Effect of friction stir processing on the stress corrosion susceptibility of beta titanium alloy in Hank's solution: passive-film chemistry and deformation response

Stress corrosion cracking (SCC) is a critical reliability concern for load-bearing biomedical titanium implants, yet the potential of friction stir processing (FSP) to mitigate SCC in metastable β-titanium alloys remains largely unexplored. This work investigated the effect of friction stir processing (FSP) on the environmental tensile response of Ti-35Nb-2Ta-3Zr by comparing hot-rolled (HR) and FSPed specimens in air and Hank's solution at room temperature. Slow strain rate tensile tests were conducted at strain rates of 10 -4 -10 -6 s -1 , together with microstructural, electrochemical, X-ray photoelectron spectroscopy (XPS), and fractographic analyses. FSP reduced the mean β-grain size from 9.41 to 4.03 μm. XPS analysis of potentiostatically formed films showed higher highest-valence oxide (HVO) and O 2- /OH - ratios for the FSPed condition. Electrochemical measurements indicated higher passive-film and charge-transfer resistances, accompanied by shallower localized-corrosion features along the examined profiles. Consequently, the FSPed exhibited a smaller environmental loss of ductility, particularly at 10 -6 s -1 , and better retention of work-hardening capacity in solution. These observations suggest an association between FSP-modified passivity and the environmental tensile response. While the results align with the involvement of localized anodic dissolution, the dominant degradation mechanism remains complex, and hydrogen-related contributions cannot be entirely ruled out. Overall, these insights support the further evaluation of FSP for biomedical β-titanium alloys under physiological-temperature and cyclic-loading conditions.