Modeling the short-term creep response in air of a Friction Stir Processed commercially pure Ti (Grade 2): effects of initial state and oxygen diffusion

This study investigates the effect of Friction Stir Processing (FSP) on the creep response in air at 550 and 600 °C of commercially pure titanium (Ti-Grade 2). FSP resulted in an inhomogeneous microstructure, which generally exhibited lower minimum creep rates compared to the base unmodified metal. Oxygen diffusion in the superficial layer of the creep samples caused a local marked increment of the hardness, a phenomenon already observed when testing the base metal. A constitutive model, which was initially developed to describe the effect of initial hardness and oxygen diffusion during the test in the unmodified grade 2 Ti, was significantly improved and implemented. The model provided an excellent description of the minimum creep rate dependence on the applied stress and temperature for unmodified and friction-stir processed materials without needing any data fitting of the creep results. In addition, the proposed model also suggests the reason for the differences in the shape of the creep curves observed when comparing short and long experiments.