Publications

Abstract not provided.

This work uses a combination of stress dependent single grain boundary Coble creep and zero-creep experiments to measure interfacial energies, along with grain boundary point defect formation and migration volumes in cubic ZrO2. These data, along with interfacial diffusivities measured in a companion paper are then applied to analyzing two-particle sintering. The analysis presented indicates that the large activation volume, v*=vf+vmprimarily derives from a large migration volume and suggests that the grain boundary rate limiting defects are delocalized, possibly due to electrostatic interactions between charge compensating defects. The discrete nature of the sintering and creep process observed in the small-scale experiments supports the hypothesis that grain boundary dislocations serve as sources and sinks for grain boundary point defects and facilitate strain during sintering and Coble creep. Model two-particle sintering experiments demonstrate that initial-stage densification follows interface reaction rate-limited kinetics.

Abstract not provided.

This work demonstrates novel in situ transmission electron microscopy-based microscale single grain boundary Coble creep experiments used to grow nanowires through a solid-state process in cubic ZrO2 between ≈ 1200 °C and ≈ 2100 °C. Experiments indicate Coble creep drives the formation of nanowires from asperity contacts during tensile displacement, which is confirmed by phase field simulations. The experiments also facilitate efficient measurement of grain boundary diffusivity and surface diffusivity. 10 mol% Sc2O3 doped ZrO2 is found to have a cation grain boundary diffusivity of [Formula presented], and surface diffusivity of [Formula presented].

Abstract not provided.

Radiation damage can cause significantly more surface damage in metallic nanostructures than bulk materials. Structural changes from displacement damage compromise the performance of nanostructures in radiation environments such as nuclear reactors and outer space, or used in radiation therapy for biomedical treatments. As such, it is important to develop strategies to prevent this from occurring if nanostructures are to be incorporated into these applications. Here, in situ transmission electron microscope ion irradiation was used to investigate whether a metallic glass (MG) coating mitigates sputtering and morphological changes in metallic nanostructures. Dislocation-free Au nanocubes and Au nanocubes coated with a Ni-B MG were bombarded with 2.8 MeV Au4+ ions. The formation of internal defects in bare Au nanocubes was observed at a fluence of 7.5 × 1011 ions/cm2 (0.008 dpa), and morphological changes such as surface roughening, rounding of corners, and formation of nanofilaments began at 4 × 1012 ions/cm2 (0.04 dpa). In contrast, the Ni-B MG-coated Au nanocubes (Au@NiB) showed minimal morphological changes at a fluence of 1.9 × 1013 ions/cm2 (0.2 dpa). The MG coating maintains its amorphous nature under all irradiation conditions investigated.

Abstract not provided.

Abstract not provided.

In this article, we studied the total ionization dose (TID) effects on the multilevel-cell (MLC) 3-D NAND flash memory using Co-60 gamma radiation. We found a significant page-to-page bit error variation within a physical memory block of the irradiated memory chip. Our analysis showed that the origin of the bit error variation is the unique vertical layer-dependent TID response of the 3-D NAND. We found that the memory pages located at the upper and lower layers of the 3-D stack show higher fails compared to the middle-layer pages of a given memory block. We confirmed our findings by comparing radiation response of four different chips of the same specification. In addition, we compared the TID response of the MLC 3-D NAND with that of the 2-D NAND chip, which showed less page-to-page variation in bit error within a given memory block. We discuss the possible application of our findings for the radiation-tolerant smart memory controller design.

Abstract not provided.

Abstract not provided.

