In metastable $\beta$ Ti alloys, the martensitic transformation from the parent bcc $\beta$ phase into a daughter orthorhombic $\alpha^{\prime\prime}$ phase and associated twinning were found to largely improve material's ductility due to the TRIP/TWIP effect. Also, in the metastable $\beta$ alloys, high levels of interstitial oxygen (in the order of 0.5-0.8 wt%) have resulted in high yield strength and deformation hardening, which also supported the ductility.

Three alloys were developed with a potential to show a combination of interstitial strengthening and TRIP/TWIP effects: Ti-xNb-7Zr-0.5O (x=23, 26, 29). We have studied these alloys by a combination of tensile testing and microstructural characterization by scanning electron microscopy: both after the tensile test and during the tensile test using an in-situ SEM deformation stage. It was found that for lower Nb content, the alloy suffers from brittleness caused by the combination of presence of athermal $\omega$ and high O content. When increasing the content of Nb, the TRIP/TWIP effect is activated resulting in high yield stress due to interstitial strengthening, combined with deformation strengthening and high ductility. On the other hand, when increasing Nb content furthermore, the stress-induced martensite is no longer observed and slip traces prevail on the surface of the in-situ sample. Also, the elastic part of this sample shows a significant non-linearity, which is often accompanied by stress-induced martensite. As the formation of the lammelar stress-induced $\alpha^{\prime\prime}$ martensite was not observed, this phenomenon must have a different cause. By transmission electron microscopy, nanodomains of O' phase were found in this alloy and it is suggested, that local transformation of these nanodomains into the nanodomains of $\alpha^{\prime\prime}$ phase occurs.