Larry E. Thomas

Void formation during early stages of passivation: Initial oxidation of iron nanoparticles at room temperature

The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies in which sample preparation artifacts have been the cause of some uncertainties. Microstructure of iron nanoparticles passivated with iron oxide shell was studied using high-resolution transmission electron microscopy and high-angle annular dark-field imaging in aberration-corrected scanning transmission electron microscopy. Voids were readily observed on both small single-crystal α-Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticles has been engineered for Co at higher temperatures [Y. Yin, R. M. Riou, C. K. Erdonmez, S. Hughes, G. A. Somorjari, and A. P. Alivisatos, Science 304, 711 (2004)], they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. There exists a critical size of ∼8n...

High-Resolution Characterization of Intergranular Attack and Stress Corrosion Cracking of Alloy 600 in High-Temperature Primary Water

Abstract Intergranular (IG) attack regions and stress corrosion cracks in Alloy 600 (UNS N06600) U-bend samples tested in 330°C pressurized water reactor water were characterized by analytical tran...

Defect clustering in GaN irradiated with O + ions

Transmission electron microscopy (TEM) is used to study microstructures formed in GaN irradiated with 600 keV O ions at room temperature. Three types of defect clusters are identified in the irradiated GaN: (1) basal-plane stacking faults with dimensions ranging from 5 to 30 nm, (2) Pyramidal dislocation loops, and (3) local regions of highly-disordered material. High resolution TEM imaging clearly reveals that one type of the basal-plane stacking faults corresponds to insertion of one extra Ga-N basal-plane in the otherwise perfect GaN lattice. The interpretation of these results indicates that interstitials of both Ga and N preferentially condense on the basal plane to form a new layer of Ga-N under these irradiation conditions. The formation of these extended defects and their interactions with the point defects produced during irradiation contribute to a dramatic increase in the dynamic recovery of point defects in GaN at room temperature.

Linking Grain Boundary Microstructure to Stress Corrosion Cracking of Cold-Rolled Alloy 690 in Pressurized Water Reactor Primary Water

Grain boundary microstructures and microchemistries are examined in cold-rolled Alloy 690 (UNS N06690) materials and comparisons are made to intergranular stress corrosion cracking (IGSCC) behavior in pressurized water reactor (PWR) primary water. Chromium carbide precipitation is found to be a key aspect for materials in both the mill-annealed and thermally treated conditions. Cold rolling to high levels of reduction was discovered to produce small IG voids and cracked carbides in alloys with a high density of grain boundary carbides. The degree of permanent grain boundary damage from cold rolling was found to depend directly on the initial IG carbide distribution. For the same degree of cold rolling, alloys with few IG precipitates exhibited much less permanent damage. Although this difference in grain boundary damage appears to correlate with measured SCC growth rates, crack tip examinations reveal that cracked carbides appeared to blunt propagation of IGSCC cracks in many cases. Preliminary results su...

Microstructure of Precipitated Au Nanoclusters in MgO

Gold nanoclusters dispersed in single crystal MgO have been prepared by ion implantation at 975 K and subsequent annealing at 1275 K for 10 h. The morphological features, size, and crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, as well as the interface structure between the Au nanoclusters and MgO, have been investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010]MgO//[010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. Above a critical size of ∼5 to 8 nm, a coherent-semicoherent interface transition is observed for the Au clusters in MgO. This critical cluster size is larger than the critical size ∼3 nm based on energetic considerations. This discrepancy is discussed with respect to the point and extended defect structures at the interface between the Au clusters and the MgO matrix. The Au clusters larger than this critic...

Microstructural evolution of Alloy 718 at high helium and hydrogen generation rates during irradiation with 600–800 MeV protons

When precipitation hardened Alloy 718 is irradiated with high-energy protons (600–800 MeV) and spallation neutrons at temperatures below ∼60∘C, it quickly hardens and loses almost all uniform elongation. It later softens somewhat at higher exposures but does not regain any elongation. This behavior is explained in terms of the evolution of Frank loop formation, disordering and eventual dissolution of the γ′ and γ″ strengthening phases, and the steady accumulation of very large levels of helium and hydrogen. These gases must be dispersed on a very fine scale in the matrix since no cavities could be found.

