Enhanced mechanical properties in metastable β titanium alloy via ω-assisted nucleation

In this study, we performed heat treatment with different solution cooling routes and aging heating rates were performed on the TB18 alloy (Ti-5.3Cr-4.9Mo-4.9V-4.3Al-0.9Nb-0.3Fe) to enhance its mechanical properties via ω-assisted nucleation. The results showed that the intragranular α plates that precipitated during aging appeared finer under slow heating than under fast heating because the ωiso particles formed during the slow heating process provide abundant ω/β interfaces, which can serve as heterogeneous nucleation sites for α precipitates and reduce the nucleation driving force. The corresponding phase transformation sequence is expected to be β + ωath → β + ωath + ωiso → β + ωiso + α → β + α during continuous heating. Further comparing the microstructures, we found that a faster solution cooling rate is more conducive to ω-assisted nucleation. During ω-assisted nucleation, the three-phase orientation relationship holds for: (112‾0)ω//(0001)α//(110)β, and [0001]ω//[112‾0]α//[11‾1]β. At the atomic scale, the ω/α interface appeared as a relatively straight atomic plane, whereas more defects, termed as disconnections, were observed at the α/β interface. Moreover, appropriate ω-assisted nucleation could enhance the ductility of the TB18 alloy without reducing its strength to obtain the optimal comprehensive mechanical properties, and the corresponding fracture mechanism was a mixed mode dominated by ductile fracture. Excessive ω-assisted nucleation could considerably strengthen the alloy but significantly deteriorated its ductility, and the mixed fracture mode was more inclined to brittle fracture.