Effect of BASCA heat treatment on microstructure of TB18 titanium alloy

Titanium alloy has the advantages of high specific strength, good corrosion resistance, etc., and has been widely used in aerospace, automotive and shipbuilding, etc. TB18 titanium alloy is a new type of sub-stable β titanium alloy designed and developed by the United Nations universities and scientific research institutes for the development of China's aerospace structural components in recent years. Compared with BT22, Ti-1023 and other titanium alloys, the alloy has better hardenability and more excellent hot workability, with its production of large-size bars after heat treatment strength of more than 1300MPa still has excellent toughness (KIC ≥ 60 MPa-m1/2), especially suitable for use as an aircraft landing gear, crossbeam and other important bearing parts of the aviation industry.
With the development of aerospace vehicles towards high speed, not only the strength level of titanium alloys is required to be increased, but also the toughness of titanium alloys has put forward higher requirements, that is, while improving the strength of titanium alloys, the toughness should be maintained at a high level. Compared with α + β heat treatment, β heat treatment will hinder dislocation movement due to the existence of grain boundaries, so that the alloy is strengthened. At the same time, due to the existence of a large number of interlaced lamellar α clusters in the intracrystalline lamellar organisation, cracks through these α clusters will change the orientation, resulting in crack paths zigzagging, branching more, the total length increases, the occurrence of rupture requires more energy, and thus the alloy has a higher fracture toughness. Therefore, β heat treatment can make titanium alloy have excellent fracture toughness while obtaining higher strength, which is an effective way to solve the contradiction between strength and toughness of titanium alloy. β solid solution slow cooling aging (BASCA) heat treatment process is a kind of β heat treatment process for high-strength titanium alloys thin-walled aerospace structural parts to reduce the damage susceptibility and make them obtain high strength and high toughness.
To this end, the effect of BASCA heat treatment on the microstructure and mechanical properties of TB18 titanium alloy was investigated, with a view to providing a basis for the development of a suitable strengthening heat treatment process.
(1) TB18 titanium alloy bar in the β-phase region of solid solution under slow cooling conditions, α-phase in the β grain boundaries and intracrystalline precipitation, intracrystalline α-phase is a point or short needle, grain boundary α-phase is basically thin film embedded in the β grain boundaries. Cooling rate has a greater impact on the α phase at the grain boundary, when the cooling rate of 1 ℃ / min, the α phase at the grain boundary with a lens-like discontinuous precipitation at the grain boundary, forming a "necklace" organisation. As the cooling rate decreases, the precipitation of the grain boundary α phase is more and more and connected to each other as a whole, and gradually coarsened to a continuous wave-like.
(2) TB18 titanium alloy bar has good aging strengthening effect, the solid solution aging treatment can improve the strength of 400-500MPa. slow cooling of the formation of grain boundary α phase is unfavourable to the alloy plasticity, toughness. With the slowing down of the cooling rate, the plasticity and toughness of the alloy are reduced, especially the fracture toughness is reduced significantly from 86.9MPa-m1/2 after air cooling to 57.8MPa-m1/2 under the condition of slow cooling at 0.25℃/min, which indicates that the fracture toughness of the alloy has a high sensitivity to the grain boundary α phase.
(3) The use of the BASCA heat treatment process enables the TB18 titanium alloy to obtain an excellent overall performance with an elongation of 8% and a fracture toughness of more than 80 MPa-m1/2 at a level of tensile strength close to 1300 MPa.