Microstructures And Corrosion-Resistant Properties Of The Al_xCoCrFeNi High-Entropy Alloys

As a type of multi-component alloys,high-entropy alloys(HEAs)break the traditional alloy design strategy,and broaden the knowledge of the central region in the multi-components phase diagrams.In recent studies,HEAs exhibit many unique properties including high strength,high ductility,and good wear resistance etc.,making them highly promising for structural applications.Nevertheless,the research of HEAs is still at its initial stage.Corrosion is one of the most severe failure types of structural materials.A satisfactory corrosion resistant property should not be ignored during exploring advanced alloys.It has been studied that the addition of passivating elements,like Cr,Ni etc.,could offer the HEAs good corrosion resistant properties.However,there are still several problems need to be solved.First,the composition-structure-property relationship in corrosion resistant HEAs has not been systematically studied and established.Second,the corrosion behavior and corrosion resistance mechanism of HEAs are unclear.Lastly,the theoretical guidance for improving the corrosion resistance through alloy design and microstructure regulation is scarce.In the present study,the alloying effects of A1 on the microstructure and corrosion resistant properties of the AlxCoCrFeNi(x = 0.3,0.5,0.7)HEAs are systematically studied.The relationships among composition,microstructure,and corrosion property are established by comprehensive microstructural characterizations and electrochemical tests.The results indicate that with the increased A1 content,the microstructures change from single FCC phase to FCC +ordered/disordered BCC phase.The formation of multi-phase microstructure leads to the segregation of elements.The increased volume fraction of the Cr-depleted BCC phase causes the decreased corrosion resistance of alloys in the 3.5 wt.%NaCl solution.Then,to reveal the corrosion resistance mechanism,the properties of passive films formed on the AlxCoCrFeNi alloys have been investigated.It has been found that with the increased Al content,the passive film contains more Al oxides and less Cr oxides.Therefore,the thickness of the passive film increases but the compactness and protectiveness decrease.After that,to reveal the controlling factor during the corrosion process,the metastable pitting behaviour of the AlxCoCrFeNi alloys has been studied.The in-situ observations of metastable pitting processes indicate that the growth and repassivation of metastable pits are ion-diffusion controlled.The process of pitting is also influenced by the property of passive film,the interaction between Cl-and the active site,and the microstructure.Follow that,an advanced in-situ electrochemical-AFM test is used to elaborate the localized corrosion behavior intuitively.It shows that with the increased Al content in the alloys,the localized corrosion processes in different ways,which changes from pitting to phase boundary corrosion,leading to the decreased resistance to localized corrosion.Lastly,based on the understanding of the corrosion mechanism in the AlxCoCrFeNi HEAs,the microstructures and elemental distributions are regulated by solution treatment.After 1250 ?,1000 h heat treatment,the microstructures are simplified.The segregations of elements have weakened and the work function differences among phases have decreased.Therefore,the corrosion properties of HEAs have been improved.According to the thermodynamic calculation,the calculated phase diagrams are reliable to predict the microstructure and phase fractions of the HEAs.Therefore,the calculated phase diagrams could be used to guide the heat treatment process to obtain the desired microstructure for corrosion resistance improvement.