The behavior of gaseous foreign species (e.g., helium and hydrogen), which are either generated, adsorbed or implanted within the structural materials (e.g., iron and zirconium) exposed to irradiation environments, is an important and largely unsolved topic, as they intensively interact with the irradiation-induced defects, or bond with the lattice atoms to form new compounds, and impose significant effects on their microstructural and mechanical properties in fission and fusion reactors. This research investigates two cases of gas diffusion in metals (i.e., the helium-point defect interactions in iron and kinetics of hydrogen desorption from zirconium hydride) through extensive experimental and modeling studies, with the objective of improving the understanding of helium effects on the microstructures of iron under irradiation and demonstrating the kinetics of hydrogen diffusion and precipitation behavior in zirconium that are crucial to predict cladding failures and hydride fuel performance.

The study of helium effects in structural materials aims to develop a self-consistent, experimentally validated model of helium - point defect, defect cluster and intrinsic defects through detailed inter-comparisons between experimental measurements on helium ion implanted iron single crystals and computational models. The combination of thermal helium desorption spectrometry (THDS) experiment with the cluster dynamic model helps to reveal the influence of impurities on the energetics and kinetics of the He-defect interactions and to realize the identification of possible mechanisms governing helium desorption peaks. Positron annihilation spectroscopy is employed to acquire additional information on He-vacancy cluster evolution, which provides an opportunity to validate the model qualitatively. The inclusion of He - self-interstitial clusters extends the cluster dynamic model while MD simulations explore the effects of dislocation loops on helium clustering. In addition, the influence of pre-existing defects on helium behavior in iron is studied by applying a hybrid model, which includes the defect evolution during neutron irradiation and the subsequent He ion implantation and THDS. These modeling predictions will be assessed in future experiments.

The hydrogen desorption process from zirconium hydride and zirconium in vacuum is also studied by coordinated experimental and modeling methods. The production and verification of the desired δ-zirconium hydride is discussed while thermal desorption spectroscopy (TDS) is employed to obtain the hydrogen desorption spectra directly. In addition, a one-dimensional two-phase moving boundary model coupled with a kinetic description of hydrogen desorption from a two-phase region of δ-ZrH1.6±n and α-Zr is developed to compare with the TDS experimental results.

The intermetallic compound, B2 NiAl, is a promising material for high temperature structural applications such as in aviation jet engines or gas turbines, provided that its high temperature mechanical properties can be improved. Although extensive efforts over the last several decades have been devoted toward enhancing ductility through alloying design and reducing impurities, as well as improving high temperature creep strength through precipitation and dispersion strengthening, these efforts have relied on traditional approaches, a combination of large grain size to limit diffusional creep and precipitation/dispersion (50 ~ 100 nm size) strengthening to limit dislocation creep, for high temperature strengthening. While traditional approaches have shown a good improvement from a relatively high temperature strengthening point of view, the size and number density of dispersoids were not able to provide sufficient strength in the high temperature creep regime. Furthermore, details of the interaction mechanism between dislocations and dispersoids are not yet well understood.

This study focuses on designing and developing advanced oxide dispersion strengthened (ODS) NiAl intermetallics with improved high temperature creep strength by incorporating a high number density (~1024 m-3) of very thermally stable Y-Ti-O nano-clusters, akin to those recently observed to improve creep strength and radiation resistance in nano-structured ferritic alloys.

Advanced ODS NiAl alloys have been produced by mechanical alloying of pre-alloyed Ni-50at%Al with Y2O3 and Ti elemental powders. The milled powders were subsequently consolidated by spark plasma sintering, with the objective of producing very high number densities of nano-sized Y-Ti-O precipitates, along with fine grain size. Advanced experimental characterization techniques, combined with microhardness strength measurement, were used to investigate the material microstructure and strength following processing and to evaluate the thermal stability during an extensive matrix of long-term thermal annealing. In particular, the size, number density and composition of nano-clusters were assessed.

While improvements in strength were obtained in the advanced NiAl ODS alloys, and the higher strength persisted through thermal annealing for 100 hrs at 1723K, characterization revealed the presence of Al in the oxide precipitate phases. The Al incorporation is believed detrimental to the formation of a high density of thermally stable Y-Ti-O nanoscale precipitates.

The mechanical properties of materials are often degraded over time by exposure to irradiation environments, a phenomenon that has hindered the development of multiple nuclear reactor design concepts. Such property changes are the result of microstructural changes induced by the collision of high energy particles with the atoms in a material. The lattice defects generated in these recoil events migrate and interact to form extended damage structures.

This study has used theoretical models based on the mean field chemical reaction rate theory to analyze the aggregation of isolated lattice defects into larger microstructural features that are responsible for long term property changes, focusing on the development of black dot damage in ferritic iron based alloys. The purpose of such endeavors is two-fold. Primarily, such models explain and quantify the processes through which these microstructures form. Additionally, models provide insight into the behavior and properties of the point defects and defect clusters which drive general microstructural evolution processes.

The modeling effort presented in this work has focused on physical fidelity, drawing from a variety of sources of information to characterize the unobservable defect generation and agglomeration processes that give rise to the observable features reported in experimental data. As such, the models are based not solely on isolated point defect creation, as is the case with many older rate theory approaches, but instead on realistic estimates of the defect cluster population produced in high energy cascade damage events. Experimental assessments of the microstructural changes evident in transmission electron microscopy studies provide a means to measure the efficacy of the kinetic models. Using common assumptions of the mobility of defect clusters generated in cascade damage conditions, an unphysically high density of damage features develops at the temperatures of interest with a temperature dependence that is much too strong.

The so-called nucleation catastrophe motivates a re-examination of the properties of interstitial defect clusters in iron. The behavior of interstitial clusters in iron is a complex puzzle, with high mobility predicted by computational techniques, much lower thermal mobility observed in electron microscopes, and a series of discrete discontinuous motions seen during in situ ion irradiation performed in a transmission electron microscope. This work has combined these observations and presented a trap mediated concept of interstitial cluster motion that has been incorporated into a larger scale kinetic model. This superior description of interstitial mobility is crucial to realizing many aspects of black dot damage structures, from saturation behavior to temperature dependence.

Another focus of this work was to analyze the assumptions widely employed in rate theory models. Cluster dynamics, the rate theory method employed in this work, is usually invoked with a number of potentially dubious assumptions regarding the mobility and interaction characteristics of defect clusters. The effects of anisotropic reaction volumes and one dimensional diffusion have both been analyzed to determine the effect they have on the development of black dot microstructures. In the trap mediated system, one dimensional diffusion proved far more significant, and the cross section for interaction between one dimensionally diffusing interstitial clusters strongly influenced the size and density of visible damage structures. The validity of the reaction rate approach to determining cluster evolution in the trap mediated environment has been established by comparison with Monte Carlo methods.

In total, this work has demonstrated the ability of mean field models to capture the key characteristics of low temperature damage microstructures in irradiated ferritic alloys when incorporating the full knowledge of interstitial cluster properties in iron, and the legitimacy of the mean field assumptions by comparison to other methods.