Solute clustering is known to play a significant role in the embrittlement of reactor pressure vessel (RPV) steels. When precipitates form they impede the movement of dislocations, causing an increase in hardness and a shift in the ductile-brittle transition temperature. Over time this can cause the steel to become brittle and more susceptible to fracture. Thus, understanding precipitate formation is of great importance to the nuclear industry. The first part of this thesis aims to isolate and better understand the thermal aging component of embrittlement in low copper, model RPV steels. Currently, relatively little is known about the effects of Ni and Mn in a low copper environment. Therefore, it is of interest to determine if Ni and Mn form precipitates under these conditions. To this end, hardness measurements and atom probe tomography were utilized to link the mechanical properties to the microstructure. After 11,690 hours of thermal aging a statistically significant decrease in hardening was observed. Consistent with hardness measurements, no precipitates were present within the matrix of the thermally aged RPV steels. The local chemistry method was then applied to investigate the very early stages of solute clustering. Association was found to be statistically significant in both the thermally aged and as-received model RPV steels. Therefore, no apparent trends regarding the changes in solute association between the as-received and thermally aged RPV steels were identified. Small, non-random clusters were observed at heterogeneous nucleation sites, such as carbide/matrix interfaces and grain boundaries, within the thermally aged material. The clusters found at the carbide/matrix interfaces were all rich in Mn and approximately 90-150 atoms in size. The clusters located along the observed low-angle grain boundary, however, were significantly larger (on the order of hundreds of atoms) and rich in Ni. Lastly, copper-rich precipitates (CRPs) and Mn- and Ni-rich precipitates (MNPs) were observed within the cementite phase of a high copper and low copper RPV steel, respectively, following long term thermal aging. APT was used to characterize these precipitates and obtain more detailed chemical information. The presence of such precipitates indicates that a range of precipitation can take place within the cementite phase of thermally aged RPV steels. The second part of this thesis aims to investigate the effects of ion irradiation on the microstructure of low copper RPV steels via APT. These steels were ion irradiated with 6.4 MeV Fe³⁺ ions with a dose rate of 1.5 x 10^-4 dpa/s at 290°C. MNPs were observed in all five of the RPV steels analyzed. These precipitates were found to have nucleated within the matrix as well as at dislocations and grain boundaries. Using the maximum separation method these MNPs were extracted and characterized. Precipitate composition, size, volume fraction, and number density were determined for each of the five samples. Lastly, several grain boundaries were characterized. Several emerging trends were observed within the samples: Ni content within the precipitates did not vary significantly once a threshold between 30-50% was reached; bulk Mn content appeared to dictate Si and Mn content within the precipitates; and samples low in bulk Ni content were characterized by a higher number density of smaller precipitates. Additionally, by regressing precipitate volume fraction against the interaction of Ni and Mn, a linear relationship was found to be statistically significant.

