Microstructures And Properties Of Carbides Reinforced Fe-Cr-Ni Series Medium-Entropy Alloys

Medium/high-entropy alloys have attracted extensive attention because of their special multi-component single-phase solid solution structure and excellent mechanical properties.However,the low yield strength and wear resistance of FCC-based medium/high-entropy alloys hinder its application in the engineering field.Carbon can exist in the alloy either as interstitial solid solution or combined with carbide-forming elements to form carbide particles,resulting in a superimposed strengthening effect of strengthening and precipitation phase strengthening.This strengthening method is an effective means of enhancing the strength and wear resistance of materials.In this paper,two kinds of carbon-containing medium-entropy alloys were prepared by powder metallurgy method.The formation and evolution of carbides during gas atomization and sintering were studied,and the effect of interstitial carbon atoms on the initial plasticity of Fe Cr Ni-based medium-entropy alloys was quantitatively calculated,the influence of carbides on the mechanical properties,friction and wear properties of the alloys was analyzed.The main research work and main conclusions of this paper are as follows:（1）The phase composition and microstructure of（Fe Cr Ni）_100-xC_xalloy powder and SPS sintered samples were studied,and the effect of carbon content on the SPS sintering densification was analyzed.Combined with the equilibrium diagram,the mechanism of carbides formation in droplets under rapid cooling condition and the evolution law of carbides during sintering were expounded.Under the rapid cooling condition,the precipitation of primary M₇C₃ carbides in the droplet of carbon-containing alloys was inhibited.The FCC phase first precipitated from the liquid phase and grew in the form of dendrite,and then the M₇C₃ carbides and FCC phase were formed between dendrites in the form of eutectic.During SPS sintering,layered carbides undergo dissolution-precipitation-ripening process,forming large size carbides dispersed in the matrix.The addition of carbon promotes the local liquid phase sintering,thus promoting the diffusion of elements,thereby increasing the density of the sintered body.Carbon can cause local liquid phase sintering of the alloy,thus promoting element diffusion and vacancy discharge,resulting in an increase in the relative densities of（Fe Cr Ni）₉₆C₄ and（Fe Cr Ni）₉₂C₈ MEAs to 98.63%and99.12%,respectively.（2）The micro and macro mechanical properties of（Fe Cr Ni）_100-xC_xalloy were tested by nanoindentation tester and mechanical testing machine equipped with video extensometer.The effect of interstitial carbon atoms on the initial elastic-plastic behavior in the micro-region of the alloy was analyzed by the load-displacement curve of indentation test.The mechanism of carbide effect on mechanical properties of alloy was revealed by dislocation configuration analysis in different strain regions of tensile specimen.Interstitial carbon atoms improve the activation volume and maximum shear stress of dislocation nucleation during the plastic initiation and hinder the local plastic flow of dislocation,thus leading to the disappearance of the ejection phenomenon in the load-displacement curve of indentation test.It is found that the heterogeneous nucleation mechanism of direct atom-vacancy exchange is the dominant mechanism that triggers the formation of dislocation core in the initial plasticity of（Fe Cr Ni）_100-xC_x alloy.The tensile yield strength of FeCrNi alloy is only 395MPa,but the elongation is as high as 60%.The deformation mechanism of Fe Cr Ni alloy is dominated by single slip dislocation slip in the low strain region and by dislocation cross slip+twin deformation in the high strain region.Due to the refined grains and dispersed carbonized grains of（Fe Cr Ni）₉₆C₄ and（Fe Cr Ni）₉₂C₈ alloys,the twin deformation is restrained,the free path of dislocation slip is shortened,and dislocation is more likely to accumulate in grain boundary and carbide,which leads to the increase of strain hardening rate and the increase of alloy yield strength to 490 MPa and 590 MPa respectively.（3）The effect of carbide on the friction and wear properties of alloy and its mechanism were explored.It is found that the specific wear rate of（Fe Cr Ni）_100-xC_x decreases with the increase of carbon content.The wear resistance of Fe Cr Ni alloy is poor because of its low hardness and lack of hard particles.The high-volume fraction of carbides in the matrix of（Fe Cr Ni）₉₆C₄ alloys effectively reduce the penetration and scraping by asperity on the surface,thus significantly reducing the degree of abrasive wear of the alloy.The carbide particles in（Fe Cr Ni）₉₂C₈ alloy with a particle size of 2μm or more are subjected to a higher holding force by the matrix and can resist the scraping and impact of the grinding ball for a long time,which makes the alloy have the best wear resistance.（4）Based on the calculation of empirical parameters of high entropy alloy,BCC-based(Fe₇₀Cr_7.5Ni_7.5Co_7.5V_7.5)_100-xC_x alloy with high Fe content was designed and prepared.The effects of carbon content on the phase composition,microstructure,mechanical properties and wear resistance of the alloy were studied,and the influence mechanism of carbides on the mechanical properties and wear resistance of the alloy was analyzed.Carbon can inhibit the formation of（Cr,V）-richσ-phase,promotes the formation of MC and M₂₃C₆ carbides with high hardness,and triggers the carbon-oxygen reaction that activates sintering,resulting in improved mechanical property.The bending strength of Fe_64.4Co_6.9Cr_6.9Ni_6.9V_6.9C₈alloy is up to 3245 MPa,which is nearly twice that of Fe₇₀Cr_7.5Ni_7.5Co_7.5V_7.5 alloy without C addition.Due to the high hardness of the matrix and the large number of diffusely distributed carbides,Fe_64.4Co_6.9Cr_6.9Ni_6.9V_6.9C₈ alloy exhibits excellent wear performance with a specific wear rate of only 1.3×10^-6 mm³/（N·m）.