Microstructural refinement and strength-ductility optimization in Ti-15Zr alloys via cobalt microalloying

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This study examines the influence of minor cobalt (Co) additions on the microstructural evolution and mechanical properties of Ti-15Zr alloys subjected to thermomechanical processing. Alloys containing up to 2 wt% Co were prepared via plasma arc melting, followed by homogenization, hot rolling, and cold rolling. Microstructural observations using optical microscopy and SEM showed that Co addition refined the prior β grain structure and promoted equiaxed α-phase formation. This refinement is attributed to solute drag during solidification, driven by Co’s high growth restriction factor (Q-value), which suppressed grain coarsening and enhanced dynamic recrystallization. Tensile testing revealed that strength increased with Co content, while ductility was preserved up to 1 wt% Co. The 1Co alloy exhibited the best strength-ductility balance in both hot- and cold-rolled states. In contrast, the 2Co alloy, although stronger, exhibited reduced elongation due to grain boundary instability, and limited strain hardening. EBSD analysis confirmed that the 1Co alloy developed a refined and isotropic α-phase structure, a weakened and broadened basal texture along the transverse direction, and favorable conditions for prismatic slip activation. These features were supported by uniform KAM distributions, indicating enhanced slip compatibility and reduced intragranular strain localization. TEM analysis further revealed uniform dislocation structures and effective dynamic recovery in the 1Co alloy, whereas the 2Co alloy exhibited dense dislocation tangles and planar strain bands, consistent with severe strain localization. Quantitative strengthening analysis showed that dislocation hardening was the dominant mechanism, while grain refinement and solid-solution strengthening provided only minor contributions to the overall strength increase. These findings demonstrate that 1 wt% Co is an optimal microalloying level for enhancing both strength and ductility in Ti-15Zr alloys, providing a practical design strategy for advanced biomedical and structural titanium applications.