Regulating stress-induced martensitic transformation through primary α phase to enhance work hardening in a Ti-7Mo-3Al-3Cr-3Nb alloy

Metastable β titanium alloys can achieve enhanced ductility through transformation-induced plasticity (TRIP) and/or twinning-induced plasticity (TWIP); however, their work-hardening rates are typically limited to ∼1–3 GPa, constraining further strengthening. In this study, a Ti-7Mo-3Al-3Cr-3Nb (wt.%) alloy with tailored primary α-phase fractions, achieved via a simple heat treatment in the α+β phase field, exhibits an ultrahigh work-hardening rate of up to ∼12.8 GPa. Meanwhile, the TRIP effect is preserved, resulting in a reasonable tensile elongation of ∼16 %. This mechanical response is associated with a uniform distribution of primary α phase within the β matrix, which modulates the stability of the retained β phase and governs the activation of stress-induced martensitic transformation. During deformation, the progressive activation of multiple α″ martensite variants, together with martensitic twinning, refines the retained β matrix and gives rise to a pronounced dynamic Hall-Petch effect at intermediate strains. At higher strain levels, the development of martensitic domains and additional twinning modes contributes to sustained plastic deformation and delayed strain localization. Meanwhile, the coordinated activation of 〈a〉 and 〈c+a〉 slip in the primary α phase improves deformation compatibility. These results demonstrate an effective microstructural pathway to enhance strain hardening in metastable β titanium alloys through controlled β→α″ martensitic transformation.