Microstructure And Mechanical Properties Of Ti₂₀Zr₂₀Nb₂₀V_xCr_40-x Refractory High-Entropy Alloys

In 1995,Professor Ye proposed the concept of high entropy alloy（HEA）,since then scholars at home and abroad began to study this kind of alloy.In 2011,Professor Senkov of the United States Air Force Laboratory produced NbMoTaW and VNbMoTaW refractory HEAs for the first time,and found that the compressive strength of these two alloys at high temperatures can reach more than 1000 MPa.This study provides scientists with a new idea of designing super alloys.Because the service temperature of traditional super alloys are difficult to exceed 1300℃,it is important to develop alloys with higher melting point,and the refractory HEAs not only have high melting point but also have excellent mechanical performance at high temperatures,which has been widely concerned by scholars.The target area of refractory HEAs is aerospace,therefor lightweight,high compressive strength and oxidation resistance have become the main concerns in the design of refractory HEAs.In this paper,five high melting point elements:Ti,Zr,Nb,V and Cr are selected,and seven groups of refractory HEAs of Ti₂₀Zr₂₀Nb₂₀V_xCr_40-x（X=5～35）are prepared by vacuum arc furnace.The microstructure and phases of as-cast and annealed alloys are observed by X-ray Diffractometer（XRD）,and scanning electron microscope（SEM）,the mechanical properties of the alloy are measured by Vickers hardness and electronic universal testing machine.The effect of annealing on microstructure and mechanical properties of alloys are also studied.The oxidation kinetics curve of alloys under 1000℃ are obtained by weighing method,and its antioxidant properties are researched.The conclusions are following.The as-cast alloy are all composed of BCC and Laves.the inter-dendrite region is rich with Nb,Ti are riched and the dendrite region is rich with Cr.What is more,the alloy tissue becomes thinner with the increase of Cr.The hardness value of all alloys are more than 390HV,Ti₂₀Zr₂₀Nb₂₀V₅Cr₃₅ has the highest hardness which is 547.41HV.V and Cr content increase can effectively enhance the alloy hardness,Cr plays more positive effect on hardness.The compressive strength of Ti₂₀Zr₂₀Nb₂₀V_xCr_40-x（X=5～35）alloys is more than 1000 MPA,and the increase of Cr makes the compressive strength increase first and then decrease.The compressive strength of Ti₂₀Zr₂₀Nb₂₀V₂₅Cr₁₅ alloy in this system can be up to 1753.5 MPa.When the alloy cracks,it has a burst sound,and its compression fracture has river pattern,cleavage steps and bright small particles.The small particles are the second phase,and they have been pulled out when the fracture broken.The alloy shows obvious brittle fracture characteristics.In Ti₂₅Zr₂₅Nb₂₅V₂₅,Ti₂₀Zr₂₀Nb₂₀V₅Cr₃₅,Ti₂₀Zr₂₀Nb₂₀V₂₀Cr₂₀,Ti₂₀Zr₂₀Nb₂₀V₃₅Cr₅ alloys,the oxidation rate of Ti₂₀Zr₂₀Nb₂₀V₃₅Cr₅ HEA with the most V is the highest,the oxidation rate constant is 3.76×10^-5mg/（cm⁴·s）,and the antioxidant performance is the worst,whereas the oxidation rate of Ti₂₀Zr₂₀Nb₂₀V₅Cr₃₅ alloy with the least V is the lowest,the oxidation rate constant is 2.68×10^-6mg/（cm⁴·s）,and the antioxidant properties of the alloy is the best.V plays a negative role in the antioxidant properties of the alloy,but Cr plays a positive role in the antioxidant properties of the alloy.The phases of annealed Ti₂₅Zr₂₅Nb₂₅V₂₅,Ti₂₀Zr₂₀Nb₂₀V₅Cr₃₅,Ti₂₀Zr₂₀Nb₂₀V₂₀Cr₂₀,Ti₂₀Zr₂₀Nb₂₀V₃₅Cr₅ alloys do not been changed and are still composed of BCC and Laves phases.However,the Ti₂₅Zr₂₅Nb₂₅V₂₅ alloy transforms from BCC into BCC+Laves.After annealing,the hardness of each alloy increases,of which the hardness of Ti₂₀Zr₂₀Nb₂₀V₅Cr₃₅ alloy reaches 636.36 HV.At the same time,the compressive strength and plasticity of the alloys reduced slightly.The compressive strength of Ti₂₀Zr₂₀Nb₂₀V₂₀Cr₂₀ alloy is the highest,its value is 1593.5MPa.The alloy is still brittle fracture.