Insights into the formation of titanium hydrides from first principles calculations

Hydride induced embrittlement is one of the main causes for the stress corrosion crack of titanium alloys. The formation process of the hydride at atomic level remains controversial as it is not accessible directly by experiments. In the present work, the formation of titanium hydride is investigated by using a first principles method. By calculating the formation energies of TiH _m with various types of H occupation state in HCP and FCC Ti matrix, we show that there exists phase equilibrium between α-Ti-H solid solution and δ/ε hydrides with χ and γ hydrides appearing as metastable phases. The energy barrier for the HCP → FCC structure transformation for the formation of the hydrides decreases with increasing H:Ti ratio m . The structure transformation is accompanied with spontaneous shift of half amounts of the H atoms from the octahedral to the tetrahedral interstices. The energy barrier for the migration of the remaining half of the H atoms from the octahedral to the tetrahedral interstices reduces with increasing m and approaches to a constant value. The hydrostatic compressive stress acting on TiH _m , resulted from the H-induced lattice dilation tendency and the constraint of the surrounding matrix, does not facilitate the HCP → FCC transformation and the migration of the H atoms. The present work sheds light on the hydride formation process at atomic level.