The concept of coating the currently used nuclear fuel cladding (zirconium-based alloy, typically Zircaloy-4 or Zirc-4) with an oxidation preventive layer is a progressing Accident tolerant Fuel (ATF) candidate alloys. The coated Zirc-4-based alloys could be a solution to suppress undesirable fast reaction kinetics with high-temperature steam. Zirc-4 has been the most preferred cladding material in pressurized water reactors (PWRs). Chromium (Cr) based alloys as a coating material provides excellent corrosion protection and good strength and wear resistance. This paper presents the surface wettability measurements and pool boiling Critical Heat Flux (CHF) for Cr-coated Zirc-4 claddings pre- and post-exposure to an ion irradiation environment. The wettability measurements, including static contact angle (contact angle, θ) and average surface roughness (surface roughness, Ra), are introduced for samples of different coating thicknesses (5–30 μm thick). The coatings fabricated by the cold spray of Cr-Al particles to 10 mm × 10 mm × 1.95 mm Zirc-4 substrates. Post fabrication, a Pilgering (cold rolling) process, was applied to finalize the coating thickness and resulted in a significant reduction in surface roughness of initially fabricated rough surfaces. The process produced three distinguished samples 5-μm unpolished (as machined), 5-μm, and 30-μm polished (cold rolled). The measurements are presented for the three surfaces and bare Zirc-4 as a baseline surface. The contact angle analyses were implemented in theoretical models from the literature to predict pool boiling CHF. Pool boiling experiments were conducted to measure the pool boiling CHF values and compare them to the predicted values. Scanning Electron Microscope (SEM) images and Energy Dispersive X-ray Spectroscopy (EDS) analysis was performed to characterize the surfaces for better understanding and interpreting the results. The SEM images showed localized surface damage due to ion irradiation. No recognized change in the measured surface roughness due to ion irradiation. The contact angles of irradiated Cr-coated surfaces are consistently higher (10°) than pre-irradiated surfaces. Decreasing the Cr-coating layer thickness resulted in lower contact angle pre- and post- ion irradiation. The predicted pool boiling CHF using the Kandlikar model is in good agreement with the experimentally measured CHF values within ±12% for all samples.

Abstract not provided.

The high-cycle fatigue life of nanocrystalline and ultrafine-grained Ni-Fe was examined for five distinct grain sizes ranging from approximately 50–600 nm. The fatigue properties were strongly dependent on grain size, with the endurance limit changing by a factor of 4 over this narrow range of grain size. The dataset suggests a breakdown in fatigue improvement for the smallest grain sizes <100 nm, likely associated with a transition to grain coarsening as a dominant rate-limiting mechanism. The dataset also is used to explore fatigue prediction from monotonic tensile properties, suggesting that a characteristic flow strength is more meaningful than the widely-utilized ultimate tensile strength.

Noble gases are generated within solids in nuclear environments and coalesce to form gas stabilized voids or cavities. Ion implantation has become a prevalent technique for probing how gas accumulation affects microstructural and mechanical properties. Transmission electron microscopy (TEM) allows measurement of cavity density, size, and spatial distributions post-implantation. While post-implantation microstructural information is valuable for determining the physical origins of mechanical property degradation in these materials, dynamic microstructural changes can only be determined by in situ experimentation techniques. We present in situ TEM experiments performed on Pd, a model face-centered cubic metal that reveals real-time cavity evolution dynamics. Observations of cavity nucleation and evolution under extreme environments are discussed.

Practical applications of nanocrystalline metallic thin films are often limited by instabilities. In addition to grain growth, the thin film itself can become unstable and collapse into islands through solid-state dewetting. Selective alloying can improve nanocrystalline stability, but the impact of this approach on dewetting is not clear. In this study, two alloys that exhibit nanocrystalline thermal stability as ball milled powders are evaluated as thin films. While both alloys demonstrated dewetting behavior following annealing, the severity decreased in more dilute compositions. Ultimately, a balance may be struck between nanocrystalline stability and thin film structural stability by tuning dopant concentration.

Nanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability. In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

This work demonstrates a novel approach to ultrahigherature mechanical testing using a combination of in situ nanomechanical testing and localized laser heating. The methodology is applied to characterizing and testing initially nanograined 10 mol % Sc2O3-stabilized ZrO2 up to its melting temperature. The results suggest that the lowerature strength of nanograined, d < 50 nm, oxides is not influenced by creep. Tensile fracture of ZrO2 bicrystals produce a weakerature dependence suggesting that grain boundary energy dominates brittle fracture of grain boundaries even at high homologous temperatures; for example, T = 2050 °C or T ≈ 77% Tmelt. The maximum temperature for mechanical testing in this work is primarily limited by the instability of the sample, due to evaporation or melting, enabling a host of new opportunities for testing materials in the ultrahigherature regime.

Abstract not provided.

Abstract not provided.

Abstract not provided.