The main properties of various titanium alloys are based on Ti-6Al-4V. On the right side, with the increase of β stable element, the processing capacity, strain rate sensitivity, heat treatment strengthening effect and room temperature strength of the alloy continue to increase; On the left side, the β transition temperature, flow stress, weldability and high temperature strength of the alloy increase with the decrease of β stable elements.
The traditional titanium alloy microstructure is described by the size and arrangement of two basic phases, α phase and β phase (α titanium based α solid solution and β titanium based β solid solution). The properties of titanium alloys mainly depend on the arrangement of α and β phases, the volume fraction and their respective properties. Compared with the body-centered cubic β, the close-packed hexagonal α phase has higher packing density and anisotropic lattice configuration, so the α phase has poor plasticity, low diffusion rate and high creep resistance. Therefore, the physical, mechanical and technological properties of different types of titanium alloys are different.
α-titanium alloys are generally single-phase alloys with moderate strength; The α+β biphase and metastable β titanium alloys can be strengthened to higher and very high strength levels, respectively. Metastable β titanium alloys obtain high strength at the cost of low ductility. Without aging, metastable β alloys have relatively good ductility similar to α and α+β.
Because the fracture toughness of titanium alloy is closely related to the microstructure and aging conditions, there is no clear relationship between the composition of titanium alloy and the fracture toughness, but the fracture toughness of coarse lamellar structure is higher than that of fine isaemic structure. This is because the layered structure can deflect the propagation crack along the different orientation of the lath bundle, resulting in the passivation of the crack front, thus absorbing additional crack propagation energy.
Close-packed hexagonal crystals have relatively low atomic diffusion and deformation capacity, which results in excellent creep resistance of the α phase. Titanium has a high affinity with oxygen atoms, which means that a very thin dense oxide layer (TiO2) can also be formed on the surface of titanium alloy in the atmosphere at room temperature, which is also the reason for the excellent corrosion resistance of titanium alloy. The deformation capacity of α-titanium alloys is extremely limited, and the work hardening capacity is very strong, which means that α and α+β alloys can only be processed at high temperatures.
