Phase transformation mechanisms, microstructural characteristics and mechanical performances of an additively manufactured Ti-6Al-4V alloy under dual-stage heat treatment

Understanding phase transformation behaviors and microstructural evolutions during optimized post-heat treatments is essential for tuning the mechanical performances of additively manufactured titanium alloys. In this work, distinctive dual-stage heat treatments were proposed for improving the microstructure and properties of the laser powder bed fusion processed Ti-6Al-4V alloy. The effects of the heat treatments on phase transformation behaviors, microstructural characteristics, mechanical and tribological properties of the alloy were investigated systematically. The dual-stage heat treatments combined with salt bath quenching proved to be an effective approach for obtaining fine lamellar α/β microstructures through the introduction of martensitic and massive phase transformations, as well as achieving optimized strength and ductility. The grain boundary populations of the alloy were strongly influenced by the phase transformation behaviors. Moreover, high-density dislocations were introduced during the dual-stage heat treatments, and the <a> type dislocations were dominant. The nonadditive strengthening mixture rule between obstacles and dislocations was substantiated in this alloy. Different wear mechanisms were found in the dual-stage heat-treated alloys depending on their microstructural characteristics. Furthermore, the underlying mechanisms of phase transformation, strengthening and toughening are also discussed.