Composition And Microstructural Design Of AlCrFeNi Eutectic High Entropy Alloy And Investigations On Its Mechanical Properties And Corrosion Resistance

Due to possessing the characteristics of eutectic alloy and high entropy alloy(HEA),AlCoCrFeNi2.1 eutectic HEA(EHEA)has shown a series of advantages such as good casting fluidity,uniform composition and structure,excellent mechanical properties and corrosion resistance,which is a potential structural material that can serve in harsh corrosive environments and showing good engineering application prospects.However,the alloy contains a large amount of expensive cobalt element with a mass fraction of 18.59%,which makes the cost of the alloy high and limits its engineering application to a certain extent.Therefore,it is urgent to design and develop a cobalt-free EHEA with high performance and low cost.In this study,based on the composition of AlCoCrFeNi2.1 EHEA,AlCrFe1.5Ni2.6 EHEA was designed with the help of valence electron concentration(VEC)empirical parameter,and the origin of good strength-ductility combination of the designed alloy as well as the corrosion resistance of the designed alloy in various corrosion solutions were revealed through various characterization and detection techniques.In addition,in order to further improve the strengthening and toughening level of AlCrFe1.5Ni2.6 EHEA,a multi-level heterogeneous lamellar structure(MHLS)was successfully constructed in the designed alloy by cold rolling and subsequent annealing treatment,and the effect of the construction of MHLS on the mechanical properties and corrosion resistance of the designed alloy was systematically clarified,which provided a certain theoretical basis and data support for the design and development of high-performance low-cost EHEA and its engineering application.The main findings are as follows:(1)The intrinsic correlation between VEC empirical parameter,microstructure,mechanical properties and corrosion resistance of alloys was established,and the feasibility of using VEC empirical parameter to assist the design of cobalt-free EHEA was verified.It was found that there was a linear correlation between the VEC value of alloys and the content of FCC phase in alloys and the VEC empirical parameter could be used to guide the composition design of cobalt-free EHEA.Compared with AlCrFeNi3.1 and AlCrFe2Ni2.1 HEAs,AlCoCrFeNi2.1 EHEA exhibited good comprehensive mechanical properties and corrosion resistance due to its special structural characteristics.The AlCrFe1.5Ni2.6 alloy with ideal eutectic structure was successfully designed by replacing Co element in AlCoCrFeNi2.1 EHEA with Fe and Ni element in equal proportion.It was found that AlCrFe1.5Ni2.6 EHEA was composed of FCC+B2 phases,while the content of two phases was 72%and 28%,respectively.The orientation relationship(OR)between two phases was K-S OR.Further,nanoprecipitates were found in both phases,among which L12 phase existed in FCC phase and disordered BCC phase riched in Cr element existed in B2 phase.(2)The mechanical properties and corrosion resistance of AlCrFe1.5Ni2.6 EHEA were studied.It was found that the tensile yield strength,ultimate strength and elongation of AlCrFe1.5Ni2.6 EHEA could reach 522 MPa,1028 MPa and 18.3%respectively,showing a good strength-ductility matching ability similar to that of AlCoCrFeNi2.1 EHEA.The high strength of the designed alloy originated from the hindering effect of a large number of heterogeneous interfaces and nanoprecipitations in the alloy on dislocation movements,while the high ductility was mainly due to the dislocation slip continuity under K-S OR.In 3.5 wt%NaCl,0.5 M H2SO4 and 0.5 M HCl solutions,AlCrFe1.5Ni2.6 EHEA showed better corrosion resistance than AlCoCrFeNi2.1 EHEA,mainly due to fact that AlCrFe1.5Ni2.6 alloy possessed the higher content of corrosion resistant FCC phase and the passive film on AlCrFe1.5Ni2.6 alloy contained the higher proportion of corrosion resistant oxides.(3)A MHLS was successfully constructed in AlCrFe1.5Ni2.6 EHEA and its effect on mechanical properties was studied.It was found that AlCrFe1.5Ni2.6 EHEA underwent phase decomposition and selective recrystallization of FCC phase after cold rolling and isothermal annealing at 800℃ for 1 h,resulting in the heterogeneity of the size and spatial distribution of the two phases and the heterogeneity of the multi-scale grain size,thus forming a MHLS in the alloy.Compared with the as-cast alloy,the yield strength and ultimate strength of MHLS HEA increased by nearly 460 MPa and 380 MPa respectively,while the elongation remained unchanged.The significant improvement in the strengthening and toughening level of MHLS HEA is mainly due to the hetero-deformation induced(HDI)strengthening and strain hardening,and the HDI stress induced by hetero-deformation is up to 55%of the flow stress.At the same time,the MHLS design could significantly hinder the growth process of microcracks in the alloy,improve the tolerance of the alloy to cracks,and avoid the rapid growth of microcracks leading to premature failure of the material.(4)The effect of the construction of MHLS on the corrosion resistance of AlCrFe1.5Ni2.6 EHEA was studied.It was found that the corrosion resistance ability of the alloy in 3.5 wt%NaCl solution was significantly improved after the construction of MHLS in the alloy.The main reason of improved corrosion resistance of MHLS HEA could be attributed to the fact that a large number of structural defects such as dislocations,subgrain boundaries,grain boundaries and twin boundaries in MHLS HEA could provide channels for the outward diffusion of elements,change the nucleation mechanism of the passive film,and promote the formation of a dense and corrosionresistant oxides-enriched passive film on the alloy surface.At the same time,the fine FCC phase riched in FeCr elements generated by the decomposition of B2 lamellae could also promote the formation of a more protective passive film on the surface of B2 lamellae,improve the passivation ability of the alloy,thus avoiding serious localized corrosion of the alloy.