<p>Nuclear fission energy has developed for more than five decades and become one of the most important low-carbon energy forms. The extreme environment in the advanced reactors, including high operating temperature and high neutron radiation dose, raises new challenges for structural materials. To date, no materials are immune to radiation damage. Bombardment by energetic particles displaces atoms from their original sites, leaves various forms of defect aggregates after cascade, and degrades the properties of the irradiated materials. FeCrAl alloys, known for their excellent high-temperature oxidation resistance, were developed under the accident tolerant fuel program in hope to replace the Zr cladding alloys in future reactors. The radiation response and mechanical properties of FeCrAl alloy have attracted great attention. The objective of this thesis is two-fold. First, investigate the high temperature mechanical behavior of coarse-grained FeCrAl alloys with and without irradiation from the perspective of small-scale testing. Second, develop a fine-grained FeCrAl alloy variant and evaluate its mechanical properties and radiation tolerance.</p> <p>Critical resolved shear stress of pristine and proton irradiated CG FeCrAl alloy was quantified at elevated temperatures. {112}<111> slip system exhibited higher irradiation induced hardening compared with the {110}<111> slip system. Gradient FeCrAl alloy was fabricated through surface mechanical grinding treatment. In situ pillar compression tests revealed an excellent combination of strength and deformability of ultra-fine-grained (UFG) FeCrAl alloys. The activation energy for plastic deformation of a nanolaminate (NL) FeCrAl alloy was determined through strain rate jump tests. Ex situ Fe-ion irradiation showed the interplay between dislocation loops and grain coarsening and their contributions to the mechanical properties of the irradiated UFG FeCrAl alloys. In situ Kr ion irradiation studies on the helium pre-injected NL FeCrAl and CG FeCrAl show that the helium induced swelling was effectively reduced in NL alloy due to their abundant grain boundaries serving as defect sinks. The findings in this thesis may provide innovative perspectives on the design and manufacture of novel FeCrAl alloys with outstanding mechanical properties and radiation tolerance.</p> <p><br></p>

Ionic transport in metal oxides is crucial for the functioning of a broad range of different components, such as heat resistant alloys designed for high temperature applications and oxide electrolytes in solid oxide fuel cells. This thesis presents results from in situ electrochemical studies of properties related to ionic transport in metal oxides that are important for their applications as protective oxides and ionic conductors. Heat resistant alloys of alumina-former type are known to form an adherent, slowly growing and protective aluminium oxide (Al2O3) scale that protects metals from chemical degradation at high temperature. In situ impedance spectroscopy was used to study highly pure and dense samples of a-alumina in the temperature range 400 – 1000 °C. It was shown that surface conduction on the sample could severely distort the measurement below 700 °C. The magnitude of the distortions appeared to be sensitive to the type of electrodes used. The use of a so-called guard electrode was shown to effectively block the surface conduction in the measurements. By varying the grain size of the sintered alpha-alumina samples, the influence of grain size on the overall conductivity of the a-alumina was studied. It was shown that the activation energy for conductivity increased as the grain size decreased. Molecular dynamics calculations were performed in order to elucidate whether Al- or O ions are dominant in the ionic conductivity of the alpha-alumina. Comparing the calculation and experimental results, the dominating charge carrier was suggested to be oxygen ions. Moreover, the ionic transport in thermally grown alumina-like oxide scales formed on a FeCrAl alloy was studied in situ by impedance spectroscopy between 600 and 1000 °C. It was shown that the properties of these scales differ largely from those of pure and dense alpha-alumina. Furthermore, the conductivity is mainly electronic, due to the multiphase/multilayer microstructure and substantial incorporation of species from the base metal. However, the diffusivity obtained from the ionic conductivity was in line with diffusion data in literature obtained by other methods such as thermogravimetry. Besides, the initial stage of oxidation of a number of Fe-, Ni- and Co-based alloys at temperatures between 500 and 800 °C was studied in situ by high temperature cyclic voltammetry, in which the oxygen activity was changed over a wide range. From the resulting voltammograms the redox reactions occurring on the alloy surface could be identified. It was concluded that the base metal oxidized readily on these alloys before a protective chromia- or alumina-like scale is formed. The base metal oxide is most likely incorporated into the more protective oxide. Further, the oxygen ionic conductivity of highly pure and fully dense yttria-stabilized zirconia produced by spark plasma sintering was studied by impedance spectroscopy. The aim was to evaluate intrinsic blocking effects on the ionic conduction associated with the space charge layer in the grain boundary region. It was observed that the ionic conductivity of the spark plasma sintered oxides is equal or slightly higher than what has been achieved by conventional sintering methods. In addition, it was shown that the specific grain boundary conductivity increases with decreasing grain size, which can be explained by a decreasing Schottky barrier height (i.e., decreasing blocking effect). The quantitative results from this work verify the space charge model describing the influence of grain size on the ionic conductivity of yttria-stabilized zirconia through dopant segregation and oxygen vacancy depletion along the grain boundaries. / QC 20100825