Corrosion Behavior And Mechanism Of FeCoCrNiMo_x High-entropy Alloys

High-entropy alloys(HEAs)are the promising candidates for many industrial fields as the structural materials because of outstanding mechanical properties.Corrosion is one of the main failure types of structural materials caused by environmental factors;therefore,it has great scientific and application value to study the corrosion behavior and mechanism of HEA.In this work,the typical FeCoCrNi alloy was alloyed with Mo element.The corrosion resistance of FeCoCrNiMOx HEAs was modified through modifying Mo content and microstructure to obtain FeCoCrNiMox HEA with excellent corrosion resistance to harsh environment.From the perspective of the intrinsic and environmental factors,the effects of alloying elements,microstructure and hydrogen on the corrosion behavior and mechanisms were studied.The relationships among compositions,microstructure and corrosion resistance are established,which lays a theoretical foundation for compositional design and microstructural modification of corrosion resistant HEAs.(1)The FeCoCrNiMox(x=0,0.1,0.3,0.6)HEAs were fabricated by arc melting.The effect of Mo content on the microstructure and corrosion behavior of as cast-FeCoCrNiMox HEAs was systematically studied by comprehensive microstructural characterization and electrochemical tests.The results indicated that FeCoCrNiMoo and FeCoCrNiMo0.1 alloys were single FCC solid solution.Precipitates enriched in Cr and Mo formed in the FeCoCrNiMo0.3 alloy,and a two-phase structure formed in the FeCoCrNiMo0.6 alloy consisting of precipitates with larger volume fractions and matrix depleted in Cr and Mo.Pitting occurred on FeCoCrNiMoo and FeCoCrNiMo0.1 alloys with single phase while FeCoCrNiMo0.3 and FeCoCrNiMo0.6 alloys suffered from preferential localized corrosion at the regions depleted in Cr and Mo.(2)The passivation behavior of the FeCoCrNiMox HEAs was investigated by a comprehensive characterization method of passive film.It had been found that the passive film had a bilayer structure composed of an outer layer with a mixed Cr/Fe oxides and hydroxides as well as MoO3,and an inner layer containing a strong enrichment of Cr oxide and hydroxide with higher Cr2O3/Cr(OH)3 ratio,also containing MoO2.The inner layer played the critical role determing the protective property of the passive film.Cr oxide formed the main part of the protective film,and Mo species incorporated into the passive film provided greater resistance to the attack of chloride ions in acid media compared to the alloy without Mo.High Cr/Mo value facilitated the passivation,and the films contained a large amount of Cr species with high ratio of Cr2O3/Cr(OH)3 partition were grown.The synergy between Cr and Mo was most operative in the FeCoCrNiMo0.1 alloy,yielding a high corrosion resistant passive film.Contrarily,the imperfect film formed on the FeCoCrNiMo0.6 alloy with lower Cr/Mo ratio.(3)The bulk FeCoCrNiMo0.1 HEA was fabricated by magnetic levitation melting,and its microstructure was further modified by heat treatment.The effect of the microstructure on the corrosion resistance and mechanical property was investigated.After homogenization at 1300℃ for 24 h,the elements and microstructures were uniformly distributed.The annealing treatments affected the grain size and grain boundaries distribution.It was demonstrated that the alloys annealed at 900℃ for 30 min and 60 min showed better pitting resistance;Meanwhile the low angle grain boundaries fraction in the alloys annealed at 900℃ for 30 min and 60 min was high,which was beneficial to coordinated the deformation of the alloy,and the excellent strength-ductility trade-off was thus achieved.(4)The failure mechanism of FeCoCrNiMo0.1 alloy under the synergistic action of hydrogen and load was studied by cathodic hydrogen charging and slow strain rate tensile test.The results showed that hydrogen seriously inhibited the formation of the passive film,and thus the corrosion resistance of FeCoCrNiMoo.i alloy decreased in 1 M NaCl solution.After hydrogen charging for 72 h at a current density of 25 mA/cm2 and subsequent simultaneous cathodic charging during slow straining rate tensile testing,hydrogen caused the reduction in tensile elongation by approximately 27.08%,indicating that the FeCoCLNiMo0.1 alloy was susceptible to hydrogen embrittlement.Hydrogen assisted localized plasticity(HELP)and hydrogen-enhanced decohesion mechanism(HEDE)were considered to account for the morphology of quasi-cleavage and intergranular fracture.Grain boundary triple junctions were observed to act as potential sites for crack initiation.The crack propagation mode was the combination of intergranular and transgranular fracture.