Microstructure And Properties Of NbMoTaWVX(X=Cr,Ti)refractory High Entropy Alloys Prepared By Powder Metallurgy

High entropy alloys,with novel design concept,develops a new metal materials research field.HEAs possess special properties,such as high strength and hardness,outstanding high temperature properties,good wear and corrosion resistance and so on,which make HEAs have been studied extensively.Since HEAs are barely 12 years old,its theory and application are very limited.In this study,senary NbMoTaWVX(X = Cr,Ti)refractory high entropy alloys were fabricated by mechanical alloying and spark plasma sintering.The main research contents are as follows:The alloying sequence and phase formation mechanism of NbMoTaWVX(X = Cr,Ti)refractory high entropy alloy powders during mechanical alloying were studied.Alloy elements with lower melting point dissolved more quickly.After ball milling for 40 h,NbMoTaWVX(X = Cr,Ti)obtained single-phase BCC structure supersaturated solid solution with lattice parameter of 3.161 ? and 3.183 ?,respectively.The average particle size of NbMoTaWVCr powders were less than 5 μm,what’s more,the average grain size was 9.7 nm and the lattice distortion was 0.418%.The average particle size of NbMoTaWVTi alloy powders were less than 10 μm,the average grain size and lattice distortion were 12.7 nm and 0.903%,respectively.The microstructure and mechanical properties of bulk NbMoTaWVCr refractoty HEA were studied.The microstructure of alloy sintered at 1400 °C consisted of disorder BCC matrix phase,V-Cr-Nb-Ta rich C15 laves phase and tetragonal structure(Ta,V)O2 oxide precipitate phase.The average grain size of BCC matrix phase was 1.24 μm.The average grain size of C15 laves phase was 0.56 μm,and its volume percentage was 8.75%;The average grain size of(Ta,V)O2 oxide precipitate phase was 0.34 μm,and its volume fraction was 5.16%.The compressive yield strength,fracture strength,fracture strain and hardness of NbMoTaWVCr were 3416 MPa,3834 MPa,10.1% and 1011 HV,respectively,which exhibited excellent mechanical properties at room temperature.The strength mechanism mainly included solid solution strengthening,grain boundary strengthening and precipitation strengthening.The compressive fracture morphologies indicated that intergranular fracture and shear fracture were two major fracture mechanisms.The microstructure and mechanical properties of bulk NbMoTaWVTi refractoty HEA were studied.The microstructure of NbMoTaWVTi alloy sintered at 1400 °C consisted of disorder BCC matrix phase and FCC TiO oxide precipitate phase.The average grain size of BCC matrix phase and oxide precipitate phase were 1.35 μm and 0.46 μm,respectively,and the volume fraction of oxide precipitate phase was 11.44%.The compressive yield strength,fracture strength,fracture strain and hardness of NbMoTaWVTi were 2709 MPa,3115 MPa,11.4% and 786 HV,respectively.The influence of ball milling time and sintering temperature on the alloy were studied.With the increase of ball milling time,the grain size of NbMoTaWVCr alloy powders refined as well as the lattice distortion increased.What’s more,the volume fraction of precipitate phase added,while the fracture strength and fracture strain reduced gradually.As the sintering temperature increased from 1200 °C to 1600 °C,the average grain size of bulk NbMoTaWVCr alloys increased,and the density,volume fraction of precipitate phase,fracture strength,fracture strain and hardness showed a decline after raising first.The density,volume fraction of precipitate phase,fracture strength,hardness reached the maximum at 1300 °C,which were 11.23 g/cm3,26.54%,4422 MPa and 1073 HV,respectively.However,the fracture strain obtained maximum 10.1% at 1400 °C.As the sintering temperature raised,the volume fraction of precipitate phase in bulk NbMoTaWVTi alloys first increased and then reduced,which reached the maximum 13.55% at 1200 °C,while the fracture strength and hardness decreased gradually.