E.A. Kenik

Detection of Single Atoms and Buried Defects in Three Dimensions by Aberration-Corrected Electron Microscope with 0.5-Å Information Limit

The ability of electron microscopes to analyze all the atoms in individual nanostructures is limited by lens aberrations. However, recent advances in aberration-correcting electron optics have led to greatly enhanced instrument performance and new techniques of electron microscopy. The development of an ultrastable electron microscope with aberration-correcting optics and a monochromated high-brightness source has significantly improved instrument resolution and contrast. In the present work, we report information transfer beyond 50 pm and show images of single gold atoms with a signal-to-noise ratio as large as 10. The instruments new capabilities were exploited to detect a buried Sigma3 {112} grain boundary and observe the dynamic arrangements of single atoms and atom pairs with sub-angstrom resolution. These results mark an important step toward meeting the challenge of determining the three-dimensional atomic-scale structure of nanomaterials.

Atom Probe Tomography of Nanoscale Particles in ODS Ferritic Alloys

An atom probe tomography characterization of the microstructure of as-processed and crept mechanically-alloyed, oxide-dispersion-strengthened (MA/ODS) ferritic alloys has been performed. The significant enrichments of Cr, W, Ti, Y, O, C and B in the vicinity of dislocations and the presence of ultrastable 4-nm-diameter Ti-, Y- and O-enriched particles appears to be responsible for their improved high temperature mechanical properties.

Tensile and Creep Properties of an Oxide Dispersion-Strengthened Ferritic Steel

The tensile and creep properties of two oxide dispersion-strengthened (ODS) steels with nominal compositions of Fe–12Cr–0.25Y2O3 (designated 12Y1) and Fe–12Cr–2.5W–0.4Ti–0.25Y2O3 (12YWT) were investigated. Optical microscopy, transmission electron microscopy, and atom probe field ion microscopy studies indicated that the 12YWT contained a high density of extremely fine Y–Ti–O clusters, compared to the much larger oxide particles in the 12Y1. The fine dispersion of particles gave the 12YWT better tensile and creep properties compared to commercial ODS alloys and ferritic/martensitic steels that would be replaced by the new ODS steel.

Atom Probe Tomography: A Technique for Nanoscale Characterization

Atom probe tomography is a technique for the nanoscale characterization of microstructural features. Analytical techniques have been developed to estimate the size, composition, and other parameters of features as small as 1 nm from the atom probe tomography data. These methods are outlined and illustrated with examples of yttrium-, titanium-, and oxygen-enriched particles in a mechanically alloyed, oxide-dispersion-strengthened steel.

Isolating the effect of radiation-induced segregation in irradiation-assisted stress corrosion cracking of austenitic stainless steels

Post-irradiation annealing was used to help identify the role of radiation-induced segregation (RIS) in irradiation-assisted stress corrosion cracking (IASCC) by preferentially removing dislocation loop damage from proton-irradiated austenitic stainless steels while leaving the RIS of major and minor alloying elements largely unchanged. The goal of this study is to better understand the underlying mechanisms of IASCC. Simulations of post-irradiation annealing of RIS and dislocation loop microstructure predicted that dislocation loops would be removed preferentially over RIS due to both thermodynamic and kinetic considerations. To verify the simulation predictions, a series of post-irradiation annealing experiments were performed. Both a high purity 304L (HP-304L) and a commercial purity 304 (CP-304) stainless steel alloy were irradiated with 3.2 MeV protons at 360 °C to doses of 1.0 and 2.5 dpa. Following irradiation, post-irradiation anneals were performed at temperatures ranging from 400 to 650 °C for times between 45 and 90 min. Grain boundary composition was measured using scanning transmission electron microscopy with energy-dispersive spectrometry in both as-irradiated and annealed samples. The dislocation loop population and radiation-induced hardness were also measured in as-irradiated and annealed specimens. At all annealing temperatures above 500 °C, the hardness and dislocation densities decreased with increasing annealing time or temperature much faster than RIS. Annealing at 600 °C for 90 min removed virtually all dislocation loops while leaving RIS virtually unchanged. Cracking susceptibility in the CP-304 alloy was mitigated rapidly during post-irradiation annealing, faster than RIS, dislocation loop density or hardening. That the cracking susceptibility changed while the grain boundary chromium composition remained essentially unchanged indicates that Cr depletion is not the primary determinator for IASCC susceptibility. For the same reason, the visible dislocation microstructure and radiation-induced hardening are also not sufficient to cause IASCC alone.

