| Beta titanium alloys exhibit good deformability and high specific strength. A good match of strength, ductility and toughness of beta titanium alloys can be obtained by solution and aging heat treatment. However, with the development of the aerospace industry, it demands higher standards for the specific strength and specific stiffiness of beta titanium alloys. In the present study, a self-designed Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy as the research material. Effects of heat treatment and hybrid effect of trace B4C and C addition on microstructure and mechanical properties of the alloy were studied systemly. The superplasticity of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy was also evaluted and the microstructure evolution during suoperplastic deformeation was ananlyszed. In addition, high temperature compressive deformation behavior was studied, and the alloy was rolled on the basis of the results of high temperature compression experiments.The heat treatment is very important to the mechanical properties of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy. The alloy has excellent ductility after solution treated at (α+β) field and β field, but lower strength. However, the strength of the alloy was significantly improved by low temperature aging, and the strength increased with decreasing of aing temperature. With the heat treatment condition of775℃/1h/AC+440℃/8h, the alloy achieved to a very high ultimate strength1715MPa, and has some ductility. With the heat treatment condition of800℃/1h/AC+520℃/8h, the alloy obtained a good match of strength and ductility, the yield strength and the elongation is1466MPa and14.5%respectively.The mechanical properties of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy are extremely sensitive to the microstructure, especialy to α phase. The primary α which produced at (α+β) field solution will suppress the growth of β grains, and the more primary α phase will lead to smaller β grain size and result in better strength and ductility of the alloy. Whereas the alloy was solution treated at β field, the β grains dramaticly grown up and it brought adverse effect on the properties of the alloy. The fine secondary α precipitated during the low temperature aging process is the main reason for the high strength improvement of the alloy. The size decreased or the volume fraction increased of secondary α phase will improve the strength of the alloy. Compared to the strength, the ductility changed in opposite way, the higher strength will lead to lower ductility.Effects of trace B4Cand C addition on microstructure and mechanical properties of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy were studied. About1vol.%of TiB and TiC produced by in situ reaction synthesis with trace B4C and C addition. TiB and TiC refined β grain size obviously. The growth of β grain can be restricted by the pinning effect of TiB and TiC when solution treated at β field, and the grian size in the alloy with B4C and C addition just noly half of the B4C and C free alloy. The number of primary α phase of the alloy increased when solution treated at (α+β) field due to the addition of B4C and C and lead to smaller β grain size. In addition, the size of secondary α phase was also refined due to the addition of B and C. The ultimate strength of the B4C and C addition alloy achieved to1834MPa with the elongation of2%. Within the same heat treatment condition, the strength of the alloy can be improved with B4C and C addition, but the ductility decreased.Superplastic deformation behavior and microstructure evolution of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy in the temperature range of650℃to800℃and in the strain rate of2.78×10-4s-1to8.33×10-3s-1were investigated. It is found that optimal superplastic elongation of455%is obtained when the alloy was tested at800℃with strain rate of2.78×10-4s-1. Superplastic deformation mechanism is mainly dislocation motion. In the superplastic deformation process, a large number dislocation produced, dislocation tangles and cutting the mother phase formed subgrains and subgrain boundaries. The subgrain grow up and the subgrain boundary rotation with continuous deformation and finally formed a new grain with a certain grain angle. The dynamic recrystallization was also found during superplastic deformation, but it is not the main deformation mechanism due to the small amount.The hot compression behavior of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy was studied by the thermal physical simulation method. The relationship between deformation temperature and strain rate was revealed. The peak stress and the flow stress decreased with increasing temperature and decreasing strain rate, and the variations of peak stress σ with (1000/T) and lnε all meet the linear relationship. The average hot deformation activation energy is271.1kJ/mol deformed at (α+β) field. The constitutive equation of the alloy hot deformed in (α+β) phase field is established. These can guide the selection of thermal deformation parameters and equipment tonnage.The Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy was rolled respectively at790℃((α+β)field) and870℃(β field) based on the results of hot compression. Tthe alloy rolling at790℃with reduction of90%and at870℃with reduction of90%shown a rolling texture that the <110>β direction parallel to the direction of rolling. The as-rolled alloy has medium strength and excellent ductility. The ultimate strength achieved to1744MPa when solution treated at800℃and aged at440℃of the alloy rolling at790℃with90%reduction, and an excellent match of strength and ductility obtained when solution treated at800℃plus aged at520℃, where the ultimated strength is1486MPa and the elongation is15.6%. |
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