The Research Of Microstructure And Mechanical Properties Of Refractory High Entropy Alloys

In recent years,it has been found that the high entropy of mixing can stabilize solid-solution-like phases with relatively simple crystal structures rather than forming the conventionally expected complex intermetallic phases.Thus,while these alloys may be compositionally complex,they are microstructurally simple.The refractory high-entropy alloy with refractory elements is mainly bcc crystal structure,and the bcc structure refractory high-entropy alloy exhibits brittleness at room temperature,which limits its further application to some extent.Through the composition design,phase transformation,and regulation of the microstructure of the alloy,the room temperature properties and high temperature strength of the refractory high-entropy alloy can be improved,so that the refractory high-entropy alloy has good room temperature and high temperature performance.The HfNbTiZr refractory high-entropy alloy was prepared by vacuum arc melting method.The microstructure of the alloy was changed by cold rolling and annealing process.The improvement of mechanical properties of HfNbTiZr refractory high-entropy alloy by precipitation was investigated.The results show that HfNbTiZr refractory high-entropy alloy not only forms solid solution phase,but also forms another Hf rich nano-hcp phase.Cold rolling and annealing at 1073 ~ 1273 K promoted the bcc phase to hcp phase transition,and the hcp volume fraction increased with the annealing temperature.Hcp nanoparticles can simultaneously increase the strength and plasticity of refractory high-entropy alloys.The HfNbTiZr refractory high-entropy alloy was cold-rolled by 77.8% and 95%,and then annealed at 1073 K and 1273 K for 1 h to study the influence of different rolling ratios on HfNbTiZr high-entropy alloy.The results show that the two-phase bcc+hcp structure is formed before and after cold rolling deformation,and no phase change occurs.Compared with the as-cast,the microhardness and tensile strength of the cold-rolled alloy were improved by 46.7% and 93.7% respectively,but the plasticity decreased.The tensile strength and fracture strain of 77.8% cold rolling and annealing at 1073 K,95% cold rolling and annealing at 1073 K refractory high-entropy alloy decreased with the increase of the deformation.77.8% cold rolling and annealing at 1273 K,95% cold rolling and annealing at 1273 K refractory high-entropy alloy also showed the same change.HfNbTaTiZr refractory high-entropy alloy was prepared by levitation melting and high temperature forging.The phase composition,microstructure,tensile properties at room temperature and high temperature(673 K,873 K,1073 K,1273 K)of as-cast and forged alloys were investigated.The results show that the HfNbTaTiZr refractory high-entropy alloy is a stable bcc structure.Compared with as-cast,forged alloys have an increase of 6% in vickers hardness and 8% in tensile strength.The fracture mechanism indicates that the ductile-brittle transition temperature of the alloy is within the range of 673 K ~ 1073 K in the high-temperature tensile process.