| Because of the characteristics like low density, high strength and well corrosion resistance, titanium and titanium alloy has a wide range of applications in aviation,aerospace, biotechnology, military and other fields. Ti-55531 alloy is a new metastableβ titanium alloy developed based on BT22 by VSMPO and Airbus conpany. The nominal composition of Ti-55531 alloy is Ti-5Al-5V-5Mo-3Cr-1Zr. This alloy has a series of excellent characteristics like low sensitive to segregation, high strength, high toughness, wide processing window and good hardenability. Ti-55531 alloy is suitable for structural parts and become one of the world`s most advanced high-toughness titanium alloy.This paper selects Ti-55531 alloy as the research object, systematic study the influence of microstructure to mechanical properties of high strength-toughness Ti-55531 alloy. And take the hot compression tests in order to study the high temperature deformation behavior of the alloy. Derive the constitutive relation and draw the processing map by use the true stress strain curve obtained in the test. Provide a reference to optimizing the parameters of alloy hot deformation and analysis the relationship between microstructure and properties of the alloy under high temperature.The obtained results are as follows:For the duplex structure Ti-55531 alloy,with the decrease of content and size of primary equiaxed α phase, more secondary lamellar α phase during aging and the secondary lamellar α phase become longer and finer. It leads to the improve of strengthand decrease of ductility. For the lamlellar structure Ti-55531 alloy, with the roughen of secondary lamellar α phase, both stength and ductility decreased.For the whole βTi-55531 alloy, the content of equiaxed α and lamellar α phase are almost zero, and alloy has low strength and well ductility. The fractre mode of duplex and lamellar structure Ti-55531 alloy is transgranular cleavage, microvoid accumulation and intergranular cracking. With the decreasing of primary α phase, the fracture shows more features of transgranular cleavage and intergranulat cracking, less features of microvoid accumulation. The fracture of fracture toughness tested duplex structure Ti-55531 alloy shows the fracture mode of fiberous zone is cleavage fracture. The fracture mode of shear lip zone is the mixed mode fracture mixed by microvoid accumulation,transgranular cleavage, intergranular cracking and quasi-cleavage.The major farcture mode is microvoid accumulation. Because the toughness of the alloy is well, no instant-off area observed.The flow curves of Ti-55531 alloy are steady state flow curve. When the strain is low, the stress increases with increasing strain, and when it reaches a peak stress, the stress decreases with increasing strain,finally stabilized to a certain valus. Derive the constitutive equations of Ti-55531 alloy according to the Arrhenius equation, and draw the processing map. Analysis the best hot deformation parameters of alloy. After compression in unstable region, the adiabatic shear band can be observed in both equaxied and duplex structure Ti-55531 alloy,the adiabatic shear band in equaxied alloy is wider than duplex alloy. The dynamic recrystallization phenomenon occurred during the compression in the stable region both in equaxied and duplex structure Ti-55531 alloy, with the higher temperature and the lower strain rate, the crystallization degree of alloy are more complete. Compare to the duplex structure, the degree of recrystallization of equaxied structure are higher. The deformation mechanism of Ti-55531 alloy in high temperature is dislocation migration. Equaxied has a better hot workability. |
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