Brian C. Jolly

Relationships Between Abrasive Wear, Hardness, and Grinding Characteristics of Titanium-Based Metal-Matrix Composites

The objective of this work was to support the development of grinding models for titanium metal-matrix composites (MMCs) by investigating possible relationships between their indentation hardness, low-stress belt abrasion, high-stress belt abrasion, and the surface grinding characteristics. Three Ti-based particulate composites were tested and compared with the popular titanium alloy Ti-6Al-4V. The three composites were a Ti-6Al-4V-based MMC with 5% TiB2 particles, a Ti-6Al-4V MMC with 10% TiC particles, and a Ti-6Al-4V/Ti-7.5%W binary alloy matrix that contained 7.5% TiC particles. Two types of belt abrasion tests were used: (a) a modified ASTM G164 low-stress loop abrasion test, and (b) a higher-stress test developed to quantify the grindability of ceramics. Results were correlated with G-ratios (ratio of stock removed to abrasives consumed) obtained from an instrumented surface grinder. Brinell hardness correlated better with abrasion characteristics than microindentation or scratch hardness. Wear volumes from low-stress and high-stress abrasive belt tests were related by a second-degree polynomial. Grindability numbers correlated with hard particle content but were also matrix-dependent.

High-Temperature Galling Characteristics of TI-6AL-4V with and without Surface Treatments

Galling is a severe form of surface damage in metals and alloys that typically arises under relatively high normal force and low sliding speed and in the absence of effective lubrication. It can lead to macroscopic surface roughening and seizure. The occurrence of galling can be especially problematic in high-temperature applications like diesel engine exhaust gas recirculation system components and adjustable turbocharger vanes, because suitable lubricants may not be available, moisture desorption promotes increased adhesion, and the yield strength of metals decreases with temperature. Oxidation can counteract these effects to some extent by forming lubricative oxide films. Two methods to improve the galling resistance of titanium alloy Ti-6Al-4V were investigated: (a) applying an oxygen diffusion treatment and (b) creating a metal–matrix composite with TiB2 using a high-intensity infrared heating source. A new oscillating three-pin-on-flat, high-temperature test method was developed and used to characterize galling behavior relative to a cobalt-based alloy (Stellite 6B, HP Alloys, Windfall, IN). The magnitude of the oscillating torque, the surface roughness, and observations of surface damage were used as measures of galling resistance. Due to the formation of lubricative oxide films, the galling resistance of the Ti alloy at 485°C, even nontreated, was considerably better than it was at room temperature. The infrared (IR)-formed composite displayed reduced surface damage and lower torque than the substrate titanium alloy.

Extrusion Development of Graphite-Based Composite Fuel for Nuclear Thermal Propulsion

Graphite-based composite fuel forms for Nuclear Thermal Propulsion (NTP) technology are being developed at ORNL. This effort involves process development for extrusion and heat treatment of intermediate length fuel elements representative of historical NERVA fuel. Earlier efforts at ORNL involved identifying extrusion feedstock materials, extrusion equipment, and fabrication techniques based on historical work. This information was utilized to procure representative materials and establish an extrusion capability. Using the new equipment and applying information learned during earlier lab scale work, several extrusion studies were conducted. The focus of these studies included determining appropriate blending methods for the feedstock materials, range of binder fraction, extrusion die and layoff table requirements, and the development of an interconnected carbide network during heat treatment. The results from these studies are reported including microstructural analysis and characterization of a high temperature heat treated element. SEM images of specimens both before and after heat treatment were used to determine changes in microstructure. Large regions of coalesced particles after heat treatment indicate that the desired interconnected carbide network was achieved. Additional characterization included density, compressive strength, and coefficient of thermal expansion (CTE). Since it is important that fuel elements from this work be representative of the original elements, these results were compared to historical values. Physical properties data from the recently fabricated element show good agreement with some historical data.

Coating Development on Graphite-Based Composite Fuel for Nuclear Thermal Propulsion

ORNL is currently recapturing graphite-based fuel forms for Nuclear Thermal Propulsion (NTP). Previous work at ORNL focused on reviewing historic fuel technology developed during the ROVER/NERVA programs and performing coating development at the lab scale. The current effort focuses on transitioning the coating work from the lab scale to an intermediate length, and coating an element with a more prototypic geometry to include a hexagonal cross section and multiple internal channels. A new vertical multi-zone furnace capable of coating 16” length elements was installed and the ability to deposit a ZrC coating along the full length of the element demonstrated. A chemical vapor deposition (CVD) process was used to apply a ZrC coating where zirconium metal was chlorinated insitu forming ZrCl4 as the Zr precursor and CH4 was used for the carbon source. The coating and additional diluent gases were directed through the internal channels using custom fixtures. While processing conditions were not fully optimized, good progress was made in coating development. Between the information gained and new capabilities installed, an excellent foundation for further fuel development and eventual qualification has been put into place.

