Study On Preparation And Technology Of Plasma Solid-state Surface Metallurgy FeCoCrNiWMo Gradient High Entropy Alloy Layer

The high-entropy alloys（HEAs）have broken through the traditional alloy design concept,obtained the superior performance than the traditional alloy,and has displayed broad application prospect.The bulk HEAs has not been produced and applied in batch because of the limitation of machining size and cost.HEA coating has good surface modification effect and low production cost,so it is an important channel for the practical application of high-entropy alloy.The plasma solid-state surface metallurgy technology is a new method to prepare HEA coating,which can provide high-melting-point metal elements on the metal surface without pollution,and realize the problem of metal surface alloying.Therefore,based on the experience of HEAs and plasma solid-state surface metallurgy,a new type of HEA layer was prepared by plasma solid surface metallurgy in this paper.In this paper,a new FeCoCrNiWMo HEA layer with metallurgical bonding and gradient structure has been obtained by Co-Cr-Ni-W-Mo five-element alloying on the surface of pure iron substrate by plasma solid-state surface metallurgy,the effects of different processing parameters（heat preservation temperature,heat preservation time,working air pressure and working voltage）on the surface morphology,cross-sectional structure,phase structure,wear resistance and high temperature oxidation resistance of the FeCoCrNiWMo HEA layer were studied by means of scanning electron microscope,X-ray diffraction,micro-hardness tester and friction and wear tester.The sputtering behavior of atoms,the interaction between particles,the diffusion and deposition of atoms during the preparation of FeCoCrNiWMo HEA were analyzed,and the formation mechanism of FeCoCrNiWMo HEA was studied.At the same time,the TiCoCrNiWMo high entropy alloy layer was prepared on the surface of TC4 alloy substrate and compared with FeCoCrNiWMo HEA layer.The main conclusions of this study are as follows:（1）FeCoCrNiWMo HEA layers with metallurgical bonding and gradient structure were successfully prepared under different process parameters,and the phase of FeCoCrNiWMo HEA layers is the same,both HCP phase and FCC phase.The high vacancy density gradient layer cannot be formed continuously in the isopotential mode,and the serious lattice distortion and the increase of atomic stacking density caused by different atomic sizes decrease the vacancy formation concentration and increase the diffusion resistance.Therefore,the FeCoCrNiWMo HEA layer obtained on the surface of pure iron substrate is a composite strengthening layer structure of deposition layer+diffusion layer,instead of traditional alloying layer（diffusion layer）.（2）The effect of different process parameters on the FeCoCrNiWMo HEA layer is mainly in the deposition layer,and has little effect on the diffusion layer.The distribution of elements in the deposition layer is uniform and there is no component segregation.The element content in the diffusion layer is distributed in gradient,and it is metallurgical bonding with substrate.The strong anti-sputtering effect results in the high sputtering rate elements are easy to be sputtered out,the smaller the atomic radius of the elements easy to diffuse into the substrate.Therefore,the Co,Cr and Ni elements with high sputtering rate and small radius tend to diffuse into the diffusion layer,while W and Mo elements with low sputtering rate and large radius tend to enrich in the deposit layer.The quality and composition of FeCoCrNiWMo HEA layer are affected by different process parameters,such as supply capacity of source electrode,absorption capacity of workpiece,diffusion rate of elements and growth rate of granular structure.（3）The micro-hardness,friction and wear properties and high temperature oxidation resistance of the substrate can be improved to different degrees by using the FeCoCrNiWMo HEA layer under different processing parameters.When the process parameters is as follows:heat preservation temperature 1300°C,heat preservation time 2h,working voltage 900V,working air pressure 30Pa,the FeCoCrNiWMo HEA layer is composed of 5.5μm deposit layer+8μm diffusion layer,the microstructure is uniform and compact,and there are no defects such as pores,cracks and impurities,the microhardness,wear resistance and high temperature oxidation resistance are 2.3,3.66and 5.5 times of the substrate respectively.（4）The surface of 1000°C TiCoCrNiWMo HEA layer is uniform and compact,and there is without composition segregation.When the heat preservation temperature is higher than 1000°C,the surface grain structure coarsens,which results in the segregation of the surface components and the property of the surface decreases significantly.The phase of TiCoCrNiWMo HEA layer is the same,and the main alloy phases are Al₁₁V,Al₉Cr₄,（Ni,Ti）,Al Mo Ti₂ and Co_1.3Ni_4.3Mo_4.6.The difference between the TiCoCrNiWMo HEA layer and FeCoCrNiWMo HEA layer is mainly reflected in the diffusion layer.Compared with Fe element,the sputtering rate of Ti element is very low,which is unfavorable to the formation of solid solution with source elements.Therefore,compared with Ti-based alloy,Fe-based alloy has a better effect on the composition gradient distribution of the new gradient HEA layer prepared by Fe-based alloy as substrate.