Modeling of Stress-Induced Titanium Hydride Formation Using the Phase Field Method

Sammanfattning

Titanium and its alloys are widely used in fields such as aerospace and biomedical applications due to their exceptional performance. However, when exposed to hydrogen-rich environments, titanium alloys can undergo hydride precipitation, a process that is significantly enhanced by applied mechanical stress. This leads to the formation of brittle titanium hydride that causes severe fractures and thus threatens safety, especially when hydrides accumulate around crack tips. Conducting realistic experiments to study this problem is expensive and time-consuming; therefore, computational simulation serves as an ideal complement to address this problem. The objective of this thesis is to develop a computational engineering tool to model stress-induced titanium hydride formation at a crack tip in the lamellar microstructure of the Ti-6Al-4V alloy. This work extends the stress-induced hydride precipitation studies conducted by Dr. Claudio F. Nigro, who investigated hydride evolution across a range of engineering metals and alloys. By examining microstructural regions such as grain boundaries, crack tips and α/β interfaces, Nigro established the engineering framework for computational studies of hydrogen-induced phase transformations. The theoretical model of this study is based on phase-field equations, which describe the spatio-temporal evolution of the titanium hydride phase. The computational implementation employs a finite volume method as the numerical approach. The results of the case studies demonstrate that hydride formation preferentially occurs in α-titanium near the crack tip and that the evolution of hydrides exhibits strong numerical stability, consistently converging to a steady state. Another major finding is that the computational model efficiently captures the dominant effect of the stress intensity factor on hydride formation, confirming the robustness of the coupled mechanical phase-field formulation. Furthermore, the introduction of lamellar anisotropy under rotation demonstrates that the model remains convergent, demonstrating its capability to predict stress-induced hydride formation in titanium alloys even when accounting for complex crystallographic orientations. The findings of this thesis provide a foundation for future studies on more complex Ti-6Al-4V microstructures, where the model can be extended to investigate hydrogen interactions under varying conditions and material parameters.