Abstract
The growing implementation of hydrogen-based energy technologies requires structural materials that can maintain their integrity during hydrogen production, storage, transport, and utilization. Titanium and its alloys are attractive for selected hydrogen-energy applications because of their high specific strength and excellent corrosion resistance. However, their performance can be compromised by hydrogen uptake. In particular, hydrogen can significantly degrade the mechanical performance of titanium through the combined effects of dissolved hydrogen and titanium hydride precipitation. The relative contributions of these mechanisms depend strongly on hydrogen concentration, crystallographic orientation, stress state, and temperature. This thesis investigates hydrogen-induced deformation and fracture mechanisms in high-purity titanium, with particular emphasis on distinguishing the effects of solute hydrogen and hydrides and clarifying how their relative contributions are governed by temperature and interact with the anisotropic deformation behaviour of the α-Ti matrix.
A multiscale experimental approach was employed. Single-crystal deformation behaviour of selected grain orientations was investigated using in-situ micropillar compression tests to determine how distinct deformation modes interact with solute hydrogen and titanium hydrides. Complementary molecular dynamics simulations of hydrogen-free α-Ti were used to support the identification of the deformation mechanisms associated with the selected loading orientations. The micromechanical experiments were complemented by synchrotron-based grazing-incidence X-ray diffraction measurements to examine the evolution of lattice spacing along different crystallographic directions during hydrogen uptake and hydride formation. At a larger length scale, the deformation and fracture behaviour of polycrystalline high-purity titanium was investigated using temperature-controlled tensile tests conducted at room temperature, 80 °C, and −100 °C, combined with in-situ scanning electron microscopy observations. Electron backscatter diffraction, scanning electron microscopy, and scanning transmission electron microscopy were further used to characterize hydride formation, dislocation structures, deformation twinning, strain localization, and crack development.
The findings establish a clear mechanistic distinction between the effects of solute hydrogen and titanium hydrides on the deformation response of α-Ti. Solute hydrogen reduced the resistance to prismatic slip, lowered the yield stress, promoted more continuous plastic flow, and increased dislocation and stacking-fault activity. In contrast, hydride precipitation increased the yield stress and strain hardening, restricted slip propagation, and promoted deformation instability and brittle failure. Hydride formation was also found to generate anisotropic transformation strains in the surrounding α-Ti matrix, producing tensile strain along the c-axis and compressive strain within the basal plane. These internal stresses modified slip-system activation and dislocation accumulation depending on the crystallographic orientation, demonstrating that hydrides influence deformation beyond their immediate interfaces. The in-situ observations further captured the evolution of deformation and damage from the earliest stages of slip and strain localization to crack initiation and subsequent propagation.
Temperature altered the balance between hydride-mediated degradation and solute-hydrogen-assisted plasticity. At room temperature, stable hydrides acted as strong barriers to deformation and preferential sites for crack initiation. At 80 °C, increased hydrogen uptake and solubility promoted a mixed deformation regime in which enhanced dislocation activity coexisted with hydride-induced strengthening, while the contribution of hydrides to brittle cracking was reduced. The complementary low-temperature experiments showed that damage development depended on both hydride distribution and the ability of the α-Ti matrix to accommodate hydride-induced strain incompatibility. In specimens containing a continuous near-surface hydride layer, cracks that exhibited local blunting at room temperature extended across the hydride–matrix interface and into the α-Ti matrix at −100 °C. After hydride redistribution, cooling reduced the strain to failure, promoted cracking within hydride-rich regions, and increased the contribution of faceted fracture.
Overall, the thesis establishes that hydrogen-assisted degradation in pure titanium is governed by the coupled effects of hydrogen state, crystallographic orientation, hydride-induced internal stress, and temperature-dependent deformation mechanisms. These findings provide a mechanistic basis for understanding the transition between hydrogen-induced softening, hardening, and embrittlement in titanium and support the assessment and safe use of titanium and its alloys in hydrogen-energy applications.