Study on performance enhancement driven by strain-induced α′ martensite transformation in a metastable dual-phase titanium alloy

This study improves ductility while maintaining the ultimate tensile strength (UTS) of a Ti-30Zr-5Al-3V alloy without changing chemical composition via microstructural control of the double-phase (α+β). The deformation mechanism of the alloy is controlled by regulating the volume fraction of the β phase, resulting in an ultra-high strain hardening capacity that enhances the material's mechanical properties. The water-quenched specimen exhibits an ultra-high strain hardening capacity attributed to the increase in both back and effective stresses as the deformation proceeds. Heterostructure deformation and the accumulation of geometrically necessary dislocations (GNDs) near grain boundaries induce back-stress strengthening. Effective stress strengthening primarily arises from dislocation reinforcement, including the dynamic Hall-Petch effect caused by strain-induced α' martensite transformation (SIMT) and the hindering effect of GNDs migration. The reorientation and merging of α grains are inconducive to high strain hardenability. However, a lower β phase volume fraction in the air-cooled specimen results in less SIMT, and even small strains could lead to significant grain coarsening. High strain hardening rates could not be maintained solely through back stress strengthening, leading to premature material failure. In comparison to the air-cooled specimen, the water-quenched specimen demonstrates a significantly increased ultra-high strain hardening capability, resulting in enhanced uniform elongation (from ∼3.4 to ∼9.6%) and overall elongation (from ∼6.2 to ∼13.3%) without reducing the UTS.