The aim of this research is to identify problematic microstructural features as hydrogen traps in linepipe steel that serve to increase the hydrogen embrittlement susceptibility. A comparison is made between the hydrogen trapping capacity and associated hydrogen embrittlement susceptibility of Ca-treated X60 grade steel skelp and X70 grade steel skelp: the latter typically being more susceptible to hydrogen-induced cracking in sour environments. Through-thickness variations in the steel skelp microstructure were characterized across multi-length scales using light optical microscopy (LOM) and scanning electron microscopy (SEM) equipped with X-ray energy dispersive spectroscopy (EDS). Key features under study include the composition, shape, and distribution of non-metallic inclusions, as well as differences in features present between the quarterline (¼ and ¾ depths) and centerline (½ depth) microstructures. The type, count, and average size of inclusions present in both steel skelp grades were analyzed using an automated SEM-EDS technique called ASPEX®. Major types of inclusions detected in both grades of steel skelp include those containing Ca, Al, Mn, Mg and Ti as major elements. Overall, the area fraction of inclusions detected in the X70 steel was larger than those detected in the X60 with the exception of Ti-containing inclusions, which had a larger area fraction within the X60 steel. Comparing the number of detected inclusions shows that there was overall slightly less Ca-containing inclusions and significantly less Ti-containing inclusions detected in the X70 steel but there was generally more Al-containing, Mg-containing, and Mn-containing inclusions than those detected in the X60 steel. Thermal desorption spectroscopy (TDS) measurements were made on samples prepared from the ¼, ½, and ¾ depths of X60 and X70 steel skelps after galvanostatic cathodic charging in an As2O3-containing solution using an applied current density of −10 mA/cm2. Hydrogen release was measured using a HYDROSEEL® probe while the sample was heated from 20°C to 650°C to detect temperature values at which hydrogen gas release peaks occurred, and thus provide information on types of reversible and/or irreversible traps present. The TDS results suggests that non-metallic inclusions indeed serve as irreversible traps along with grain boundaries and dislocations, which serve as reversible traps. Hydrogen permeation measurements were also made on samples prepared from the ¼, ½, and ¾ depths after galvanostatic cathodic charging in an As2O3-containing solution using an applied current density of −10 mA/cm2. Hydrogen gas release was measured using a HYDROSEEL® probe while the sample remained at room temperature (~20°C), providing information regarding the potency of reversible hydrogen traps when subjected to a flux of hydrogen. Only reversible traps can be detected at room temperatures due to their low binding energies. Higher temperatures are required to overcome the larger binding energies associated with irreversible traps. The hydrogen permeation results indicate no significant effect of through-thickness variations in the X60 steel, but the centreline depth of the X70 steel skelp trapped a larger quantity of hydrogen than either of the two quarterline depths, indicating the presence of a distinct problematic trap. The X70 steel skelp was also observed to trap more hydrogen than the X60 steel skelp. The observed hydrogen trapping capacity was linked to the hydrogen embrittlement susceptibility by comparing the uniaxial tensile behaviour of centreline samples with and without hydrogen charging applied as a pre-treatment step. Hydrogen charging was achieved by galvanostatic cathodic polarization at an applied current density of −10 mA/cm2 for 24 h in an NH4SCN-containing solution while simultaneously loading the samples to 85% of the yield strength using a proof ring tensile test cell. An increase in hydrogen embrittlement as a result of pre-charging was confirmed through tensile plots by comparing the area of reduction and failure strain of charged samples to uncharged samples. A decrease in both values was observed in the charged samples indicating a loss in ductility as a result of hydrogen charging. Fracture surfaces were imaged using SEM and inclusions of interest were analyzed for elemental composition using EDS. Inclusions observed along the fracture surfaces include oxysulfides of Ca and Al, oxides of Mg, Al-Ca-Si oxides, and Al2O3-containing inclusions which are likely to be heterogeneous Al-Ca-O inclusions. / Thesis / Master of Applied Science (MASc)

