Karen Kruska

Examinations of oxidation and sulfidation of grain boundaries in alloy 600 exposed to simulated pressurized water reactor primary water

High-resolution characterizations of intergranular attack in alloy 600 (Ni-17Cr-9Fe) exposed to 325°C simulated pressurized water reactor primary water have been conducted using a combination of scanning electron microscopy, NanoSIMS, analytical transmission electron microscopy, and atom probe tomography. The intergranular attack exhibited a two-stage microstructure that consisted of continuous corrosion/oxidation to a depth of ~200 nm from the surface followed by discrete Cr-rich sulfides to a further depth of ~500 nm. The continuous oxidation region contained primarily nanocrystalline MO-structure oxide particles and ended at Ni-rich, Cr-depleted grain boundaries with spaced CrS precipitates. Three-dimensional characterization of the sulfidized region using site-specific atom probe tomography revealed extraordinary grain boundary composition changes, including total depletion of Cr across a several nm wide dealloyed zone as a result of grain boundary migration.

Applicability of post-ionization theory to laser-assisted field evaporation of magnetite

Analysis of the detected Fe ion charge states from laser-assisted field evaporation of magnetite (Fe3O4) reveals unexpected trends as a function of laser pulse energy that break from conventional post-ionization theory for metals. For Fe ions evaporated from magnetite, the effects of post-ionization are partially offset by the increased prevalence of direct evaporation into higher charge states with increasing laser pulse energy. Therefore, the final charge state is related to both the field strength and the laser pulse energy, despite those variables themselves being intertwined when analyzing at a constant detection rate. Comparison of data collected at different base temperatures also shows that the increased prevalence of Fe2+ at higher laser energies is possibly not a direct thermal effect. Conversely, the ratio of 16O+:(16O2+ + 16O+) is well correlated with field strength and unaffected by laser pulse energy on its own, making it a better overall indicator of the field evaporation conditions. Plotti...

Understanding surface oxidation in stainless steels through 3D FIB sequential sectioning

Focused Ion Beam (FIB) 3D sequential sectioning has been used to reconstruct the morphology of surface oxides in 304 stainless steels in nuclear reactors in order to characterize environmental degradation. Several coupon specimens with prior cold work and oxidized under applied stress have been studied in detail. A novel statistical approach based on more than 250k individual measurements will be presented as an alternative to conventional methods.

Background Recovery through the Quantification of Delayed Evaporation Multi-Ion Events in Atom-Probe Data

The reliable quantitative analysis of oxides by atom probe tomography (APT) is an on-going issue. Analysis conditions have been shown to dramatically affect the measured composition, while rarely achieving a completely satisfactory measurement. In most cases, the optimal analysis conditions have been identified as a combination of low laser energy and high electric field. However, these analysis conditions also result in a relatively high probability for multi-ion evaporation within a single pulse, which can degrade the overall analysis and possibly introduce an unintentional compositional bias.

Characterizing Environmental Degradation in PWRs by 3D FIB Sequential Sectioning

Modern dual-column FIB-SEMs are capable of automatically milling and acquiring images that can be used to reconstruct sample volumes in 3D. We will demonstrate that this technique, applied to the environmental degradation of materials in nuclear reactors, is capable of revealing features, phases and/or defects in 3D with nm resolution. In this paper, we have used this technique to characterize surface oxidation and cracking in Zr alloys, the effect of cold work on the oxidation resistance of austenitic steels and crack growth in welded 316L stainless steel.

Carbon Contamination During Ion Irradiation - Accurate Detection and Characterization of its Effect on Microstructure of Ferritic/Martensitic Steels

Accelerator-based ion beam irradiation techniques have been used to study radiation effects in materials for decades. Although carbon contamination induced by ion beams in target materials is a well-known issue in some material systems, it has not been fully characterized nor quantified for studies in ferritic/martensitic (F/M) steels that are candidate materials for applications such as core structural components in advanced nuclear reactors. It is an especially important issue for this class of material because of the strong effect of carbon level on precipitate formation. In this paper, the ability to quantify carbon contamination using three common techniques, namely time-of-flight secondary ion mass spectroscopy (ToF-SIMS), atom probe tomography (APT), and transmission electron microscopy (TEM) is compared. Their effectiveness and shortcomings in determining carbon contamination are presented and discussed. The corresponding microstructural changes related to carbon contamination in ion irradiated F/M steels are also presented and briefly discussed.

A Precession Electron Diffraction and EELS Study of Beta-phase Evolution in Nano-crystalline Mg-9 wt.% Al Thin Films during Heat Treatment

Modern transmission electron microscopy (TEM) and spectroscopy techniques routinely probe the microstructure and local compositions in nanocrystalline materials using various analytical techniques such as electron energy-loss spectroscopy (EELS) or energy dispersive x-ray spectroscopy (EDX) maps. Electron diffraction is another technique the can help to identify phases in materials, but in applying such a technique, it is cumbersome to obtain statistically relevant information from nanomaterials. With the development of precession electron diffraction (PED) mapping and the use of image correlation for pattern identification, novel information can be obtained relatively rapidly that covers a much larger number of grains [1]. Combined with elemental mapping, this provides a powerful technique to characterize the distribution of phases and chemical distribution in nanomaterials.

Dynamical TEM Investigation of Solidification Kinetics in AZ91 Mg Alloys

The recent developments in climate and environmental politics have made the quest for new lightweight materials that will decrease a cars overall weight and fuel consumption a focus of interest for the automotive industry. In this regard, magnesium alloys have a high strength-to-weight ratio and are therefore a great candidate material to replace structural Al alloys and steels to reduce component weight. Al-containing Mg alloys have good mechanical strength and corrosion resistance at room temperature (RT), but these properties deteriorate at the elevated temperatures (60 200°C) that automotive components are commonly exposed to [1]. Understanding the microstructural evolution in this system is therefore key to the use of these lightweight alloys in the future.