Microstructure And Properties Of TiCrNbMoTa(Al) Refractory High-entropy Alloys Prepared By Mechanical Alloying And Hot Pressing

High entropy alloy is a new type of alloy formed by the doping of multiple primary elements,which has four major characteristics,namely high entropy effect,lattice distortion effect,slow diffusion effect and"cocktail"effect.Compared with conventional alloys,the multi-component refractory high-entropy alloys with high melting point（generally greater than 1600℃）and lower density elements to meet the needs of aero-engine hot-end components generally have good mechanical properties over a wide temperature range and excellent oxidation resistance,but the significantly different melting points between the multi-component elements pose a great challenge to the conventional melt-casting preparation of the alloy.In order to overcome the tissue defects easily produced by the melt-casting process,superfine crystalline（to be supplemented with supporting materials later）refractory high-entropy alloys with relatively uniform tissue composition are obtained.In this paper,TiCrNbMoTa（Al）alloy specimens were prepared by mechanical alloying+vacuum hot pressing sintering,and the effects of different ball milling times（20 h,40 h,60 h,80 h）and different compositions of low-density doping elements Al（x=0,0.2,0.4）on the equimolar ratio TiCrNbMoTamulticomponent refractory high entropy alloy were investigated by XRD,SEM,EDS,etc.The effect of organization and properties.The results of the study showed that the ball milling of the unadulterated Al mixed element TiCrNbMoTapowder with equal molar ratio up to 30 h can basically achieve alloying,and the structure of the ball milled powder is BCC supersaturated solid solution.As the ball milling time was increased from 20 h to 80 h,the lattice constant of the BCC phase formed by the ball milled powder increased from 3.10（?）to 3.216（?）and then decreased to 3.213（?）.Four powders of equal molar ratios with different ball milling times were sintered by vacuum hot pressing at 1300°C and 45MPa pressure for 60 min to obtain an ultra-fine grain intermetallic compound particle-reinforced TiCrNbMoTarefractory high-entropy alloy body with the BCC phase as the matrix,the Laves phase NbCr₂and the intermetallic compound CrTi₄precipitated phase,and the composition of the matrix phase was close to the nominal composition.With an increase of the ball milling time from 20 h to 80 h,the volume fraction of BCC solid solution phase in the specimens increased from 70.52%to 78.64%,and the volume fraction was decreasing to 62.17%when the ball milling time exceeded80 h.The content of Laves phase NbCr₂ decreased from 23.26%to 13.62%;while the content of intermetallic compound CrTi₄increased from 5.21%to The grain size of Laves phase NbCr₂ increased from 238.32 nm to 416.67 nm in the specimens,and the grain size of the 80 h powder preparation specimens decreased significantly to 250.48nm,while the intermetallic compound CrTi₄ gradually decreased from 368.68 nm to333.34 nm;the densities of the alloys increased from 94.6%to The dense density of the alloy increased from 94.6%to 98.4%,which was close to full dense.The Vickers hardness of the hot-pressed specimens increased significantly when the ball milling time was extended from 20 h to 60 h.The Vickers hardness of the hot-pressed specimens increased further with the increase of the ball milling time,and the Vickers hardness of the hot-pressed specimens reached 798.1 HV at 60 h.After the ball milling time exceeded 80 h,the Vickers hardness decreased to 778.3 HV.The fracture toughness of the alloy decreased from 6.23 MPa·m^1/2 at 60 h to 6.16 MPa·m^1/2 at 80h.The compressive strength of the alloy increased from 2483.145 MPa at 20 h to3335.5 MPa at 80h.The alloy had the best overall mechanical properties with compressive strength,Vickers hardness,and fracture toughness of 2917.48 MPa,798.1 HV,and 6.23 MPa·m^1/2,respectively,after 60 h of ball milling.The average grain sizes of TiCrNbMoTaAl_x（x=0,0.2,0.4）mixed powders were13.4 nm,15.2 nm,and 14.5 nm after ball milling for 60 h.The lattice constants were in the range of 3.22～3.29（?）.After hot pressing and sintering,all the powders formed a white BCC solid solution phase as the matrix and a gray Laves phase NbCr₂（Cr-rich phase）and the black intermetallic compound CrTi₄（Ti-rich phase）as the precipitating phase,TiCrNbMoTaAl_x ultrafine crystal refractory high entropy alloy was formed.With the increase of Al content from x=0 to x=0.4,the BCC solid solution phase of the specimen increased from 78.46%to 85.38%,and the Laves phase NbCr₂ and intermetallic compound CrTi₄ content decreased from 13.62%and7.92%to 9.17%and 5.45%,respectively.With the increase of Al content,the alloy densities increased from 97.5%for Al₀ to 98.2%for Al_0.4,which was close to full densities;the specimen Vickers hardness and fracture toughness gradually increased from 798.10 HV and 6.23 MPa·m^1/2 for Al0 alloy to 828.44 HV and 6.98 MPa·m^1/2for Al_0.4 alloy.The best organization and comprehensive mechanical properties are obtained at x=0.4,when the volume fractions of BCC solid solution phase,Laves phase NbCr₂,and intermetallic compound CrTi₄ are 85.38%,9.17%,and 5.45%,respectively;the compressive strength and hardness and fracture toughness are3030.34 MPa,828.44 HV,and 6.98 MPa·m^1/2.