Tuning microstructure and improving the corrosion resistance of Ti-6Al-3Nb-2Zr-1Mo alloy using the electron beam freeform fabrication

Herein, the microstructure and corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy manufactured by the electron beam freeform fabrication (EBF³) have been thoroughly investigated. The evident prior-β grain boundary, the inside of which is composed of predominant α-lath and slight β-phase, is achieved. The EBF³-manufactured alloy displays a significantly improved corrosion resistance compared to the wrought alloy, especially the XY-plane. The X-ray photoelectron spectroscopy results reveal the formation of a more stable oxide film for the EBF³-manufactured alloy due to a higher fraction of phase boundaries induced by the fine α-laths; besides, the heterogeneous distribution of compositions between α- and β-phases can be effectively alleviated, resulting in the absence of the strong galvanic corrosion, which has been illustrated combined experiments and ab-initio calculations. The electrochemical analyses indicate that all samples exposed to the artificial seawater exhibit a typical spontaneous passive behavior, deriving from the presence of a compact oxide film, while a active–passive transition behavior is observed in 5 M HCl, stemming from the formation of a non-protective porous oxide film, which is well interpreted using the mixed potential theory. In addition, the oxygen reduction reaction, determined by diffusion and charge-transfer processes, is the predominant cathodic reaction in the artificial seawater, whereas, in 5 M HCl, the cathodic reaction is primarily associated with the hydrogen evolution reaction controlled by the discharge step.