Study On Fabrication,microstructures And Properties Of Ultrafine WC-Al_xCoCrCuFeNi Cemented Carbides

Due to high hardness,good fracture toughness and wear resistance,WC-Co cemented carbides have been widely used in the industry.However,Co is toxic,magnetic and a strategic resource,and the high-temperature oxidation resistance and corrosion resistance of cemented carbide with Co binder phase are poor.The development of binder phases with good performances as substitutes of Co has always been a research hotspot in the field of cemented carbide,and high-entropy alloy(HEA)provides a new idea for solving the above problems with its excellent properties.Therefore,in the present work,Al_xCoCrCuFeNi high-entropy alloys were used as binder phases to fabricate high-performance ultrafine WC-HEA cemented carbide by spark plasma sintering(SPS)without adding extra grain growth inhibitors.The SPS sintering process of ultrafine WC-HEA cemented carbide was optimized.The influences of Al_xCoCrCuFeNi high-entropy alloy binder phases on the microstructures and mechanical properties of WC-HEA cemented carbides were studied.The high-temperature oxidation resistance and corrosion resistance properties of the WC-HEA cemented carbides were analyzed,which aims to provide technical reference for the development of HEA binder phases with excellent performance and expand the application of WC-HEA cemented carbides.Firstly,Al_xCoCrCuFeNi high-entropy alloys were prepared by mechanical alloying.The results show that with the increase of Al content,the major phase of the Al_xCoCrCuFeNi high-entropy alloys transfers from FCC phase to BCC phase,and it becomes a single BCC phase solid solution.Due to the solid solution strengthening of Al atoms,and higher strength and hardness of BCC phase,the hardness and the compressive strength of SPSed Al_xCoCrCuFeNi high-entropy alloys enhance with an increase of Al content,while the fracture strain decreases gradually.The Al_xCoCrCuFeNi high-entropy alloys exhibit good thermostability.Annealing treatment in a range of 500?800? has little effect on the microstructures of Al_xCoCrCuFeNi high-entropy alloys,moreover,a reasonable annealing process can improve the hardness and the compressive strength of HEAs containing Al,and the fracture strain remains almost unchanged after annealing.The wetting behaviour of Al_xCoCrCuFeNi/WC system was measured by using a modified sessile drop method.The HEA/WC system exhibits good wettability with the final equilibrium contact angles less than 5°,and the higher the Al content,the worse wettability of HEA.In the initially rapid spreading stage,the wetting of HEA/WC is driven by the adsorption of Cr atoms at the solid-liquid interface,and the contact angle decreases rapidly.In the subsequently steady spreading stage,when x?0.5,the dissolution rate of WC in molten HEA is faster than the adsorption rate of Cr,and the dissolution of WC largely determines the wetting of HEA/WC;When x?1.0,interfacial reaction is dominant in competition with the dissolution of WC,and a continuous reaction layer forms at the interface.Thus,the wetting of HEA/WC mainly attributes to interfacial reaction.The SPS sintering process of ultrafine WC-HEA cemented carbide without adding extra grain growth inhibitor was optimized by taking Al CoCrCuFeNi high-entropy alloy binder phase as an example.When sintering at 1250? for 5 min under a pressure of 30 MPa,WC-10 wt.%Al CoCrCuFeNi cemented carbide shows excellent mechanical properties.The relative density reaches 98.3%,and the hardness and fracture toughness are 1922±15.3 HV₃₀and 10.41±0.12 MPa?m^1/2,respectively.The results,which are based on the study of WC-10wt.%Al_xCoCrCuFeNi cemented carbides,further indicate that the performances of WC-HEA are related to the wettability,mechanical properties,and sintering characteristics of the Al_xCoCrCuFeNi high-entropy alloy binders.The density of WC-HEA alloys,which depends on the wettability and sluggish diffusion effect of HEA binder,is the main reason for the hardness difference among the WC-10 wt.%Al_xCoCrCuFeNi cemented carbides,while the wettability of HEA/WC is the decisive factor for the fracture toughness.The Al_xCoCrCuFeNi high-entropy alloy binder phases significantly inhibit the growth of WC grains during sintering,and the average WC grain size of WC-10 wt.%HEA cemented carbides are less than 43%of that for WC-10 wt.%Co under the same conditions.It should be attributed to the sluggish diffusion effect of HEA binders and the complexion phase forming at WC/HEA interface.The increase of Al content in Al_xCoCrCuFeNi weakens the sluggish diffusion effect,therefore,the resistance of W/C atom diffusion in HEA binder phase to achieve the growth of WC grains by dissolution-precipitation reduces,and the average WC grain size of WC-10 wt.%Al_xCoCrCuFeNi cemented carbide increases.Both high-temperature oxidation resistance and corrosion resistance in 3.5wt.%Na Cl solution of the WC-HEA cemented carbides are better than those of WC-Co.The oxidation of WC-HEA is controlled by reaction at the interface,and the selective oxidation of HEA binder exists.The HEA binder phase with excellent anti-oxidation suppresses the inward diffusion of oxygen to the oxide layer/matrix interface,and further oxidation of the matrix is prevented,which improves the oxidation resistance of WC-HEA cemented carbides.The high-temperature oxidation resistance of WC-HEA can be improved by increasing the content of HEA binder phase or Al content in binder phase.During the corrosion process of WC-HEA cemented carbides,galvanic corrosion happens between WC hard phase and HEA binder phase,and a reduction reaction occurs at the WC cathode,while the HEA binder dissolves as the anode.Furthermore,Cu-rich phase with poor corrosion resistance in the HEA is corroded first.The passive film on the surface of HEA binder hinders dissolution of the anode and enhances corrosion resistance of WC-HEA cemented carbides.Additionally,there is no strong positive correlation between the corrosion resistance of WC-10 wt.%Al_xCoCrCuFeNi cemented carbides in 3.5wt.%Na Cl solution and the Al content in HEA binder phase.