Showing papers by "Gary S. Was published in 1997"

The role of coincidence-site-lattice boundaries in creep of Ni-16Cr-9Fe at 360 °C

Abstract: The objective of this study is to understand and quantify the role of the coincidence-site-lattice boundary (CSLB) population on creep deformation of Ni-16Cr-9Fe at 360 °C. It is hypothesized that an increase in the CSLB population decreases the annihilation rate of dislocations in the grain boundary, leading to an increase in the internal stress and a decrease in the effective stress. The result is a reduction in the creep strain rate. The role of CSLBs in deformation is, thus, to increase the internal stress by trapping run-in lattice dislocations at the grain boundaries as extrinsic grain boundary dislocations (EGBDs), creating backstresses on following dislocations rather than annihilating them, as in the case of high-angle boundaries (HABs). The hypothesis was substantiated by showing (1) that dislocation absorption kinetics differ substantially between a CSLB and an HAB, and (2) that the CSLB fraction strongly affects the internal stress in the solid. Dislocation absorption kinetics were measured by comparing EGBD density in transmission electron microscopy (TEM). Results showed that CSLBs contain an EGBD density which is 3 times higher than HABs at 1.25 pct strain. Internal stress was measured by the stress dip test and was found to be ≈ 30 MPa higher in the CSLB-enhanced sample. Steady-state creep rates of Ni-16Cr-9Fe in 360 °C argon were also found to be strongly affected by the grain boundary character distribution. Increasing the CSLB fraction by approximately a factor of 2 resulted in a decrease in steady-state creep rates by a factor of 8 to 26 in coarse-grain (330 µm) samples and a factor of 40 to 66 in small-grain (35 µm) samples. It is postulated that annihilation of EGBDs only occurs at triple lines where at least two HABs intersect. By using a geometric relationship to evaluate the probability of EGBDs annihilating at a triple line, the model predicts a non-linear dependence of the creep rate with CSLB fraction, yielding excellent correlation with measurement. The model provides a physical basis for measurements which show that increasing the CSLB fraction by only moderate amounts can greatly reduce the steady-state creep rate in Ni-16Cr-9Fe.

Microstructure evolution during irradiation

Abstract: The symposium focused on the microstructural changes produced in semiconductors, metals, ceramics and polymers by irradiation with energetic particles. The symposium provided an opportunity to bring together those working in different materials systems and revealed that there are a remarkable number of similarities in the changes produced by irradiation in the different classes of materials. Experimental, computational and theoretical contributions were intermixed throughout the sessions, which provided an opportunity for these groups to interact. Separate abstracts were prepared for most papers in this volume.

Effect of ion bombardment on in-plane texture, surface morphology, and microstructure of vapor deposited Nb thin films

Abstract: Niobium films were deposited by physical vapor deposition (PVD) and ion-beam-assisted deposition (IBAD) using ion energies of 0, 250, 500 and 1000 eV, and R ratios (ion-to-atom arrival rate ratio) of 0, 01, and 04 on (100) silicon, amorphous glass, and (0001) sapphire substrates of thickness 50–1000 nm Besides a {110} fiber texture, an in-plane texture was created by orienting the ion beam with respect to the substrate The in-plane texture as measured by the degree of orientation was strongly dependent on both ion-beam energy and the R ratio In fact, the degree of orientation in the films followed a linear relationship with the energy per deposited atom, En The grain structure was columnar and the column width increased with normalized energy The surface morphology depended on both the normalized energy of the ion beam and the film thickness All films had domelike surface features that were oriented along the ion-beam incident direction The dimension of these features increased with normalized en

Beam-broadening effects in STEM/EDS measurement of radiation-induced segregation in high-purity 304L stainless steel

Abstract: Radiation-induced segregation (RIS) is the spatial redistribution of elements at defect sinks such as grain boundaries and free surfaces during irradiation This phenomenon has been studied in a wide variety of alloys and has been linked to irradiation-assisted stress corrosion cracking (IASCC) of nuclear reactor core components Therefore, accurate determination of the grain boundary composition is important in understanding its effects on environmental cracking Radiation-induced segregation profiles are routinely measured by scanning-transmission electron microscopy using energy-dispersive X-ray spectroscopy (STEM-EDS) and Auger electron spectroscopy (AES) Because of the narrow width of the segregation profile (typically less than 10-nm full width at half-maximum), the accuracy of grain boundary concentration measurements using STEM/EDS depends on the characteristics of the analyzing instrument, specifically, the excited volume in which x-rays are generated This excited volume is determined by both electron beam diameter and the primary electron beam energy Increasing the primary beam energy in STEM/EDS produces greater measured grain boundary segregation, as the reduced electron beam broadening a smaller excited volume In this work, the effect of beam broadening is assessed on segregation measurements in a 304L stainless steel sample irradiated with 32 MeV protons at 400 C to doses of 30 and 01 dpa The STEM/EDS measurements are also compared to measurements made using AES

Microstructure and mechanical properties of IBAD niobium films on sapphire substrates

Abstract: The effect of ion bombardment during film deposition on the microstructure, surface morphology, residual stress, hardness and interfacial fracture energy was determined for Nb films on sapphire substrates. In the present study, niobium films of nominal thickness 100 nm were deposited on (0001) sapphire substrates by Physical Vapor Deposition (PVD) and by Ion Beam Assisted Deposition (IBAD) at an ion energy E = 1000 eV and an ion-to-atom arrival rate ratio R = 0.4. The microstructures of the PVD and IBAD films were very similar, but the IBAD film had a much rougher surface. Ion bombardment also changed the residual stress state from tensile (224 MPa) for the PVD film to compressive (−400 MPa) for the IBAD film. The IBAD film had a lower hardness (5.7 GPa) than the PVD film (10.9 GPa) as determined by nanoindentation. An attempt was made to measure the fracture energy of both the PVD niobium-sapphire and IBAD niobium-sapphire interfaces by scratch test and nanoindentation. The PVD sample failed in a brittle manner in both tests and interfacial fracture energies were able to be estimated. By contrast, the IBAD sample failed in a ductile manner in both tests, precluding the determination of the interfacial fracture energy.