Publications

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A novel amorphous silicon oxycarbide dispersion-strengthened (SiOC-DS) austenitic steel has been fabricated via a powder metallurgy process. The microstructure of dispersion particles has been characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), revealing that amorphous SiOC nanoparticles with an average particle size of 30 nm were homogeneously distributed in the austenite grains with a sub-micrometer grain size. The high strength and hardness of SiOC-DS may be attributed to grain boundary strengthening, as well as dispersion strengthening via dislocation–particle interactions that were revealed by TEM investigations. In situ ion irradiation experiments showed that amorphous SiOC particles were stable after irradiation of 3.7 dpa, and the SiOC/steel interface can be an effective sink for the annihilation of irradiation defects. The excellent mechanical and irradiation properties of SiOC-DS austenitic steel make it a promising structural material for nuclear applications.

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The ability of nanoporous metals to avoid accumulation of damage under ion beam irradiation has been the focus of several studies in recent years. The width of the interconnected ligaments forming the network structure typically is on the order of tens of nanometers. In such confined volumes with high amounts of surface area, the accumulation of damage (defects such as stacking-fault tetrahedra and dislocation loops) can be mitigated via migration and annihilation of these defects at the free surfaces. In this work, in situ characterization of radiation damage in nanoporous gold (np-Au) was performed in the transmission electron microscope. Several samples with varying average ligament size were subjected to gold ion beams having three different energies (10 MeV, 1.7 MeV and 46 keV). The inherent radiation tolerance of np-Au was directly observed in real time, for all ion beam conditions, and the degree of ion-induced damage accumulation in np-Au ligaments is discussed here.

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Nanocrystalline metals typically have high fatigue strengths but low resistance to crack propagation. Amorphous intergranular films are disordered grain boundary complexions that have been shown to delay crack nucleation and slow crack propagation during monotonic loading by diffusing grain boundary strain concentrations, which suggests they may also be beneficial for fatigue properties. To probe this hypothesis, in situ transmission electron microscopy fatigue cycling is performed on Cu-1 at.% Zr thin films thermally treated to have either only ordered grain boundaries or amorphous intergranular films. The sample with only ordered grain boundaries experienced grain coarsening at crack initiation followed by unsteady crack propagation and extensive nanocracking, whereas the sample containing amorphous intergranular films had no grain coarsening at crack initiation followed by steady crack propagation and distributed plastic activity. Microstructural design for control of these behaviors through simple thermal treatments can allow for the improvement of nanocrystalline metal fatigue toughness.

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The effect of ion irradiation on the microstructure of oxide dispersion strengthened (ODS) MA956 steel, before and after friction stir welding (FSW), was studied. Both the base material (BM) and welded stir zone (SZ) were irradiated with 5 MeV Fe ++ ions at 450 °C up to 25 displacements per atom (dpa). Characterization was performed using scanning transmission electron microscopy (STEM) and atom probe tomography (APT), with particular emphasis on the Y–Al–O dispersoid characteristics and dislocation microstructures. After irradiation, the dispersoids in the BM increased in diameter and decreased in number density, which was explained by an Ostwald ripening mechanism. FSW caused significant coarsening and agglomeration of the dispersoids. After irradiation, both the diameter and number density of the SZ dispersoids increased, which was explained by an irradiation-enhanced diffusion mechanism. Dislocation loop and network behavior was also characterized and large dislocation loops of ≈20 nm diameter formed by 1 dpa in both the BM and SZ samples, whereas the network density remained nearly constant with irradiation.

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