-EMS on grain nucleation and growth was investigated using Gibbs free energy theory and component undercooling.Furthermore,a formation model for the white band was established,and the mechanism of white band formation was elucidated according to the changes in the solute-enriched layer,solute precipitation,and diffusion.
基金the support of the National Natural Science Foundation of China(51871069,52071093)the Zhejiang Province Key Research and Development Plan,China(2021C01086)Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities(3072022GIP1004)。
文摘The mechanics-corrosion and strength-ductility tradeoffs of magnesium(Mg)alloys have limited their applications in fields such as orthopedic implants.Herein,a fine-grain structure consisting of weak anodic nano-lamellar solute-enriched stacking faults(SESFs)with the average thickness of 8 nm and spacing of 16 nm is constructed in an as-extruded Mg96.9Y1.2Ho1.2Zn0.6Zr0.1(at.%)alloy,obtaining a high yield strength(YS)of 370 MPa,an excellent elongation(EL)of 17%,and a low corrosion rate of 0.30 mm y−1(close to that of high-pure Mg)in a uniform corrosion mode.Through scanning Kelvin probe force microscopy(SKPFM),one-dimensional nanostructured SESFs are identified as the weak anode(∼24 mV)for the first time.The excellent corrosion resistance is mainly related to the weak anodic nature of SESFs and their nano-lamellar structure,leading to the more uniform potential distribution to weaken galvanic corrosion and the release of abundant Y^(3+)/Ho^(3+)from SESFs to form a more protective film with an outer Ca_(10)(PO_(4))_(6)(OH)_(2)/Y_(2)O_(3)/Ho_(2)O_(3) layer(thickness percentage of this layer:72.45%).For comparison,the as-cast alloy containing block 18R long period stacking ordered(LPSO)phase and the heat-treated alloy with fine lamellar 18R-LPSO phase(thickness:80 nm,spacing:120 nm)are also studied,and the characteristics of SESFs and 18R-LPSO phase,such as the weak anode nature of the former and the cathode nature of the latter(37-90 mV),are distinguished under the same alloy composition.Ultimately,we put forward the idea of designing Mg alloys with high mechanical and anti-corrosion properties by constructing"homogeneous potential strengthening microstructure",such as the weak anode nano-lamellar SESFs structure.
