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 omega-assisted nucleation. The results showed that the intragranular alpha plates that precipitated during aging appeared finer under slow heating than under fast heating because the omega iso particles formed during the slow heating process provide abundant omega/beta interfaces, which can serve as heterogeneous nucleation sites for alpha precipitates and reduce the nucleation driving force. The corresponding phase transformation sequence is expected to be beta + omega ath -> beta + omega ath + omega iso -> beta + omega iso +alpha -> beta + alpha during continuous heating. Further comparing the microstructures, we found that a faster solution cooling rate is more conducive to omega-assisted nucleation. During omega-assisted nucleation, the three-phase orientation relationship holds for: (1120)omega//(0001)alpha//(110)beta, and [0001]omega//[1120]alpha//[111]beta. At the atomic scale, the omega/alpha interface appeared as a relatively straight atomic plane, whereas more defects, termed as disconnections, were observed at the alpha/beta interface. Moreover, appropriate omega-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 omega-assisted nucleation could considerably strengthen the alloy but significantly deteriorated its ductility, and the mixed fracture mode was more inclined to brittle fracture.