| New strategic missiles and military aircraft,as the killer weapons of modern warfare,are important guarantees for national defense and security,and their research and development work is increasingly valued by countries around the world.The performance of missile propulsion systems and aircraft engines,as the core components of missiles and fighter jets,becomes a crucial part of their development process.Due to the stable and excellent mechanical properties of nickel based high-temperature alloy GH4720 Li in complex environments such as high temperature and pressure,it is widely used in the manufacturing of key components for power devices in weapons and equipment such as fighter jets or next-generation strategic missiles.Therefore,studying the dynamic mechanical properties of GH4720 Li alloy under working conditions and even wide temperature and high strain rate conditions is of great practical significance.This dissertation investigates the mechanical properties of GH4720 Li alloy under high strain rate and wide temperature range conditions,and analyzes the variation law between the third strain aging property of GH4720 Li alloy and temperature and strain rate based on the relevant theory of material strain aging property;Based on the phenomenological theory of constitutive equations,a constitutive equation was established that can accurately describe the mechanical performance change curve of GH4720 Li alloy under high strain rate and wide temperature range conditions.The main research work and conclusions of the dissertation are as follows:(1)This dissertation conducted isothermal uniaxial compression tests on GH4720 Li alloy with a temperature range of 20-1000℃ and a strain rate range of 0.01-5000/s using Gleeble-3800 thermal simulator,separated Hopkinson pressure bar and other testing equipment.Based on the Stress–strain curve obtained from the test,the influence of temperature and strain rate on the rheological properties of GH4720 Li alloy was analyzed.It was found that the stress value of GH4720 Li alloy did not follow the alloy softening effect within a certain range of temperature and strain rate,but increased with the increase of temperature,and an abnormal mechanical phenomenon occurred.In addition,it was also found that the flow stress temperature relationship curve of GH4720 Li alloy showed a bell shaped change,and a stress peak appeared at a certain temperature range.The peak stress will move towards the hightemperature zone with the increase of strain rate,and the temperature width of the bell curve will also change with the increase of strain rate.(2)The microstructure changes of GH4720 Li alloy specimens were observed through electron microscopy,energy spectrum testing,XRD and other tests.The results showed that the content of Fe element in the precipitate of the test piece under high strain rate conditions was significantly higher than that under low strain rate conditions,while the content of Cr(Mo)element was the opposite.Fe element can increase the mechanical strength of materials,while Cr(Mo)element can reduce the mechanical properties of alloys.This is because a large amount of Cr(Mo)rich precipitates make the matrix brittle,and cracks are prone to appear in the Cr(Mo)precipitates.In addition,due to the flat and intact fracture surface of Cr(Mo)precipitates,they absorb less energy during the fracture process,which further reduces the mechanical properties of GH4720 Li alloy.The above factors are the main reasons for the mechanical anomaly of GH4720 Li alloy stress.(3)The variation curve between flow stress and temperature of GH4720 Li alloy is a bell shaped curve,and the highest point of the bell shaped curve is the peak stress;When the strain rate increases,the peak stress will shift towards the high-temperature zone,and the temperature width of the bell curve will decrease.The rheological properties described above become the criterion for determining the third strain aging behavior of GH4720 Li alloy.Through observation of the microstructure of GH4720 Li alloy,it was found that the main reason for the third type of strain aging behavior is the pinning effect of solute atoms and precipitated relative moving dislocations.(4)Based on the trend of stress-strain changes in GH4720 Li alloy,the Johnson Cook constitutive equation parameters of GH4720 Li alloy material were modified;Based on experimental phenomena and data,segmented constitutive equations were established to mainly describe the elastic-plastic properties of materials,and two phenomenological constitutive equations were established to consider the coupling effects of various experimental parameters(temperature,strain rate,strain).The fitting accuracy of the constitutive equation for the stressstrain curve of GH4720 Li alloy was verified by comparing the theoretical stress values calculated from the constitutive equation with the experimental stress values.The results indicate that all four constitutive equations mentioned above can effectively predict the trend of stress changes in GH4720 Li alloy.The research results of this dissertation can provide useful reference for the application of GH4720 Li nickel based alloy in missiles and fighter jets. |
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