Fundamental influence of hydrogen on various properties of α-titanium

First-principles calculation reveals that the Ti–H interactions are energetically favorable with negative heats of formation, and H atoms could occupy octahedral and tetrahedral interstitial sites of α-Ti simultaneously due to their small energy difference. Calculation also shows that hydrogen concentration plays an important role in determining brittle/ductile behavior of Ti–H phases, and densities of states suggest that a bonding transition from mainly covalent to mainly metallic appear for Ti–H phases when the H/Ti ratio reaches about 1/8. The calculated results agree well with experimental observations and could clarify the controversies of the Ti–H system in the literature.