Gradient microstructure-mediated superior torsional properties in a metastable β Ti-55531 alloy

This study explores a gradient microstructure design strategy for Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) alloy, utilizing high-frequency electromagnetic induction quenching (HFEIQ) to establish a surface-to-core microstructural gradient that optimizes torsional properties. Through synergistic integration of HFEIQ and aging treatments, we fabricated a hierarchical gradient microstructure characterized by nanoscale secondary α (α_s) lamellae at the surface to enhance strength, while retaining semi-equiaxed primary α (α_p) phases in the core to preserve ductility. The optimized gradient structure achieved remarkable mechanical improvements, with the HFEIQ-5.6 s specimen demonstrating a maximum shear stress (τ_max) of 1158.08 MPa—representing an 8.75 % enhancement over conventional bimodal microstructures—while retaining 9.63 % shear ductility, thereby achieving an exceptional strength-ductility balance. Microstructural analysis reveals that the gradient structure promotes dislocation accumulation at α_s/β_r (retained β matrix) interfaces and deformation twinning within α_s lamellae, enabling coordinated plastic deformation. These mechanisms, tailored by the gradient design, are critical for the enhanced properties. However, prolonged HFEIQ time (5.7 s) results in the coarsening of α_s lamellae, which alleviates stress concentration and consequently reduces twin formation, thereby diminishing the strengthening effect. This work demonstrates that precisely controlled gradient microstructures can optimize the mechanical response of Ti-55531 alloy, offering a promising pathway for advanced structural applications.