Preparation,Microstructure And Mechanical Properties Of Powder Metallurgy TiVNbTa (Al) Refractory High Entropy Alloys

As a new type of alloy,high entropy alloys（HEAs）attract more and more researchers’attention.The refractory high entropy alloy has a series of excellent properties such as high strength,high hardness,good wear resistance,excellent high temperature mechanical properties and physical properties.In the present study,TiVNbTa and TiVNbTaAl_0.25refractory high entropy alloys were fabricated by mechanical alloying（MA）and spark plasma sintering（SPS）,and their microstructures and mechanical properties were studied.The mechanical alloying process of two refractory high-entropy alloys TiVNbTa and TiVNbTa Al_0.25 were studied.After ball milling for 40 h,both TiVNbTa and TiVNbTaAl_0.25obtained single-phase BCC structure solid solutions.Among them,the particle size of TiVNbTa alloy powder is mostly in the range of 5-15μm,the lattice constant is 3.24?,the average grain size is 16.8 nm,the lattice distortion is 0.77%;the particle size of TiVNbTa Al_0.25 alloy powder is about 5μm,the lattice constant,average grain size,and lattice distortion are 3.23?,21.5 nm,and 0.76%,respectively.The microstructures and mechanical properties of TiVNbTa and TiVNbTaAl_0.25refractory high entropy alloys were studied.The two kinds of bulk alloys sintered at 1100°C by spark plasma sintering are composed of the matrix of BCC structure and the precipitated phase of FCC structure.The matrix elements are relatively uniform and their sizes are all below 1μm.The precipitated phase is dispersed at the grain boundaries and inside the grains,which was enriched with Ti and C,N,and O elements and is characterised to be a complex of TiN+TiC+TiO.The average size of the grain boundary precipitates is less than 300 nm and that of the intragranular precipitates is less than 25 nm.Both the TiVNbTa and TiVNbTaAl_0.25HEAs have excellent room temperature mechanical properties:the compressive yield strength(ζ_0.2),plastic strain（ε_p）,fracture strength(ζ_bc)and hardness of the TiVNbTa alloy are 1506.33MPa,33.17%,2967.88 MPa and 460.9 HV,respectively;The compressive yield strength(ζ_0.2),plastic strain（ε_p）,fracture strength(ζ_bc)and hardness of TiVNbTaAl_0.25 were increased to1755.62 MPa,36.19%,3321.32 MPa and 498.4 HV,respectively.The sintered samples showed a typical plastic fracture morphology which consisted of a large number of dimples.The sintering temperature had a great influence on the microstructures and mechanical properties of the TiVNbTa and TiVNbTaAl_0.25 refractory high entropy alloys.The grain size and precipitated phase size of the two alloys grow up with the increase of sintering temperature,and the volume fraction of precipitated phase gradually decreases.The obvious growth of matrix grains occured at the sintering temperature of 1300°C.The plasticity of the two alloys increases first and then decreases with the increase of the sintering temperature,while the strength decreases first and then increases.Both the HEAs sintered at 1000°C and1100°C exhibited the best comprehensive mechanical properties.The addition of Al increases the strength,hardness and ductility of the TiVNbTa high entropy alloy and decreases the density of the alloy.With the increasing content of O and N,the plasticity of TiVNbTa high entropy alloys deteriorates,while the strength and hardness increase slightly.Compared with the corresponding alloy fabricated by vacuum arc melting,the compressive yield strengths of TiVNbTa and TiVNbTa Al_0.25 refractory high entropy alloys prepared by MA-SPS are improved by more than 400 MPa,and the plasticity of the alloy is slightly reduced,two high entropy alloys prepared by MA+SPS have better comprehensive mechanical properties.