Mechanical properties and deformation mechanisms of metastable β Ti-12Mo alloy fabricated by in-situ alloying-based additive manufacturing

The strength-ductility trade-off in additively manufactured (AM) β Ti alloys remains a significant challenge. In this study, we employed a cost-effective in-situ alloying laser powder bed fusion approach with optimized processing parameters to fabricate a nearly fully dense, chemically homogeneous β Ti-12Mo alloy. We then examined how solution-treatment duration influences the tensile behavior of the AM Ti-12Mo alloy. The optimally solution-treated alloys exhibited high tensile yield strength (725–741 MPa) and commendable ductility (22–36 %) along both the 0° and 90° orientations relative to the build direction. Focusing on the underlying deformation mechanisms perpendicular to the build direction, we report a uniform elongation of 17.9 % and a pronounced strain hardening rate (∼2300 MPa at 4 %), which we elucidate via in-situ high-energy synchrotron X-ray diffraction and microstructural characterization. The high yield strength is primarily attributed to the presence of Mo-lean embryonic athermal ω nanoparticles. During plastic deformation, both twinning and phase transformation contribute to the high strain hardening rate. At the early stage (strain < 1.9 %), deformation is dominated by {332}< 113 >_β twinning, whereas at later stages, the deformation-induced α'' phase becomes significant. The volume fraction of α'' phase increases with strain, supporting the continuous hardening. Notably, irrational {112}< 751 >_β, secondary {112}< 111 >_β, and {130}< 310 >_α'' nano-twins confine the primary structures to nanograins and sustain strain hardening. This study sheds light on designing high-performance β Ti-12Mo alloy via AM followed by heat treatment.