Hydrogen embrittlement (HE), which results in an unpredictable failure of metals, has been a major limitation in the design of critical components for a wide range of engineering applications, given the near-ubiquitous presence of hydrogen in their service environments. However, the exact mechanisms that underpin HE failure remain poorly understood. It is known that hydrogen, when free to diffuse in these materials, can tend to concentrate at a crack tip front. In turn, this facilitates crack propagation. Hence one of the proposed strategies for mitigating HE is to limit the content of freely diffusing hydrogen within the metal atomic lattice via the introduction of microstructural hydrogen traps. Further, it is empirically known that the introduction of finely-dispersed distribution of nano-sized carbide hydrogen traps in ferritic steel matrix can improve resilience to HE. This resilience has been attributed to the effective hydrogen trapping of the carbides. However, conclusive atomic-scale experimental evidence is still lacking as to the manner by which these features can impede the movement of the hydrogen. This lack of insight limits the further progress for the optimisation of the microstructural design of this type of HE-resistant steel. In order to further understand the hydrogen trapping phenomenon of the nano-sized carbide in steel, an appropriate characterisation method is required. Atom probe tomography (APT) has been known for its powerful combination of high 3D spatial and chemical resolution for the analysis of very fine precipitates. Furthermore, previous studies have shown that the application of isotopic hydrogen (²H) loading techniques, combined with APT, facilitates the hydrogen signal associated to fine carbides to be unambiguously identified. However, the considerable experimental requirements as utilised by these previous studies, particularly the instrumental capability necessary for retention of the trapped hydrogen in the needle-shaped APT specimen, limits the study being reproduced or extended. In this APT study, a model ferritic steel with finely dispersed V-Mo-Nb carbides of 10-20 nm is investigated. Initially, existing specialised instrumentation formed the basis of a cryogenic specimen chain under vacuum, so as to retain loaded hydrogen after an electrolytic charging treatment for APT analysis. This work confirms the importance of cryogenic treatment for the retention of trapped hydrogen in APT specimen. The quality of the obtained experimental data allows a quantitative analysis on the hydrogen trapping mechanism. Thus, it is conclusively determined that interior of the carbides studied in this steel acts as the hydrogen trapping site as opposed to the carbide/matrix interface as commonly expected. This result supports the theoretical investigations proposing that the hydrogen trapping within the carbide interior is enabled by a network of carbon vacancies. Based on the established importance of the specimen cold chain in these APT experiments, this work then successfully develops a simplified approach to cryo-transfer which requires no instrumental modification. In this approach there is no requirement for the charged specimen to be transferred under vacuum conditions. The issue of environmental-induced ice contamination on the cryogenic sample surface in air transfer is resolved by its sublimation in APT vacuum chamber. Furthermore, the temperature of the transferred sample is able to be determined independently by both monitoring changes to vacuum pressure in the buffer chamber and also the thermal response of the APT sample stage in the analysis chamber. This simplified approach has the potential to open up a range of hydrogen trapping studies to any commercial atom probe instrument. Finally, as an example of the use of this simplified cryo-transfer technique, targeted studies for determining the source of hydrogen adsorption during electropolishing and electrolytic loading process are demonstrated. This research provides a critical verification of hydrogen trapping mechanism of fine carbides as well as an achievable experimental protocol for the observation of the trapping of individual hydrogen atoms in alloy microstructures. The methods developed here have the potential to underpin a wide range of possible experiments which address the HE problem, particularly for the design of new mitigation strategies to prevent this critical issue.

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This dissertation discusses several recently developed experimental techniques for controlling the motion of neutral atoms. While laser cooling and evaporative cooling have been extremely successful and have been in widespread use for many years, these techniques are only applicable to a few atomic species. Supersonic beams provide a general method of producing cold atoms in the co-moving frame, but their speeds are typically several hundreds of meters per second in the lab frame. Methods to slow and control atoms cooled by supersonic expansion are detailed. A method for controlling the velocity of a cold beam of ground state helium using specular reflection from single crystal surfaces is demonstrated. The velocity of the beam is shown to be continuously tunable, and beam velocities as slow as 265m/s are created from an initial beam speed of 511 m/s. Magnetism is a nearly universal atomic phenomenon, making magnetic control of atomic motion a very general technique. Magnetic stopping of supersonic beams of metastable neon and molecular oxygen is demonstrated using a series of pulsed electromagnetic coils. Neon is slowed from 446 m/s to 56 m/s, and oxygen is slowed from 389 m/s to 83 m/s, removing over 95% of the kinetic energy. The experimental technique is described in detail, and the theory and principle are discussed. An experiment for slowing and trapping of atomic hydrogen isotopes at around 100 mK using a room temperature apparatus is described. A method for further cooling of magnetically trapped hydrogen ensembles, single-photon cooling, is proposed. / text