Ce travail de thèse propose une investigation des dynamiques régissant la réactiond’hydrogénation du dioxyde de carbone sur une nanoparticule modèle de rhodium àl’échelle moléculaire. Cette recherche s’inscrit dans le contexte de la valorisation du CO2 parla voie catalytique. Une meilleure compréhension de la relation entre la structurecristallographique de la surface et les mécanismes réactionnels permettrait, à terme,d’améliorer l’optimisation des matériaux catalytiques. Dans ce but, la réaction est étudiée àl’échelle moléculaire sur une nanoparticule modèle de rhodium. Sa surface est observée àl’aide de l’ensemble des techniques d’émission de champ :la microscopie d’émission dechamp (FEM), la microscopie ionique à effet de champ (FIM), la sonde atomique unidimensionnelle(1DAP) et la sonde atomique tomographique (APT). Afin d’identifier lesdifférents phénomènes observés, ces derniers sont comparés à d’autres systèmes réactifsimpliquant l’O2, le N2O et le CO, sur du rhodium, du platine et un alliage de platine rhodium(10% en masse de rhodium).Dans ce travail nous avons observé et caractérisé l’adsorption dissociative du CO2 sur lerhodium et sa réaction avec l’hydrogène. À l’aide d’études comparatives avec les autressystèmes, des phénomènes réactifs ont été identifiés. Les traitements des donnéesrecueillies indiquent qu’à 700 K, les gaz CO2 et H2 réagissent via un processus en surface etforment les produits CO(g) et H2O(g). Cette réaction est connue sous le nom de « réaction dugaz à l’eau inverse ». Cette réaction s’accompagne de transformations observables par FEM.La présence d’atomes d’oxygène adsorbés provoque une augmentation du travail de sortiedes électrons de la plupart des facettes cristallographiques de la surface de rhodium, ce quise traduit par un assombrissement global de l’image FEM. Cependant, certaines régionsparticulières correspondant aux facettes {113} présentent une tendance inverse. Grâce audéveloppement d’une nouvelle méthodologie adaptant la sonde atomique tomographique àl’étude de processus dynamiques, il a été possible de relier les observations faites par FEM àla diffusion d’atomes d’oxygène sous la surface. Cette diffusion n’est pas homogène etdépend de la structure cristallographique des facettes. L’oxygène, formé à la suite d’uneadsorption dissociative sur les facettes du type {012}, ne pénètre le coeur de phase qu’autravers de certaines régions telles que les {113}, les {011} et les {111}. La construction desdiagrammes d’adsorption du CO2 et les diagrammes de phase cinétique du systèmeH2+O2/Rh ont permis de mettre en évidence les mouvements des atomes d’oxygène entre lasurface et la sous-surface. Finalement ces observations ont été étendues à d’autresmatériaux :Le platine et l’alliage PtRh. L’accumulation d’atomes d’oxygène sur et sous lasurface peut donc être observée en temps réel et à l’échelle moléculaire. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The cast products Mg–Sn based alloys are promising candidates for automobile industries, since they provide a cheap yet thermally stable alternative to existing alloys. One drawback of the Mg–Sn based alloys is their insufficient hardness. The hardenability can be improved by engineering the microstructure through additions of Zn to the base alloy and selective aging conditions. Therefore, detailed knowledge about the microstructural characteristics and the role of Zn to promote precipitation hardening is essential for age hardenable Mg-based alloys. In this work, microstructural investigation of the Mg–1.4Sn–1.3Zn–0.1Mn (at.%) precipitation system was performed using TEM. The chemical composition of the precipitates was analyzed using EDS. APT was employed to obtain precise chemical information on the distribution of Zn in the microstructure. It was found from microstructural studies that different precipitates with varying sizes and phases were present; lath-shaped precipitates of the Mg2Sn phase have an incoherent interface with the matrix, unlike the lath-shaped MgZn2 precipitates. Furthermore, nano-sized precipitates dispersed in the microstructure with short-lath morphology can either be enriched with Sn or Zn. On the other hand, APT analysis revealed the strong repulsion between Sn and Zn atoms in a portion of the analysis volume. However, larger reconstruction volume required to identify the role of Zn is still limited to the optimization of specimen preparation.

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No description available.

Metallic bone implant devices are commonly used to tackle a wide array of bone failures in human patients. The success of such implants relies on the biomechanical and functional bonding between the living bone tissue and implant, a process defined as osseointegration. However, the mechanism of osseointegration is still under debate in the scientific community. One efficient method to help understand this complex process is to characterize the interface between human bones and implant devices after the osseointegration has been established, while another approach is to visualize mineralization in real-time under simulated body conditions. Both of these approaches to understand mineralization have been explored in this thesis. Firstly, due to the inhomogeneous nature of bone and complex topography of implant surfaces, a suitable sample geometry for three-dimensional (3D) characterization was required to fully understand osseointegration. Electron tomography has been proven as an efficient technique to visualize the nanoscale topography of bone-implant interface in 3D. However, resulting from the thickness and shadowing effects of conventional transmission electron microscope (TEM) lamellae at high tilt angles and the limited tilt-range of TEM holders, “missing wedge” artifacts limit the resolution of final reconstructions. In Chapter 3, the exploration of a novel sample geometry to explore osseointegration is reported. Here, on-axis electron tomography based on a needle-shaped sample was applied to solve the problem of the “missing wedge”. This resulted in a near artifact-free 3D visualization of the structure of human bone and laser-modified titanium implant, showing bone growth into the nanotopographies of the implant surface and contributing to the evolution of the definition of osseointegration towards nano-osseointegration. One of the key issues regarding the mechanism of osseointegration that remains is that of the chemical structure at the implant interface, namely distribution of calcium-based and carbon-based components at the interface and their origins. Thus, the second objective of this thesis aimed to push characterization techniques further to four dimensions (4D), by incorporating chemical information as the fourth dimension after the spatial X,Y,Z coordinates. In Chapter 4, correlative 4D characterization techniques including electron energy-loss spectroscopy (EELS) tomography and atom probe tomography (APT) and other spectroscopy techniques were used to probe the nanoscale chemical structure of the bone-implant interface. This work uncovered a transitional biointerphase at the bone-implant interface, consisting of morphological and chemical differences compared to bone away from the interface. Also, a TiN layer between the surface oxide and bulk metal was identified in the laser-modified commercial dental implant. Both findings have implications for the immediate and long-term osseointegration. Since bone formation at the implant interface is a dynamic process, which includes calcium phosphates (CaP) biomineralization as a basis of these reactions, the third objective of this work focused on exploring real-time mineralization processes. Liquid-phase transmission electron microscopy (LP-TEM) is a promising technique to enable real-time imaging with nanoscale spatial resolution and sufficient temporal resolution. In Chapter 5, by using this technique, we present the first real-time imaging of CaP nucleation and growth, which is a direct evidence to demonstrate that CaP mineralization occurs by particle attachment. Overall, this thesis has applied state-of-the-art advanced microscopy techniques to enhance the knowledge and understanding of osseointegration mechanisms by investigating established biointerfaces and real-time mineralization. The developed correlative 4D tomography workflow is transferable to study other interfacial applications in materials science and biological systems, while the LP-TEM work forms a basis for further mineralization research. / Thesis / Doctor of Philosophy (PhD)

