| Traditional alloy design concepts,which mainly consist of one or two elements and add other specific elements to improve properties,have been unable to meet the urgent need for higher performance materials in various fields.Facing the great challenge of high-performance materials in the field of modern material science,the design concept of high-entropy alloys was put forward in 2004.Its unique alloy design concept has the clustering effect of various alloy elements.With its excellent properties such as high strength,high density,high hardness,good plasticity,wear resistance and corrosion resistance,it attracts wide attention in the fields of aerospace,ships,nuclear energy,military industry and electronics.In particular,the refractory high-entropy alloys（RHEAs）with body-centered cubic（BCC）structure consisting of refractory metal elements exhibits good mechanical properties in extreme environments.However,mechanical properties and ignition behavior of high-entropy alloys under dynamic loading in low temperature and wide strain rate range are scarce.It is necessary to carry out systematic quasi-static and dynamic mechanical properties tests to reveal the correlation between low-temperature dynamic mechanical properties and impact ignition behavior of high-entropy alloys.In this subject,TiZrHf based high-entropy alloys was taken as research object,TiZrHf Cu0.5,TiZrHf Ni0.5 and TiZrHf Al0.5alloy ingots were prepared by vacuum arc melting,and the dynamic properties and impact ignition behavior of TiZrHf based high-entropy alloys at different temperatures（-40～20℃）were systematically studied.The original specimens were characterized by SEM,XRD and EDS.Quasi-static/dynamic compression experiments of high-entropy alloys were carried out by universal testing machine and SHPB experimental system,respectively.The influence laws of temperature,trace element type and strain rate on dynamic compression mechanical properties of high entropy alloys were analyzed.The samples after dynamic compression were characterized by scanning electron microscope（SEM）,and the deformation mechanism of high-entropy alloys at different temperatures was analyzed.Based on the impact ignition experimental platform built by ourselves,the influence of trace element changes and temperature on ignition behavior was analyzed.The impact ignition reaction mechanism of TiZrHf high-entropy alloys was analyzed by combining theoretical analysis with numerical simulation.The main work and research achievements of this paper are as follows:（1）The basis for selecting principal and trace elements of high-entropic alloy was put forward.The phase structure of TiZrHf based high entropy alloys was determined by mixing enthalpy,mixing entropic and atomic compatibility.The phase structure and microstructure morphology of the specimen were tested.The ultimate strength of TiZrHf Cu0.5,TiZrHf Ni0.5 and TiZrHf Al0.5 are almost the same at room temperature of 20℃.The elastic modulus and ultimate strength of TiZrHf Cu0.5and TiZrHf Ni0.5 specimens were significantly increased with decreasing temperature,while the toughness of TiZrHf Al0.5 at low temperature was increased.The deformation mechanism of high-entropic alloy at low temperature shows dual fracture modes of toughness and brittleness.（2）Based on the quasi-static and dynamic compression experimental data,the parameters（strain hardening term coefficient,strain rate hardening coefficient and temperature term coefficient）of the Johnson-Cook constitutive model of TiZrHf based high entropy alloys at different temperatures（-40～20℃）and strain rates(300～2600s-1)were fitted;By comparing the predicted results of the fitting equation with the experimental results,it was found that the original Johnson-Cook constitutive model can accurately reflect the mechanical behavior of the alloy at low strain rates.With the decrease of temperature and the increase of strain rate,the coupling effects of strain,strain rate and temperature on the constitutive model need to be considered.（3）On the basis of the original Johnson-Cook constitutive model,considering the coupling effect of strain,strain rate and temperature,and through ABAQUS simulation software,the finite element model of dynamic compression was established,and the numerical simulation and experimental results of different strain rates and temperatures were compared.The predicted results obtained by fitting the modified constitutive model in the strain rate range of2400s～2600s-1 and the temperature range of-40～0℃matched the experimental results well,which can accurately reflect the dynamic mechanical properties of TiZrHf at high strain rates and low temperatures.（4）The ignition behavior of TiZrHf high entropy alloy at low temperature was studied by using a self built ignition behavior experimental system.The results showed that the ignition threshold of TiZrHf Ni0.5 decreases with the decrease of temperature,the reaction duration was longer and the reaction was more intense,and the ignition behavior shows an obvious strain rate dependence;At the same temperature（-40℃）and strain rate,the ignition incubation time of TiZrHf Ni0.5,TiZrHf Cu0.5 and TiZrHf Al0.5 is 100μs、125μs and 175μs.The ignition duration and intensity from high to low were TiZrHf Cu0.5,TiZrHf Ni0.5 and TiZrHf Al0.5,respectively.The mechanical properties of materials directly affected their impact ignition behavior.When the total strain energy accumulation exceeded the surface of crack nucleation,cracks can appear.The energy accumulation at the crack tip promoted the generation of hot spots and then induces ignition.The ductile brittle transition of the specimen under low temperature conditions made the crack tip more prone to generate hot spots and induce strong impact ignition reaction. |
