Fe-stabilized duplex α/β microstructure containing γ titanium hydride in Ti grade 2 obtained by volumetrically incomplete phase transition

Highlights: • Uncommon duplex α/β microstructure in Ti grade 2 can be created by annealing slightly below the β transformation temperature. • Volumetrically incomplete phase transition creates a microstructure that can have detrimental effects. • Residual β phase is stabilized by Fe impurity content via its fast diffusion at elevated temperatures. • Presence of stable γ titanium hydride is confirmed in primary α grain interiors and at α/β interfaces. -- Abstract: The aim of this work is to characterize microstructure of commercially pure titanium grade 2 (Ti grade 2), which is formed by volumetrically incomplete α/β phase transition. This duplex α/β microstructure is formed by cooling Ti grade 2 once the β phase is nucleated at sites where nucleation is more favorable (e.g. grain boundaries, areas with higher Fe concentration). After cooling, the remaining small fraction of β phase is stabilized by the iron content, which was introduced to the β phase via its diffusion during annealing. The morphology and distribution of the secondary α phase lamellae and the residual β phase greatly affects the achievable ductility and character of the fracture of this material. Observed fracture mechanism significantly differs from the common one, as the samples are fractured by a large number of parallel cracks. Apart from the residual β phase, locally stabilized down to the room temperature, the presence of face-centered tetragonal γ titanium hydride (γ-TiH) was observed. Although the main nucleation site of these hydrides was found to be the α/β interface, the presence of γ-TiH was also observed in α grains interior. This work aims to discuss negative effects of volumetrically incomplete phase transition in Ti grade 2, as well as to provide detailed microstructural characterization.