Microstructure And Mechanical Properties Of Ti₂Al_0.5-xSi_xNbV_0.5Cr Refractory High-Entropy Alloys

Refractory high entropy alloy is a branch of high entropy alloys,and its constituent elements are mainly high-melting-point elements.Because of its higher melting point and higher temperature strength than traditional nickel-based superalloys,it has attracted widespread attention from scholar at home and abroad.In 2011,professor Senkov of the U.S.Air Force Laboratory firstly produced NbMoTaW and VNbMoTaW refractory high-entropy alloys,and found that the compressive strength of these alloys can reach more than 1000 MPa at high temperatures.This research provides scientists with new ideas for designing superalloys.The target of application of refractory high-entropy alloy is as aerospace material,but it is still a long way from practical application.High density,high brittleness of refractory high-entropy alloy at room temperature and poor resistance to high temperature oxidation are waiting to be solved.In this subject,six elements of Ti,Al,Nb,V,Cr,and Si were selected,and as-cast Ti2Al0.5-xSixNbV0.5Cr(x=0,0.1,0.3,0.5)refractory high-entropy alloys were prepared by using the vacuum arc melting furnace.X-ray diffraction(XRD),field emission scanning electron microscope(SEM)were used to observe the microstructure and phase structure of the as-cast and annealed(800?,1000?,1200? for 24 h)alloys.The Vickers hardness tester,electronic universal testing machine and Gleeble 3500 thermal simulation testing machine were used to measure the mechanical properties of alloys.The antioxidant experiment was performed in the air atmosphere at 800? and 1000?.The annealing experiment was performed on the alloys using the Muffle furnace to study the microstructure and mechanical properties of the alloys after annealing.The studies found that the phase structure of as-cast Ti2Al0.5-xSixNbV0.5Cr(x=0,0.1,0.3,0.5)refractory high-entropy alloys changed from single BCC phase to BCC+Silicide+Cr2Nb three phases.With the proportion of Si element increasing,the content of silicide phase gradually increase and the content of Cr2Nb phase increases first and then decreases.Among the four alloys,Ti2Si0.5NbV0.5C alloy has the largest hardness value of 586.9 HV,and Ti2Al0.2Si0.3NbV0.5Cr alloy has the largest room temperature compressive fracture strength of 1970.7 MPa.Ti2Al0.4Si0.1NbV0.5Cr alloy has the best comprehensive performance,with has the yield strength of 1303.1 MPa,the fracture strength of 1665.0 MPa and the fracture strain of 22.4%.All four alloys have high-temperature strength,Ti2Al0.2Si0.3NbV0.5Cr alloy has the largest high temperature strength(419.7 MPa)at 1000?,Ti2Si0.5NbV0.5Cr alloy has the lowest high temperature strength(295.5 MPa).The strength of Ti2Al0.2Si0.3NbV0.5Cr alloy gradually decreases with temperature increasing and the compressive strength of alloy were 878.8 MPa at 800? and 566.1 MPa at 900?,respectively.The four alloys have good resistance to high temperature oxidation,after oxidation at 800? for 100 h,the Ti2Al0.2Si0.3NbV0.5Cr alloy has the best resistance to high temperature oxidation with the average oxidation rate of 0.019 mg/cm2·h,Ti2Si0.5NbV0.5Cr alloy has poor resistance to high temperature oxidation with the average oxidation rate of 0.043 mg/cm2·h.After oxidation at 800? for 100 h,Ti2Al0.2Si0.3NbV0.5Cr alloy has the best resistance to high temperature oxidation with the average oxidation rate of 0.175 mg/cm2·h,the Ti2Si0.5NbV0.5Cr alloy has poor resistance to high temperature oxidation at 1000?.After performing oxidation experiment at 75 h,the oxidized sample was directly crushed with light pressure.Ti2Al0.5-xSixNbV0.5Cr(x=0,0.1,0.3,0.5)alloys were annealed at different temperatures,the microstructure uniformity and room temperature compression performance were significantly improved.With the increase of annealing temperature,the compressive strength of Ti2Al0.5NbV0.5Cr alloy increases first and then decreases,the fracture strength reaches the maximum value(1769.6 MPa)after annealing at 800? and the plasticity decreases;The compressive strengths of Ti2Al0.4Si0.1NbV0.5Cr,Ti2Al0.2Si0.3NbV0.5Cr and Ti2Si0.5NbV0.5Cr alloy gradually increase with the annealing temperature increasing,and the fracture strength of Ti2Al0.2Si0.3NbV0.5Cr alloy reaches 2271.7 MPa afer annealing at 1200?,and the fracture strain value of alloy increases.