Enhancement of strength-ductility synergy in high-strength metastable β-titanium alloys through boron microalloying

Metastable β-titanium (Ti) alloys have always attracted great attention due to the available ultra-high strength via precipitation strengthening, while they often succumb to a strength-ductility trade-off known as improving strength but degrading ductility. In this study, we exploit a novel strategy to address the issue in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β-Ti alloy through the trace boron addition. It is found that an ultra-high strength ∼1600 MPa with elongation of ∼9 % is synchronously achieved with the addition of minor boron, corresponding to a ∼7 % increase in strength and ∼70 % improvement in ductility compared to the boron-free counterpart. Microstructural characterizations manifest that boron addition promotes almost complete transformation from abnormal β-grain region to normal β-grain region, which possesses better deformation accommodation capacity at the aged state. Meanwhile, TiB whiskers are produced and adhered by a layer of α_s-film, creating a sandwiched structure architected by TiB whisker/α_s-film/the aged β-matrix. This unique structure not only effectively retards the deteriorative development of highly localized deformation propagating from the aged β-matrix, but also sufficiently activates dislocation slip within TiB whiskers. These findings demonstrate a counterintuitive result that TiB intermetallic compound well-known as the susceptive brittleness can be harnessed in ductilizing materials, and also provide a practical strategy for enhancing plasticity of high-strength Ti alloys by microalloying design.