Microstructure And Properties Of NbMoTiVSi_x Refractory High-entropy Alloys

Refractory high-entropy alloys（RHEAs）has high melting point and high entropy alloy characteristics,exhibits excellent high-temperature performance,and is expected to become a new generation of high-temperature structural materials due to the excellent mechanical properties at high temperatures.In this paper,NbMoTiVSix（x=0,0.25,0.5）high-entropy alloy was prepared by vacuum arc melting.The microstructure,mechanical properties and oxidation performance were studied and analyzed.（1）There is a single BCC phase with coarse grains in the NbMoTiV RHEA.But in the NbMoTiVSix（x=0.25,0.5）RHEAs are BCC+M5Si3（M=Nb,Mo,Ti,V）dual-phase structures.The BCC solid solution phase is enriched with Mo,Nb and V,the M5Si3 phase is enriched with Si and Ti.With the increase of Si content,the M5Si3 phase changes from lath to chrysanthemum and is uniformly distributed in the BCC around the solid solution phase.（2）With the increase of Si content,the hardness of NbMoTiVSix（x=0,0.25,0.5）HEAs increased from 391.1 HV to 660.7 HV.The compressive strength increased from 1747 MPa to 2038 MPa,while the compressive fracture strain decreased from 30%to 16%Which is attributed to the formation of the M5Si3 phase as a hard phase.The fracture morphology observation found that the fracture mechanism of the NbMoTiVSix RHEAs changed from ductile-brittle mixed fracture to quasi-cleavage-brittle fracture（3）The NbMoTiVSix（x=0,0.25,0.5）RHEAs has excellent mechanical properties.With the increase of Si element,the yield strength at 1000℃ is increased from 590 MPa to 1109 MPa.The combination of the two-phase structure enables the RHEAs to possess excellent performance at high temperature,with a large number of dislocations in the BCC with irregular distribution,dislocation tangle,dislocation pinning and nanosilicide.Which make the NbMoTiVSix RHEAs show excellent strength at high temperature.（4）The oxidation kinetics curves of the NbMoTiVSix RHEAs are roughly parabolic.With the increase of Si content,the oxidation rate constant of the alloy decreases from 52.6346 mg2/cm4·h to 28.8323 mg2/cm4·h,The main oxidation products of NbMoTiV RHEA are Ti3O5,Nb2O5,NbO2,and the oxidation products of NbMoTiVSix（x=0.25,0.5）RHEAs are mainly SiO2,Nb2O5,NbO2.After adding Si element,which is distributed between dendrites in the matrix,and the oxygen potential of Si element is the smallest,the generated SiO2 is densely distributed among dendrites in a network shape,which can effectively hinder the inward diffusion of oxygen element.Meanwhile,the higher the content of Si,the more channels that hinder the inward diffusion of oxygen,and the better the oxidation resistance of the NbMoTiVSix（x=0.25,0.5）RHEAs.