Influence of Zr and Sn alloying on the deformation mechanism of Ti-12Mo-4Al metastable β titanium alloys: An atomic-scale study

Abstract

The influence of zirconium and tin additions on the mechanical response of Ti-12Mo-4Al alloys was examined through tensile experiments, electron back-scattered diffraction (EBSD) analysis, fractography, and SQS-based first-principles calculations. The base alloy exhibits pronounced strain hardening with a peak hardening rate of 17.25 GPa and deforms primarily through stress-induced martensitic transformation. In contrast, Zr and Sn additions markedly suppress strain hardening, which becomes nearly negligible at 5 wt.% Zr or 3 wt.% Sn, accompanied by a transition from mixed-mode fracture to dimple-dominated ductile fracture. EBSD analysis further indicates that the primary mode of deformation shifts with composition, changing from martensite formation to dislocation-mediated slip. To clarify the atomic-scale origin of this transition, generalized stacking fault energy calculations were performed for the {110} <111> and {112} <111> slip systems in the β phase. The results show that both Zr and Sn reduce the unstable stacking fault energies of these slip systems, thereby lowering the critical resolved shear stress for slip initiation. This reduced barrier promotes dislocation-mediated plasticity over martensitic transformation, shifting the dominant deformation mode toward slip-dominated plasticity. These findings provide an atomistic basis for tuning deformation behavior in metastable β-Ti alloys through Zr and Sn alloying.