In recent years, advanced high κ gate dielectrics are under serious consideration to replace SiO2 and SiON in semiconductor industry. Hafnium-based dielectrics such as hafnium oxides, oxynitrides and Hf-based silicates/nitrided silicates are emerging as some of the most promising alternatives to SiO2/SiON gate dielectrics in complementary metal oxide semiconductor (CMOS) devices. Extensive efforts have been taken to understand the effects of hydrogen impurities in semiconductors and its behavior such as incorporation, diffusion, trapping and release with the aim of controlling and using it to optimize the performance of electronic device structures. In this dissertation, a systematic study of hydrogen trapping and the role of carbon impurities in various alternate gate dielectric candidates, HfO2/Si, HfxSi1-xO2/Si, HfON/Si and HfON(C)/Si is presented. It has been shown that processing of high κ dielectrics may lead to some crystallization issues. Rutherford backscattering spectroscopy (RBS) for measuring oxygen deficiencies, elastic recoil detection analysis (ERDA) for quantifying hydrogen and nuclear reaction analysis (NRA) for quantifying carbon, X-ray diffraction (XRD) for measuring degree of crystallinity and X-ray photoelectron spectroscopy (XPS) were used to characterize these thin dielectric materials. ERDA data are used to characterize the evolution of hydrogen during annealing in hydrogen ambient in combination with preprocessing in oxygen and nitrogen.

Certains éléments des internes de cuve des réacteurs à eau pressurisée (REP) en acier inoxydable austénitique 316L, présentent un endommagement prématuré par corrosion sous contrainte assistée par l’irradiation. Ce phénomène est complexe puisqu’il implique le couplage entre le matériau lui-même, l'état de contrainte, l’irradiation et l’environnement. L’irradiation neutronique couplée à un facteur environnemental, l’hydrogène, pourrait jouer un rôle dans ce phénomène. Ainsi, les travaux présentés dans ce manuscrit porte sur l’étude des effets des défauts induits par l’irradiation sur le piégeage de l’hydrogène dans un acier inoxydable austénitique 316L au cours de son oxydation en milieu primaire des REP. Grâce à des implantations ioniques permettant de reproduire le même type de défauts que ceux induits par les neutrons, les interactions hydrogène - défauts ont tout d’abord été étudié avec une approche modèle par chargement cathodique en deutérium, traceur isotopique de l’hydrogène. Cela a permis de mettre en évidence le piégeage de cet élément au niveau des défauts induits par l’implantation ionique et particulièrement des cavités. Un modèle de résolution numérique des équations de McNabb et Foster permettant de simuler la diffusion et le piégeage de l’hydrogène dans un matériau a confirmé ce résultat. Ensuite des essais d’oxydation en milieu primaire simulé des REP à 320 °C ont été réalisés afin de comparer les couches d’oxyde formées entre matériaux implantés et non implantés, ainsi que leur prise d’hydrogène. Ces essais ont permis de mettre en avant le piégeage de l’hydrogène dans l’alliage sous l’oxyde et, dans une moindre mesure, au niveau des défauts d’implantation plus en profondeur. / Some components of vessel internals in pressurized water reactor (PWR) made of stainless steel, have shown cracks induced by Irradiation-Assisted Stress Corrosion Cracking (IASCC). This complex phenomenon originates from the coupling between the material itself, a tensile stress state, environmental conditions and irradiation. This PhD thesis aims at studying the influence of hydrogen in IASCC and particularly its interactions with the defects created by neutron irradiation in a 316L austenitic stainless steel. Thanks to ion implantation, defects similar to neutron irradiation-induced defects were created. As a first step, hydrogen - defects interactions were studied with a model approach consisting in deuterium cathodic charging. Deuterium was used as an isotopic tracer for hydrogen. This technique allowed to highlight hydrogen trapping by implantation-induced defects (mostly by the cavities) in 316L stainless steel. Simulation of hydrogen diffusion and trapping in the studied materials with a numerical resolution model of McNabb and Foster’s equations confirmed the experimental results. Then, oxidation tests were performed in PWR simulated primary environment at 320 °C in order to study the effects of irradiation-induced defects on the oxidation and the hydrogen uptake of the 316L stainless steel. The major highlight of these experiments was the observation of hydrogen accumulation in the alloy beneath the oxide, due to trapping by vacancies created by oxidation process and by ion implantation. In addition, hydrogen trapping was observed deeper in the alloy and it was attributed to the cavities induced by implantation.