Microstucture Control And Performance Optimization Of Ti55531 Titanium Alloy

Ti-5Al-5Mo-5V-3Cr-1Zr(Ti55531) is a new type β titanium alloy. The alloy has high strength and toughness which is suitable for making large load-bearing components. So it has a very broad perspective in aerospace. The microstructure of the alloy is very sensitive to heat treatments due to its high degree of alloying. Thus, the systematic study on the effecs of heat treatment on the microstructure of Ti55531 alloy is benefit for optimization of the microstructure and improvement of the mechanical properties of the alloy. In this thesis, the Ti55531 alloy was heat treated at different conditions, and the effects of the heat treatments on the microstructures of alloy was investigated. More over, the mechanical properties of the alloy with different microstructures were evaluated, aiming at building the relationship between the microstructure and mechanical properties. The main conclusions are as follows:(1) The Ti55531 alloy was annealed in β single-phase region and(α+β) double phase regions, respectively. The effects of the annealed temperature on its microstructures and mechanical properties were investigated. The results show that, the microstructure mainly consists of primary equiaxed α phase and secondary acicular α phase and β phase when the alloy was annealed in the double phase region. While, the alloy only consists of widmanstatten structure when it annealed in the single-phase region. The alloy, which was annealed at 830°C for 1.5 h, and then cooled to 580°C in the furnace and holded for 8 h. exhibits an excellent comprehensive mechanical properties, which tensile strength and elongation reach 1107 MPa and 21.1%, respectively.(2) The Ti55531 alloy was solid solution treated under different temperatures and then aged at 580°C. The effects of the treatment on microstructures and mechanical properties of Ti55531 alloy were studied. The results show that the microstructure have acicular secondary phase precipitation after the alloy is solid solution and aging treated. When it was solution treat at to 780°C for 1 h, and then aged at 580°C for 8 h, the alloy exhibits an excellent comprehensive mechanical properties, which tensile strength and elongation reach 1304 MPa and 13.8%, respectively.(3) The relationship between microstructure and mechanical properties of Ti55531 alloy were quantitatively or semi quantitatively studied. The results show that the strength of the alloy depends on the individual strength and volume fraction of each constitution phase, which meets the mixture law of composite materials. The plasticity of the alloy depend on the volume fraction of the primary α phase and secondary α phase. The ductility of alloy reduces with the increase of secondary α phase content. Nevertheless, the plasticity of the alloy is raised with the increase of primary α phase. The strength of the alloy meets the mixture law of composite materials.