This work is a thermodynamic and kinetic study of the Fe-Cr-Ni system as the core of stainless steels. The Fe-Cr, Fe-Ni and Cr-Ni systems were studied intensively using both computational and experimental techniques, including CALPHAD (CALculation of PHAse Diagrams), phase field simulation, ab initio modeling, calorimetry, and atom probe tomography. The purpose of this thesis is to reveal the complexity of the phase transformations in the Fe-Cr-Ni system via the integrated techniques. Due to the importance of the binary Fe-Cr system, it was fully reassessed using the CALPHAD technique by incorporating an updated description of the lattice stability for Fe down to zero kelvin. The improved thermodynamic description was later adopted in a phase field simulation for studying the spinodal decomposition in a series of Fe-Cr binary alloys. Using atom probe tomography and phase field simulation, a new approach to analyze the composition amplitude of the spinodal decomposition was proposed by constructing an amplitude density spectrum. The magnetic phase diagram of the Fe-Ni system was reconstructed according to the results from both ab initio calculations and reported experiments. Based on the Inden-Hillert-Jarl magnetic model, the thermodynamic reassessment of the Fe-Ni system demonstrated the importance of magnetism in thermodynamic and kinetic investigations. Following this, the current magnetic model adopted in the CALPHAD community was further improved. Case studies were performed showing the advantages of the improved magnetic model. Additionally, the phase equilibria of the Fe-Cr-Ni ternary were discussed briefly showing the need of thermodynamic and kinetic studies at low temperatures. The “low temperature CALPHAD” concept was proposed and elucidated in this work showing the importance of low temperature thermodynamics and kinetics for designing the new generation of stainless steels. / <p>QC 20120612</p> / Hero-m

Stainless steels are very important engineering materials in a variety of applications such as in the food industry and nuclear power plants due to their combination of good mechanical properties and high corrosion resistance. However, ferrite-containing stainless steels are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation of the ferrite phase, where it decomposes into Fe-rich ferrite (α) and Cr-rich ferrite (α'). The phase separation is accompanied with a severe loss of toughness. Therefore, the upper service temperature of ferrite-containing stainless steels in industrial applications has been limited to around 250°. In the present work, Fe-Cr based steels were mainly investigated by atom probe tomography. A new method based on the radial distribution function (RDF) was proposed to quantitatively evaluate both the wavelength and amplitude of phase separation in Fe-Cr alloys from the atom probe tomography data. Moreover, a simplified equation was derived to calculate the amplitude of phase separation. The wavelength and amplitude was compared with evaluations using the auto-correlation function (ACF) and Langer-Bar-on-Miller (LBM) method, respectively. The results show that the commonly used LBM method underestimates the amplitude of phase separation and the wavelengths obtained by RDF shows a good exponential relation with aging time which is expected from the theory. The RDF is also an effective method in detecting the phenomena of clustering and elemental partitioning. Furthermore, atom probe tomography and the developed quantitative analysis method have been applied to investigate the influence of different factors on the phase separation in Fe-Cr based alloys by the help of mainly mechanical property tests and atom probe tomography analysis. The study shows that: (1) the external tensile stress during aging enhances the phase separation in ferrite. (2) Phase separation in weld bead metals decomposes more rapidly than both the heat-affected-zone metals and the base metals mainly due to the high density of dislocations in the welding bead metals which could facilitate the diffusion. (3) The results show that Ni and Mn can enhance the phase separation comparing to the binary Fe-Cr alloy whereas Cu forms clusters during aging. (4) Initial clustering of Cr atoms was found after homogenization. Two factors, namely, clustering of Cr above the miscibility gap and clustering during quenching was suggested as the two responsible mechanisms. (5) The homogenization temperatures significantly influence the evolution of phase separation in Fe-46.5at.%Cr. / <p>QC 20140910</p> / Spinodal Project