Study On Microstructure And Tribo-Corrosion Resistance Mechanism Of Nb-Containing Eutectic High-Entropy Alloy Coatings By Laser Cladding

With the rapid development of science and technology in human society,the component damage or failure caused by severe working conditions of corrosion and wear interaction has become one of the key problems restricting the development and operation of high-tech equipments.It is urgent to develop novel coating design strategies and explore their tribo-corrosion resistance mechanism.The eutectic high-entropy alloys（EHEAs）system achieved the balance of strength and toughness and excellent tribo-corrosion resistance by coupling the design concepts of eutectic alloys and high-entropy alloys.Therefore,the EHEAs provided a breakthrough design idea for the new-generation coating systems under severe working conditions.However,the current researchs on EHEAs mainly focused on bulk materials,and there are few researchs on EHEA coatings have been reported.Moreover,the solidification mechanism and forming mechanism of coatings were significantly different from those of bulk materials.Thus,the research on EHEA coatings faced numerous difficulties,such as the design of eutectic composition,unclear microstructure evolution mechanism,and imperfect corrosion and resistance mechanism.In this work,the microstructure,evolution mechanism,properties and strengthening mechanism of the EHEA coatings were studied in detail.Two EHEA coating systems of FCC+Laves type Co Cr Fe_0.5Ni_1.5Mo_0.1Nb_x and BCC/B2+Laves type Al Co Cr Fe Mn_0.5Mo_0.1Nb_xwere designed.The EHEA coatings with hypoeutectic,eutectic and hypereutectic micromorphologies were prepared.The microstructure evolution and tribo-corrosion resistance mechanism of the EHEA coatings were revealed,and the“structure-performance”interaction mechanism was defined.The research results provided some theoretical basis and technical supports for the study of laser-cladded EHEA coatings.The following results were obtained:The eutectic compositions of the two EHEA coating systems were determined as Co Cr Fe_0.5Ni_1.5Mo_0.1Nb_0.68 and Al Co Cr Fe Mn_0.5Mo_0.1Nb_0.65 by dilution rate control,binary eutectic composition calculation,thermodynamic solidification simulation and microstructure characterization.The two EHEA coating systems were composed of Nb-poor solid solution phase（FCC and BCC/B2）and Nb-rich Laves phase.The central region of the EHEA coatings had a typical nano-lamellar eutectic microstructure or hypoeutectic/hypereutectic microstructure,while the interface region showd a columnar crystal microstructure.The combined effects of principal atomic radius difference,mixing enthalpy,molten pool temperature gradient,solidification rate and cooling rate contributed to the formation of different microstructures in the central and interface regions of the coatings.The corrosion and wear behavior and dual-phase interaction promotion mechanism of Co Cr Fe_0.5Ni_1.5Mo_0.1Nb_x（x=0.55 hypoeutectic,0.68 eutectic and 0.8 hypereutectic）HEA coating system were proposed.The corrosion resistance of Nb_0.55,Nb_0.68 and Nb_0.8 coatings in3.5 wt.%Na Cl solution increased with the increase of Nb content.The passivation films of the three coatings were composed of oxides and hydroxides of the coating elements.With the increase of Nb content,the hardness and the wear resistance of the three coatings increased,and the main wear mechanism of the coatings changed from adhesive wear to abrasive wear.No obvious corrosion phenomenon occurred during the tribo-corrosion process of the three coatings in 3.5 wt.%Na Cl solution,and the main corrosion elements tended to concentrate in the adhesion layer.The excellent tribo-corrosion resistance of the coatings was due to the interaction between FCC and Laves phases.Under the tribo-corrosion condition,the in-situ nano-lamellar FCC and Laves phases induced the nano-structure effect,thereby inhibiting the expansion of pits and coating corrosion.Furthermore,the FCC phase with good toughness and the Laves phase with excellent load-carrying capacity formed a complementary effect.The increase of Laves phase also inhibited the formation of adhesive layer,which reduced the accumulation of main corrosion elements during long-term tribo-corrosion process.The tribo-corrosion resistance and corresponding mechanism of Al Co Cr Fe Mn_0.5Mo_0.1Nb_x（x=0.4 hypoeutectic,0.65 eutectic and 0.8 hypereutectic）HEA coatings were studied.The dry friction wear rates of of Nb_0.4,Nb_0.65 and Nb_0.8 coatings all reached the order of 10^-6 mm³/N·m,and the Nb_0.65 coating had the best wear resistance.After wear test,multi-principal oxide layers composed of the oxides of Al,Co,Cr,Fe,Mn and Nb were formed on the three coatings,and undulating oxide structure was formed.The three coatings showed slight wear after the tribo-corrosion test in 3.5 wt.%Na Cl solution,and the Nb_0.65 coating had the lowest wear rate.Both the BCC/B2 phase and the Laves phase had high hardness and load-carrying capacity.The synergistic effect between the two phases provided longitudinal load-carrying capacity and transverse scratch resistance.The Laves phase improved the corrosion resistance of the HEA coatings,especially the passivation film performance.In addition,the oxidation behavior of the coatings formed passivation protection against corrosive media and lubrication effect against the wear process,which further improved the comprehensive performance of the tribo-corrosion resistance.Cold-sprayed Cu Al9 aluminum bronze transition coating,cold-sprayed pure Al transition coating and laser-cladded Ni₂Mn Cu Sn Al_0.1 multi-principal element alloy transition coating were prepared on magnesium-lithium（Mg-Li）alloy,respectively.Gradient Co Cr Fe_0.5Ni_1.5Mo_0.1Nb_0.68 and Al Co Cr Fe Mn_0.5Mo_0.1Nb_0.65 EHEA coatings were successfully prepared by high-speed laser cladding induced in-situ bidirectional diffusion of the transition coatings.The microstructure evolution mechanism of the gradient EHEA coatings under the joint effect of thermodynamic mixing enthalpy,atomic radius difference and laser energy distribution was revealed.The gradient transition microstructure characteristics significantly inhibited the dilution of substrate elements（Mg and Li）to the EHEA coatings,while the transition regions were mainly formed by the metallurgical reaction between transition layer elements and substrate elements.The corrosion current densities of the EHEA coatings were nearly three orders of magnitude lower than that of Mg-Li alloy substrate.The wear resistance of the EHEA coatings was more than 30 times higher than that of Mg-Li alloy.Therefore,the EHEA coatings have significantly improved the corrosion and wear resistance of Mg-Li alloy substrate.