Segregation-induced hydrogen embrittlement in titanium

Although titanium offers an optimal combination of strength, low weight, and toughness for various applications, it suffers from a drawback: loss of ductility upon exposure to hydrogen. In this work, we couple CALPHAD-integrated density-based thermodynamic modelling of hydrogen segregation with an experimentally calibrated fracture model to investigate its on crack propagation in titanium. Here we propose to model the crack propagation path as a quasi-interface with slightly opened structure and reduced atomic density, enabling interstitial hydrogen segregation. The atomic density is then directly linked with the damage parameter. We found that hydrogen segregation in titanium undergoes a significant transition such that above a threshold of only few atomic percent hydrogen in the solid solution, the interfacial hydrogen concentration exceeds 20 at.%. Integrating this information into our fracture model, the material damage evolution could be explained by a segregation-affected Griffith crack energy, resulting in material decohesion. We found that the segregation transition and subsequent embrittlement effects are critically sensitive to the temperature in the system. The present results suggest a mechanism underlying the sudden loss of fracture toughness during crack propagation, in relation to the ductile-to-brittle transition observed in titanium alloys exposed to hydrogen. The proposed CALPHAD-integrated chemo-mechanical framework can be further generalised for studying more complex failure mechanisms in various materials.