Microstructure And Properties Of Ti₂ZrHf_0.5VNb_0.5X（X=Al,Cr,N） Refractory High Entropy Alloys

The demand of all countries in the aerospace and military industry is getting higher and higher,which puts forward more stringent requirements for high-temperature materials.The alloys are not only required to have excellent comprehensive mechanical properties at room temperature（RT）and high temperature（HT）,but also to pursue its lightweight,high strengthening,excellent oxidation resistance and thermal stability and other properties.Due to the density and melting point of the elements used in traditional superalloys,the composition design has reached the bottleneck,and various processes are needed to improve the properties of the alloys.Thereinto,refractory high-entropy alloys（RHEAs）were designed using high-melting-point metals,which combined the advantages of the high entropy alloy and the refractory alloy,and possessed high strength,oxidation resistance and thermal stability at HT.Among them,the characteristics of low density and high strength of RHEAs containing Ti also made them have great potential in light and high strength superalloys.Therefore,the Ti₂Zr Hf_0.5VNb_0.5-X（X=Al,Cr,N）RHEAs were designed and prepared.The microstructure,mechanical properties at room temperature（RT）and high temperature（HT）,oxidation resistance and thermal stability of the alloys were systematically studied,and the effects of different microstructure and strengthening methods on the properties of the alloys were also discussed.（1）Based on the empirical parameter method of HEAs,and the values of and of d-orbit energy levels,quinary Ti₂Zr Hf_0.5VNb_x RHEAs were designed and prepared.All the alloys displayed a simple BCC structure.The microstructures of the alloys changed from the initial single-phase columnar structure（x=0）to dendrite microstructure（x>0）.The yield strength of the alloys decreased slightly at RT,and increased at 873 K and 1073 K.Among them,Ti₂Zr Hf_0.5VNb_0.5（Nb0.5）alloy has excellent comprehensive properties.Nb0.5 alloy exhibited specific yield strength（SYS）of 145.6 k Pa·m³·kg^-1 with ductility over 50%at RT,and the yield strength at 873 K can reach 716 MPa.（2）Al element was added in Ti₂Zr Hf_0.5VNb_0.5 alloy to improve the properties.With the increase of Al content,the Ti₂Zr Hf_0.5VNb_0.5Al_x series alloys resultd in ordering without obvious long range element diffusion.The alloys were transformed from single-phase BCC structure to single-phase B2 phase structure of Ti₂Zr Hf_0.5VNb_0.5Al₁（Al1）alloy.The transition from short-range order to long-range order resulted in a large increase in the strength of the alloy at RT,873 K and 1073 K with tiny loss of ductility.Among them,Al1 alloy exhibited excellent lightweight and high strength properties,and the SYS at RT and 873 K is up to 232.4k Pa·m³·kg^-1,and 179.2 k Pa·m³·kg^-1,respectively.In addition,the composite oxide layer composed of Al₂O₃ and（Zr,Hf）V₂O₇ after oxidation improves the oxidation resistance of the alloy to a certain extent,and the mass gain per unit surface area decreases to 112.06 mg/cm²after 50 h oxidation.3)Cr element was added to Nb0.5 alloy to obtain eutectic refractory high entropy alloy with good mechanical properties and thermal stability.The substitution of Cr for V leaded to the microstructure of the Ti₂Zr Hf_0.5V_1-xNb_0.5Cr_xalloys transform from single BCC phase dendrite structure to BCC+Cr₂（Zr,Hf）-type Laves dual phases eutectic structure.Thereinto,Ti₂Zr Hf_0.5V_1-xNb_0.5Cr_1.25（Cr1.25）alloy exhibited full eutectic structure,which was a rarely reported eutectic RHEA.The yield strength of the alloys increased from 970 MPa to 1320 MPa at RT due to the appearance of dual-phase eutectic structure.However,the yield strength of the alloys decreased from 716 MPa to 314 MPa at 873 K due to the decrease of melting point of eutectic alloys,and the mechanical properties at HT were not satisfactory.However,the eutectic structure improved the thermal stability of the alloy at 873 K and 1073 K,and the composite oxides of Cr₂O₃ and Cr（V/Nb）O₄ formed by oxidation of the alloy with Cr addition had better protective properties.After oxidation for 50 h,the mass gain per unit surface area decreased to59.27 mg/cm²,which was superior to Al1 alloy.（4）The interstitial element N was added to the Nb0.5 alloy in order to improve the strength and the thermal stability of the alloy under the joint action of interstitial strengthening and second phase strengthening.The results showed that most of the interstitial solution of N element was in the matrix BCC phase after the addition of N element,and only a small amount of nitride was formed in the Ti_1.8Zr Hf_0.5VNb_0.5（Ti N）_0.2（N20 alloy）.This obvious interstitial reinforcement and hindrance of dislocation movement result in the maximum increase of the yield strength at RT and HT.Among them,Ti_1.9Zr Hf_0.5VNb_0.5（Ti N）_0.1（N10 alloy）with a density of 6.62 g/cm³ showed the most excellent comprehensive properties,and the values of the specific yield strength（RT）,ductility（RT）,the yield strength at 873 K are 228.0 k Pa·m³·kg^-1,43.7%and 916 MPa,respectively.N20 alloy with a density of 6.64 g/cm³ exhibited the highest yield strength and specific yield strength at RT,which are 2104 MPa and 316.9k Pa·m³·kg^-1,respectively.In addition,its yield strength at 873 K is as high as 1157 MPa,which was higher than that of Al1 alloy.In terms of the thermal stability of the alloy,the series of alloy precipitated nitride after a long time annealing,the thermal stability was better than the ordered alloys with Al addition,and weaker than the refractory eutectic high entropy alloys with Cr addition.