Preparation And Microstructure And Properties Of CoCrFeNiAl_xTi_y High-entropy Alloy

High entropy alloy is a new alloy system developed in recent years.It breaks through the traditional concept of alloys based on a single element.It consists of multiple elements and the atomic concentration of each element is 5%to 35%.And through the change of preparation method,the optimization of alloy composition and heat treatment,the excellent properties of high strength,high hardness,high temperature resistance and wear resistance can be obtained,so it has broad application prospects.The predecessors have done more research on its phase formation and transformation characteristics,and its microstructure and properties at room temperature,but less research on its microstructure and properties in high temperature environments,Such as friction and wear under high temperature conditions,mechanical properties,etc.In this thesis,the microstructure of CoCrFeNiAl_x high entropy alloy with different Al content is studied,and the hardness and friction and wear properties of the alloy at high temperature are studied.In addition,the effect of Ti addition on microstructure and properties of CoCrFeNiAl_xTi_y high entropy alloy was also studied.In this thesis,Hvax-3 non-consumable vacuum arc melting furnace was used to prepare high entropy alloy,and the microstructure of the alloys was analyzed by metallographic microscope,Smart Lab X-ray diffractometer,and Phenom Prox surface morphology analyzer.The hardness was measured with HXD-1000TMC micro-Vickers hardness tester.The WDW-200E 200 KN electronic universal testing machine was used to test its compression characteristics,and the UMT-5 surface fatigue testing machine was used to test the change of wear resistance at different temperatures.Research indicates:1.For CoCrFeNiAl_x（x=0,0.25,0.5,0.75,1）high entropy alloy,with the increase of aluminum content,the microstructure of the alloy gradually changed from dendrite to equiaxed crystal,and the phase composition of the alloy consists of a single phase solid solution（FCC）.structural phase)gradually transformed into a multiphase solid solution（a small amount of FCC structural phase and other BCC structural phases）.2.CoCrFeNiAl_x（x=0,0.25,0.5,0.75,1）high-entropy alloys have excellent mechanical properties.As the Al content increases,the hardness tends to increase.In addition,with the increase of Al content,the yield strength of the alloy showed a trend of high-low-high.The yield strength increased from 156 MPa to 1250 MPa,then decreased to 875 MPa and finally increased to 1300 MPa.The breaking strength showed a low-high trend,the breaking strength decreased from 705MPa to 465MPa,and finally increased to3261MPa.3.At 400℃and 600℃,the friction coefficients of Al₁ HEA are 0.52 and 0.49,respectively,and the wear rates are 3.7×10^-4mm³/（N·m）and 7.9×10^-5mm³/（N·m）.The wear performance is better than that of Al_0.5 high-entropy alloy,the friction coefficient is0.82 and 0.9,and the wear rate is 9.0×10^-4mm³/（N·m）and 6.0×10^-4mm³/（N·m）.It is also better than Al_0.75 HEA,its friction coefficient is 0.55 and 0.52,respectively,and the wear rate is 3.8×10^-4mm³/（N·m）and 2.4×10^-4mm³/（N·m）.4.The addition of titanium refines the structure of CoCrFeNiAl_x（x=0,0.25,0.5,0.75,1）high-entropy alloys,and the addition of titanium forms a new structural phase—BCC2 structure,and this structural phase is similar to Different B2 structure phases appear in CoCrFeNiAl_x high-entropy alloys.In addition,the addition of titanium element improves the hardness and yield strength of the alloy.5.The friction coefficient and wear rate of Al_0.75Ti_0.25 HEA at 600℃are 0.40 and8.9×10^-5mm³/（N·m）,respectively,and its wear resistance is better than that of Al_0.75HEA.The friction coefficient and wear rate are 0.52 and 2.4×10^-4mm³/（N·m）,respectively.