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 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 . 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 and approaches to a constant value. The hydrostatic compressive stress acting on TiH, 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.

中文翻译：

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从第一原理计算深入了解氢化钛的形成

氢化物脆化是钛合金应力腐蚀裂纹的主要原因之一。原子水平上氢化物的形成过程仍然存在争议，因为它无法通过实验直接获得。在目前的工作中，使用第一原理方法研究了氢化钛的形成。通过计算HCP和FCC Ti基体中各种H占据态的TiH的形成能，表明α-Ti-H固溶体与δ/ε氢化物之间存在相平衡，其中χ和γ氢化物以亚稳相出现。形成氢化物的 HCP → FCC 结构转变的能垒随着 H:Ti 比例的增加而降低。结构转变伴随着一半的氢原子从八面体间隙自发转移到四面体间隙。剩余一半H原子从八面体间隙迁移到四面体间隙的能垒随着增加而减小并接近恒定值。由于H引起的晶格膨胀趋势和周围基体的约束，作用在TiH上的静水压应力不利于HCP→FCC转变和H原子的迁移。目前的工作揭示了原子水平上的氢化物形成过程。