Study On In-Situ Synthesis Of High Entropy Alloy Coating On Copper Surface

Although high-entropy alloys(HEAs) have now become a new kind of materials, the name has not been clearly de?ned since its inception. Normally, HEAs are de?ned in terms of the ?ve containing elements in equal or near equal atomic percent(at.%). The atomic percentage of each principal element is between 5at.% and 35 at.%, whilst that of each minor element is below 5at.%. HEAs have much large con?gurational entropy than conventional alloys, compared at the liquid or random solid solution states. HEAs usually intend to form FCC and/or BCC solid solutions rather than intermetallic compounds or other complex ordered phases. With suitable alloy design, it has have been found that HEAs are of promising properties, such as high hardness, excellent resistance to temper softening, exceptional high-temperature strength, outstanding wear resistance, good structural stability, good corrosion and oxidation resistance. As one of the severe plastic deformation(SPD) approaches, surface mechanical attrition treatment(SMAT), have been used for in-situ synthesis of nanometer surface layer with contamination-free, porosity-free, and no obvious interface with the substrate, by flying balls impacting the surface of material repeatedly. After pure Cu subjected to SMAT, the characterize of surface structure was of rheological microstructure, which should shorten the diffusion distance between atoms. Otherwise there are more defects in the nanocrystalline layer, providing numerous channel for atom diffusion. It has been shown that the balls and the container walls always are coated with the processed powder during mechanical alloying(MA).Therefore, MA and SMAT approaches can be used to in-situ preparation of alloy coating on bulk metal surface.In this work, high entropy alloy coatings were in-situ synthesized on the copper surface using SMAT and MA method. The characterize of the coating microstructures, phases, and compositions were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDS). Effect of process parameters on the formation of coatings and the formation mechanism of the coatings were also studied. The main conclusions are as follows:A series of equiatomic binary to quinary alloys(NiCo, NiCoFe, NiCo FeCr, Ni CoFeCu and NiCoFeCrCu) were prepared by MA. The results show that a simple FCC solid solution is formed in all the studied systems. The more alloy components, the longer the time it takes to form a solid solution.It is revealed that increasing of milling time can enhance the thickness and densification of the coating. With the process parameters: addition of NiCo FeCrCu HEAs powders to the vessel, the ball-to-powder weight ratio of 10:1, rotation speed of 500 rpm, the most dense and uniform coating, with an average thickness of 40μm, were synthesized after MA for 5h, metallurgically bonding with the copper substrate. The formation mechanism of the coating can be simpli?ed as follows:(1)Particles hammered into substrate.(2)Powder deposited on substrate through cold welding.(3)Element diffused between the coating and the substrate.(4)Work hardening leads to spalling fatigue.Equiatomic NiCo alloy original mixed powders were added during SMAT on the copper matrix surface. The results show that a relatively dense and uniform NiCo composite gradient coating with an average thickness of 100μm was closely bonded on the pure copper substrate after SMAT for 4h. The early sandwich lamellar structure of the composite layer eventually evolved into the uniform alloying composite layer.NiCoFeCuCr HEAs powders were added to pure copper plate during SMAT. After SMAT for 30 min, then removing the powders and SMAT for 30 min, the powders embed into the substrate and form a non-uniform coating which have obvious difference of thickness, but combined compactly with copper matrix. In order to acquire more dense and uniform HEAs coating, the second crycle was conducted, the average thickness of the coating up to 50μm. However the surface work hardening resulted in the cracks in the coating.