Penetrative Internal Oxidation from Alloy 690 Surfaces and Stress Corrosion Crack Walls during Exposure to PWR Primary Water

Analytical electron microscopy and three-dimensional atom probe tomography (ATP) examinations of surface and near-surface oxidation have been performed on Ni-30%Cr alloy 690 materials after exposure to high-temperature, simulated PWR primary water. The oxidation nanostructures have been characterized at crack walls after stress-corrosion crack growth tests and at polished surfaces of unstressed specimens for the same alloys. Localized oxidation was discovered for both crack walls and surfaces as continuous filaments (typically <10 nm in diameter) extending from the water interface into the alloy 690 matrix reaching depths of ~500 nm. These filaments consisted of discrete, plate-shaped Cr2O3 particles surrounded by a distribution of nanocrystalline, rock-salt (Ni-Cr-Fe) oxide. The oxide-containing filament depth was found to increase with exposure time and, at longer times, the filaments became very dense at the surface leaving only isolated islands of metal. Individual dislocations were oxidized in non-deformed materials, while the oxidation path appeared to be along more complex dislocation substructures in heavily deformed materials. This paper will highlight the use of high resolution scanning and transmission electron microscopy in combination with APT to better elucidate the microstructure and microchemistry of the filamentary oxidation.

Microstructural features of Al-implanted 4H–SiC

The microstructural features of highly-damaged 4H-SiC implanted with Al ions at 450 K have been studied using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Conventional TEM images reveal that the crystalline SiC domains are highly strained/distorted when the relative disorder on the Si sublattice ranges between about 0.4 and 0.8, as determined by Rutherford backscattering spectrometry in channeling geometry (RBS/C). As the relative disorder approaches to 1.0, the high strain contrast appears to be relieved and localized amorphized domains are observed. Plasmon-loss energy shows a red-shift following the implantation, and the magnitude of the red-shift increases with increasing relative disorder. Based on the red-shift, the estimated volume expansion is {approx} 8% for highly-damaged crystalline SiC and {approx} 16% for the amorphous state. Energy-loss near-edge-structure (ELNES) of both the C and Si K-edge reveals the existence of Si-Si and C-C bonding in the Al-implanted SiC.

High‐Resolution Characterizations of Grain Boundary Damage and Stress Corrosion Cracks in Cold‐Rolled Alloy 690

Unidirectional cold rolling has been shown to promote intergranular stress corrosion cracking (IGSCC) in alloy 690 tested in PWR primary water. High-resolution scanning (SEM) and transmission electron microscopy (TEM) have been employed to investigate the microstructural reasons for this enhanced susceptibility in two stages, first examining grain boundary damage produced by cold rolling and second by characterization of stress corrosion crack tips. The degree of permanent grain boundary damage from cold rolling was found to depend directly on the initial IG precipitate distribution. Cold rolling to high levels of reduction was discovered to produce small IG voids and cracked carbides in alloys with a high density of grain boundary carbides. For the same degree of cold rolling, alloys with few IG carbides exhibited much less permanent damage. Although this difference in grain boundary damage appears to correlate with measured SCC growth rates, crack tip examinations reveal no interaction between the preexisting voids and cracked carbides with the propagation. In many cases, these features appeared to blunt propagation of IGSCC cracks. High-resolution characterizations are described for cold-rolled alloy 690 CRDM tubing and plate materials to gain insights into IGSCC mechanisms.

Electrochemical Interpretation of a Stress Corrosion Cracking of Thermally Treated Ni base Alloys in a Lead Contaminated Water

Since the PbSCC(Lead stress corrosion cracking) of alloy 600 tubing materials was reported by Copson and Dean in 1965, the effect of lead on a corrosion film and cracking morphology have been continually debated. An electrochemical interaction of lead with the alloying elements of SG tubings was studied and the corrosion products were analyzed. It was found that lead enhanced the anodic dissolution of alloy 600 and alloy 690 in the electrochemical test. The lead preferentially dissolved the Cr from the corrosion film of alloy 600 and alloy 690 in alkaline water. The lead ion seemed to penetrate into the TG crack tip and react with the corrosion film. A selective Cr depletion was observed to weaken the stability of the passive film on the alloys. Whereas passivity of Ni became stable in lead containing solution, Cr and Fe passivity became unstable.