Influence of Zr and Sn alloying on phase stability, microstructure, and mechanical behavior of Ti-12Mo-4Al metastable β titanium alloys: An atomic-scale study

The stabilization of the metastable β phase in titanium alloys is crucial for achieving superior mechanical properties, yet the atomistic mechanisms by which neutral elements like Zr and Sn enhance β stability remain contentious. Here, using the same Ti-12Mo-4Al alloy matrix to ensure identical chemical and structural conditions, we introduce Zr and Sn as separate solute additions to enable a direct and controlled comparison of their individual effects on β-phase stability. By integrating first-principles calculations with a mapping-SQS modeling strategy and targeted experiments, we quantify the distinct thermodynamic and kinetic pathways through which each element interacts with the β lattice. Both Zr and Sn enhance β-phase stability by simultaneously suppressing α^′ and ω formation, with Sn exhibiting superior thermodynamic and kinetic efficacy. We show that Sn is significantly more effective than Zr: it raises the kinetic barrier for martensitic transformation by >200 meV/atom at only 3 wt.%, and reverses the thermodynamic stability at 7 wt.%, effectively preventing the transformation. Differential charge density analysis reveals the electronic origins of this disparity: Zr acts through localized bond reinforcement, whereas Sn induces a profound, delocalized Ti-Sn coupling that greatly enhances lattice rigidity. These mechanisms cooperatively alter β stability in Ti-Mo-(Zr/Sn) alloys, establishing a quantitative link between alloy composition, bonding character, and transformation energetics. Our findings provide an electronic-structure framework for designing β-titanium alloys with tailored metastability.