Preparation And Properties Of NiCo(CrTiV,FeAlCu) High-Entropy Alloys And Coatings

Because high-entropy alloys and high-entropy alloy coatings have high hardness,high strength,good wear resistance and corrosion resistance,they have been becoming one of research hotspots in the field of materials science.In this work,bulk NiCrCoTiV high-entropy alloy was designed firstly,the NiCrCoTiV and FeNiCoAlCu high-entropy alloy coatings were prepared by laser cladding.The performance of high-temperature tribology,high temperature stability and corrosion resistance of high-entropy alloy coatings were studied in detail.At the same time,the interface characteristics,including interfaces between coating and substrate,and between different phases,were investigated by technologies of TEM,STEM,etc.Then,the technology of in-situ TEM tensile testing was innovatively applied to quantificationally characterize the fracture behavior of Ni-Cr-Co-Ti-V-Al high-entropy alloy coating.The NiCrCoTiV high-entropy alloy has a relatively complex phase composition,but its phase number is still less than Gibbs equilibrium phase number.After heat-treatment at500?,600?and 700?,its phase composition is almost invariant,however,the grain size of the second phase is refined to a certain extent and with the increase of heat-treatment temperature,the degree of refinement increases,thus the corrosion resistance is further improved and after heat-treatment at 700?,the NiCrCoTiV high-entropy alloy shows the best corrosion resistance in the solution of 1 mol/L H₂SO₄.Ni-Cr-Co-Ti-V-Al high-entropy alloy coating has improved the oxidation resistance and wear resistance of TC4 titanium alloy greatly.High-temperature heat-treatment?at 900?for8h?doesn't change the phase composition and microstructure of high-entropy alloy coating.The NiO oxidation film on high-entropy alloy coating and the alloying elements?Al,Cr?are the main reasons for good high-temperature oxidation performance of high-entropy alloy coating.The crystal structure and elementary composition of the interface beween high-entropy alloy coating and TC4 titanium alloy matrix are quite different and the interface blocks the diffusion and transfer of elements between substrate and coating;The interfaces of?Co,Ni?Ti₂ compound/?-Ti phase and?Co,Ni?Ti₂ compound/BCC phase are incoherent or semi-coherent boundarys without obvious orientation relationship.By laser remelting,the phase composition of Ni-Cr-Co-Ti-V high-entropy alloy coating is not changed,but the microstructure is changed obviously.Owing to laser remelting,the volume fraction of Ti-rich phase in laser-remelted Ni-Cr-Co-Ti-V high-entropy alloy coating and the degree of solid solution increases clearly,and the grains are also refined,which leads to the increase of micro-hardness.In turn,its wear-resisting performance has been improved.Fe-rich NiCrCoTiV_x high-entropy alloy coatings have 60at.%Fe element,much higher than the original design value?5%³5%?,which makes these coatings high efficiency and applicability.With the increase of V content,the phase composition of Fe-rich NiCrCoTiV_x high-entropy alloy coatings doesn't change but the grain size of Fe₂Ti-type Laves phase decreases gradually.The refined grain size Fe₂Ti-type Laves phase and V content are immediate reason for the different wear resistance,corrosion resistance and thermal stability of high-entropy alloy coatings.FeNiCoAlCu high-entropy alloy coating has a good thermal stability and high-temperature wear resistance.The formation of oxidation film at high temperature and the phase transformation from BCC to FCC at 780?can directly affect the high-temperature tribological property of high-entropy alloy coating.The addition of graphite can inhibit the formation of BCC solid solution but promote the formation of FCC solid solution.The graphite/FeNiCoAlCu high-entropy alloy coating with 8wt.%graphite addition has the best wear resistance at room temperature and 600?.Excessive graphite addition will lead to the appearance of needle-flake structure,which reduces the wear resistance of high entropy alloy coating.Dislocation motion and crack behavior including initiation and instability expansion are observed clearly during the in-situ TEM tensile process of laser-cladded Ni-Cr-Co-Ti-V-Al high-entropy alloy coating.The motion and pile-up of dislocations can induce the formation of the crack source,and the axial tension induces the formation of amorphous phase;the disappearance and regeneration of dislocations may be the main reason for the phase transformation of high-entropy BCC solid solution.The quantitative analysis results of tensile failure show that the measured value of the modulus?E?,the fracture strength??_f?and the total elongation of the fracture??_f?are¹08GPa?2.70GPa and 2.6%,respectively.