Achieving strength-ductility synergy in a laser-powder bed fused near-α titanium alloy through well-crafted heat treatments

The design of a two-step heat treatment (solutionizing and aging) protocol for laser-powder bed fused Ti-6Al-2Sn-4Zr-2Mo (L-PBF-Ti-6242) and its effect on microstructure and mechanical properties are studied. The heat treatment is designed considering the kinetics of α’ to β transformation and nanotwin annihilation with an emphasis on preserving the unique, ultrafine, and hierarchical microstructure. The solutionizing temperature of 900 °C for 10 and 20 min is selected based on the α’ to β transformation percentage. The aging temperatures of 300 °C and 350 °C for 12–72 h are chosen considering the kinetics of nanotwin annihilation. A total of 15 conditions are evaluated from these solutionizing and aging parameters including as-built, only solutionized, solutionized-aged, and direct aging. The as-built microstructure has the highest strength and lowest ductility due to the ultrafine acicular α’ martensite and dense dislocation network. The solutionized microstructure has α/α’and β phases with different dislocation densities. This condition results in the lowest strength and highest ductility, governed by the presence of the bcc phase (β), and high mean effective slip length in the α/α’ phase. The aging process (following the solutionizing step) results in changes in dislocation substructure in the α/α’ phase which leads to an increase in strength, controlled by a reduction in mean effective slip length. The formulated two-step heat treatment leads to the best strength-ductility synergy in this study