Research On Corrosion And Antibacterial Properties Of Fe-based Medium Entropy Alloys

Medium and high entropy alloys have multi-component characteristics,and there are differences in the dimension and chemical bonding of each component.Therefore,the microstructure of the alloy can be changed by adjusting the type and content of the alloy elements,thus exhibiting different properties from those of traditional alloys and attracting significant interest.In this paper,the medium entropy alloys Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8and Fe_54.1Mn_23.2Si_9.1Cr_9.8C_3.8 were prepared by water-cooled copper crucible magnetic levitation melting and negative-pressure copper mold suction casting method.Firstly,the density of states of alloy was calculated by first principles calculation method,and the corrosion mechanism was analyzed by electrochemical test and the alloy composition with excellent corrosion resistance was optimized.Then the alloy Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 was used as the matrix,by varying the contents of Cu and Ag elements in the alloy,the phase structure,microstructure,corrosion resistance and antibacterial properties of the alloy were analyzed.The alloy composition has been optimized to provide excellent corrosion resistance and antibacterial properties.The medium-entropy alloy Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 has a single-phase FCC structure,and with the increase of Mn content,the alloy Fe_54.1Mn_23.2Si_9.1Cr_9.8C_3.8 hasε-martensite generation,bipolar FCC+HCP structure.Using the method of the first principles calculation,alloy Fe_54.1Mn_23.2Si_9.1Cr_9.8C_3.8,Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 density of states at Fermi level value were 337.077 electrons/e V,214.145 electrons/e V.The alloy Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 has a smaller density of states,more stable structure and weaker ability to lose electrons.It is not easy to lose electrons during the reaction,which gives the alloy better stability.Combined with the electrochemical test results of the alloy in 3.5%Na Cl solution,the alloy Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 has a more positive corrosion potential,a smaller corrosion current density and has a better corrosion resistance.The alloy Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8 was used as the matrix and Cu and Ag elements were added to prepare the alloys(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_99.5-xCu_xAg_0.5（x=1,2,3,4,5at.%）,(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_88-xCu₁₂Ag_x（x=1,2,3,4,5 at.%）.The entropy values of the alloysΔS_mixare all between 1R and 1.5R.Since the alloys have a negativeΔH_mix,δas well as VEC are smaller,which facilitate the formation of a solid solution structure.With the addition of Cu and Ag,the FCC1+FCC2 phases are produced in the alloys(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_99.5-xCu_xAg_0.5（x=3,4,5 at.%）,(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_88-xCu₁₂Ag_x（x=1,2,3,4,5 at.%）.As the binary mixing enthalpies of the Cu and Ag elements with the major group elements in the alloy are positive,phase separation occurs,leading to the formation of the FCC2 phase by enrichment of Cu and Ag between the dendrites.The corrosion behaviors of the alloy in 3.5%Na Cl solution and Hank’s solution were studied.The results show that with the increase of Cu content,the corrosion potential and polarization resistance of the alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_99.5-xCu_xAg_0.5（x=1,2,3,4,5 at.%）increase first and then decrease,and the corrosion current density decreases first and then increases,indicating that the corrosion resistance of the alloy increases first and then decreases.The addition of the appropriate amount of Cu can enhance the corrosion resistance of the alloy,while an excess of Cu will make the alloy less resistant to corrosion.With the increase of Ag content,the corrosion potential and polarization resistance of the medium entropy alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_88-xCu₁₂Ag_x（x=1,2,3,4,5 at.%）increase,the corrosion current density decreases,and the corrosion resistance of the alloy increases.With the presence of high levels of Cu in the alloy,corrosion behaviour will preferentially occur at the edges of the second phase particles where the Cu content is high.However,due to the chemical stability of Ag compared to Cu and its corrosion resistance,as the elemental content of Ag increases,Ag is dispersed in the form of its oxide on the passivated film of the alloy.Due to the presence of Cr and Si elements in the matrix alloy,a dense Cr₂O₃ and Si O₂ protective layer can be formed during the corrosion process,which improves the stability of the passivation film and increases the corrosion resistance of the alloy.The alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_97.5Cu₂Ag_0.5has the best corrosion resistance in Hank’s solution,and the corrosion current density is 1.172×10^-7 A·cm^-2.The antibacterial properties of the alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_99.5-xCu_xAg_0.5（x=1,2,3,4,5 at.%）after solution+antibacterial aging treatment,and the alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_88-xCu₁₂Ag_x（x=1,2,3,4,5 at.%）without heat treatment in Escherichia coli were studied.The results show that the solid solution+antimicrobial ageing treated alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_99.5-xCu_xAg_0.5（x=1,2,3,4,5 at.%）produces a significant Cu-rich phase,which effectively enhances the antimicrobial properties of the alloy,(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_94.5Cu₅Ag_0.5 had the strongest antibacterial activity,and the antibacterial rate reached 99.94%.Due to the high Cu and Ag content in alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_88-xCu₁₂Ag_x（x=1,2,3,4,5at.%）,there is an obvious Cu and Ag rich phase without solid solution aging treatment.The higher Ag content,the stronger antibacterial properties of the alloy,where alloy(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)₈₃Cu₁₂Ag₅ has an antibacterial rate of 99.97%.