Abstract: To overcome the strength-plasticity trade-offin the structural titanium alloys, a novel metastable β ti-tanium alloy Ti-5Mo-4Cr-1V-1Zr (Ti-5411) with high strength and high plasticity was designed by the d-electrons theory, average electron-to-atom ratio $ (\overline{\mathrm{e} / \alpha})$ and atomic radius difference $(\overline{\Delta r})$ theory. Com-bined in-situ scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), the defor-mation mechanisms of the novel Ti-5411 metastable β titanium alloy were systematically investigated. The results show that the Ti-5411 alloy exhibits excellent yield strength (～689 MPa), tensile strength (～930 MPa) and total elongation (～39%). The in-situ tension indicates that slip activities, crystal rota-tion, stress induced martensite (SIM) α'' transformation and {332} <113> deformation twin are the major deformation mechanisms of Ti-5411 alloy. Besides, with the increase of strain degree (0-0.5 mm displace-ment), deformation twins increase, widen and interlace. At 0.35 mm tensile displacement, the orientation of the β grains rotates ～6.65 °to accommodate the increased macrostrain. Additionally, martensite α'' also assists the nucleation of twins. Some {332} <113> twins grow and merge by consuming martensite α'' during deformation, and the residual martensite α'' remains in the merged twins.

Key words: Metastable β titanium alloy, In-situ tension, Slip activity, Stress induced martensite, Deformation twin