Abstract
Titanium and its alloys possess excellent strength-to-weight ratio and corrosion resistance, making them highly desirable structural materials. Hexagonal close-packed (hcp) α-Ti alloys are also among the most corrosion resistant structural materials available for application in aggressive environments. In these alloys, a sufficiently-high trace level of β-stabilizing elements can lead to the retention of a low volume fraction of β-phase, in the form of small β pockets that are in the range of 100 nm and typically located at triple points across the microstructure. The Hydrogen solubility in β-Ti is ~200x higher than in α-Ti at room temperature, which can cause the hydrogen concentrations in α phase to possibly quickly become sufficiently high to cause hydride formation. Elucidating the hydrogen behavior and the hydride formation mechanisms in CP-Ti in relation to the presence of β-pockets, especially at interphase and grain boundaries, are hence of interest to further understand the formation of undesired hydrides. Additionally, knowledge on the deformation behavior of hydrides and their interaction with the parent Ti matrix can help with design approaches to alleviate hydrogen embrittlement of these alloys. In this study, we employ cryogenic sample preparation via focused ion beam (FIB) for atom probe tomography (APT) to analyze the α-α and α-β sections of the abutting grain boundary of a β-pocket in a Grade 2 CP-Ti, as well as the α-β phase boundary. Fe and H were enriched across the β-pocket and at the grain boundary but no segregation was seen at the α-β phase boundary. We propose that β-stabilizing impurities have an indirect effect on the hydrogen embrittlement as they stabilize the β-pockets, which along with the grain boundaries are the key factors for the formation of hydrides. Subsequently, the deformation behavior of the formed hydrides and their interaction with the parent Ti matrix were investigated by in-situ deformation and synchrotron x-ray diffraction. High internal and interphase stresses were shown to be generated within and around hydrides due to the volume expansion induced by the formation of hydrides. The initially wide dispersion of hydrides d-spacings significantly contributes to the peak broadening, in contrast to the common knowledge that it is caused by the grain size and dislocations. This work aims to bridge the atomic-scale formation and macro-scale deformation of hydrides to understand the hydrogen embrittlement of CP-Ti.