Why is neutral tin addition necessary for biocompatible β-titanium alloys?–Synergistic effects of suppressing ω transformations

Well-designed β-Ti alloys exhibit a unique combination of low elastic modulus and high strength, making them promising candidates for the next generation of biocompatible implant materials. A recently PMDA-approved β-Ti alloy, stabilized by the addition of a β-phase stabilizing element (Nb), contains a small amount of Sn, which is not a β-stabilizer, to prevent the formation of brittle ω phase. Here, focusing on the Ti–V–Sn alloy system, we have elucidated two-fold roles of the neutral Sn addition in β-Ti alloys. First, although it cannot be completely suppressed by the β-stabilizing elements alone, co-alloying with Sn dramatically enhances the capability of preventing the local collapsing of the (111)_β planes, the elementary process of β↔ω transformation, due to the emergence of many-body effects. Second, Sn atoms serve as anchors for adjacent β-stabilizing elements, preventing the phase separation at intermediate temperatures. These two synergistic effects lead to a comprehensive suppression of all modes of the ω phase transformation.