On the mechanism of radiation-induced segregation in austenitic Fe-Cr-Ni alloys

Abstract The relative importance of the vacancy and interstitial contributions to radiation-induced segregation (RIS) in Fe–Cr–Ni alloys is studied to better understand the mechanisms causing changes in grain boundary composition and to improve the capability to predict RIS in austenitic Fe–Cr–Ni alloys. The primary driving mechanism for segregation in Fe–Cr–Ni alloys is shown to be the inverse Kirkendall (IK) mechanism, specifically the coupling between alloying elements and the vacancy flux. To study grain boundary segregation, seven alloys were irradiated with 3.2 MeV protons at temperatures from 200°C to 600°C and to doses from 0.1 to 3 dpa. Grain boundary compositions were measured using both Auger electron spectroscopy (AES) and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM/EDS). Grain boundary compositions were compared to model predictions that assume segregation was driven either by preferential interaction of solute atoms with the vacancy flux alone or in combination with binding of undersized solutes to the interstitial flux. Calculations that assume the segregation is caused by preferential interaction of solute atoms with the vacancy flux generally followed the trends of the segregation measurements. However, the inclusion of interstitial binding to the IK model causes poor agreement between model predictions and segregation measurements, resulting in severe overprediction of Ni enrichment and Fe depletion. Comparisons of segregation models with RIS in alloys irradiated with neutrons also show that preferential interaction of solutes with the vacancy flux sufficiently describes segregation in Fe–Cr–Ni alloys.

Large Discrete Resistance Jump at Grain Boundary in Copper Nanowire

Copper is the current interconnect metal of choice in integrated circuits. As interconnect dimensions decrease, the resistivity of copper increases dramatically because of electron scattering from surfaces, impurities, and grain boundaries (GBs) and threatens to stymie continued device scaling. Lacking direct measurements of individual scattering sources, understanding of the relative importance of these scattering mechanisms has largely relied on semiempirical modeling. Here we present the first ever attempt to measure and calculate individual GB resistances in copper nanowires with a one-to-one correspondence to the GB structure. Large resistance jumps are directly measured at the random GBs with a value far greater than at coincidence GBs and first-principles calculations. The high resistivity of the random GB appears to be intrinsic, arising from the scaling of electron mean free path with the size of the lattice relaxation region. The striking impact of random GB scattering adds vital information for understanding nanoscale conductors.

Binder deformation in WC-(Co, Ni) cemented carbide composites

The microstructural responses to monotonic and cyclic compressive loading of three WC-(Co,Ni) alloys have been characterized and measured by high voltage transmission electron microscopy and neutron diffraction. A base alloy comprising WC-17 wt pct Co was prepared and evaluated, along with two alloys in which the binder composition was altered by replacing 15 pct and 30 pct of the total cobalt by nickel. Results are presented for strains of 0, 0.75, and 5.0 pct, and for two fatigue stress levels, both at 0.5 million cycles. Predominant binder-deformation mechanisms were observed to shift with increasing Ni content from the fcc-hcp martensitic transformation to slip plus twinning over the composition range studied. In the base alloy, 44.5 vol pct of the binder had transformed at the highest strain level, while only 11.4 pct transformation occurred at the same strain in the 30 pct Ni-binder alloy. The shift in binder plasticity mechanisms and the corresponding changes in composite stressstrain behavior have been discussed with respect to several theories on the role of binder deformation in cermet mechanical response.

Radiation-induced solute segregation in a low swelling 316 stainless steel

Ni ion irradiations were carried out on a high-Ti, Si heat and a low-Ti, Si heat of 316 ss. The large difference in swelling behavior was confirmed, and the damage stages were studied using an electron microscope with energy dispersive x-ray analysis capabilities for determining Si segregation at dislocation loops. Possible mechanisms for the effect of Si segregation at dislocations on void growth are discussed. (DLC)

Structure and phase stability in a cast modified-HP austenite after long-term ageing

Abstract Phase transformations in a cast HP series alloy after long-term ageing were determined by analytical electron microscopy (AEM). Beyond the chromium and niobium carbides normally expected for the as-cast material, an eta phase enriched in Nb and Si is present, indicating phase instability at high temperatures (∼1000 °C).