Composition design of a novel Ti-6Mo-3.5Cr-1Zr alloy with high-strength and ultrahigh-ductility

The strength-elongation to fracture (ε_f) trade-off and low strain hardening rate have been a longstanding dilemma in titanium alloys. In this work, we innovatively manufactured a novel Ti-6Mo-3.5Cr-1Zr alloy via the introduction of a stress-induced strengthening phase into the material. Stress-induced ω (SIω) phase transformation was expected to replace the α'' martensitic transformation that resulted in the low yield strength of titanium alloys. The obtained alloy exhibited an extremely high strain hardening rate of up to ∼1820 MPa. The true peak tensile strength and ε_f reached ∼1242 MPa and ∼40%, respectively. The intrinsic mechanisms underlying the simultaneous improvement of strength and ductility of the material were systematically investigated via in-situ and ex-situ characterizations. In-situ electron backscatter diffraction (EBSD)/digital image correlation (DIC) results showed that SIω phase transformation dominated the early stage of plastic deformation (1.5%–3%) and promoted the strain partitioning between the stress-induced bands and β matrix. Subsequently, the formation of {332}<113> β twins and ω twins was observed via ex-situ EBSD. In-situ transmission electron microscopy results revealed that dislocation pile-up (DPU) occurred at the SIω/β interface. The coupling effects associated with the transformation induced plasticity (TRIP), twinning induced plasticity (TWIP), and DPU mechanisms contributed to the enhanced strength and ε_f of the designed titanium alloy.