Microstructure, Morphology, and Nanomechanical Properties Near Fine Holes Produced by Electro-Discharge Machining

Fine holes in metal alloys are employed for many important technological purposes, including cooling and the precise atomization of liquids. For example, they play an important role in the metering and delivery of fuel to the combustion chambers in energy-efficient, low-emission diesel engines. Electro-discharge machining (EDM) is one process employed to produce such holes. Since the hole shape and bore morphology can affect fluid flow, and holes also represent structural discontinuities in the tips of the spray nozzles, it is important to understand the microstructures adjacent to these holes, the features of the hole walls, and the nanomechanical properties of the material that was in some manner altered by the EDM hole-making process. Several techniques were used to characterize the structure and properties of spray-holes in a commercial injector nozzle. These include scanning electron microscopy, cross sectioning and metallographic etching, bore surface roughness measurements by optical interferometry, scanning electron microscopy, and transmission electron microscopy of recast EDM layers extracted with the help of a focused ion beam.

Development of a High-Temperature Repetitive-Impact Apparatus for Exhaust Valve Material Testing

Some types of mating parts, such as exhaust valves and seats in diesel engines, must perform at elevated temperatures and in oxidizing environments, while at the same time resisting the effects of repetitive impacts and interfacial slip. The wear that takes place under such situations is the net result of a complex process that involves plastic deformation, tangential shear, and oxidation. Tribolayers form, are removed, and reform. An apparatus was designed to simulate the key aspects of elevated temperature wear of exhaust valves. Three degrees of motion: impact, slip, and rotation were taken into account. Two different test geometries were developed: (a) inclined cylindrical pins striking rounded corners, and (b) actual exhaust valve sealing surfaces striking the edges of flat blocks. The features of the high-temperature repetitive impact device are described, and examples of the wear scars produced by the two test configurations are presented.Copyright

Carbothermic Synthesis of ~820- m UN Kernels. Investigation of Process Variables

This report details the continued investigation of process variables involved in converting sol-gel-derived, urainia-carbon microspheres to ~820-μm-dia. UN fuel kernels in flow-through, vertical refractory-metal crucibles at temperatures up to 2123 K. Experiments included calcining of air-dried UO3-H2O-C microspheres in Ar and H2-containing gases, conversion of the resulting UO2-C kernels to dense UO2:2UC in the same gases and vacuum, and its conversion in N2 to in UC1-xNx. The thermodynamics of the relevant reactions were applied extensively to interpret and control the process variables. Producing the precursor UO2:2UC kernel of ~96% theoretical density was required, but its subsequent conversion to UC1-xNx at 2123 K was not accompanied by sintering and resulted in ~83-86% of theoretical density. Decreasing the UC1-xNx kernel carbide component via HCN evolution was shown to be quantitatively consistent with present and past experiments and the only useful application of H2 in the entire process.

Recapturing Graphite-Based Fuel Element Technology for Nuclear Thermal Propulsion

ORNL is currently recapturing graphite based fuel forms for Nuclear Thermal Propulsion (NTP). This effort involves research and development on materials selection, extrusion, and coating processes to produce fuel elements representative of historical ROVER and NERVA fuel. Initially, lab scale specimens were fabricated using surrogate oxides to develop processing parameters that could be applied to full length NTP fuel elements. Progress toward understanding the effect of these processing parameters on surrogate fuel microstructure is presented. I. Introduction HE research presented in this report is a collaborative effort between Oak Ridge National Laboratory (ORNL) and NASA to recapture manufacturing technology for full length ROVER/NERVA graphite composite fuel elements. Nuclear thermal propulsion (NTP) fuel development has been intermittently ongoing since the late 1950’s and many of the original materials used in the early fuel development are no longer available. Also, the processing capability and the art associated with the production of full-length elements have been lost. The focus of the collaboration is to recapture the capability and expertise to produce representative fuel element test samples and iteratively scale up to full-length elements. To maximize efficiency, the work was separated into two tasks, extrusion development and coating development, which were conducted in parallel. At this stage in the program, the extrusion development task is focused on recreating a representative blend of materials, evaluating blending methods, and establishing an extrusion capability. The coating task is focused on developing processing conditions and equipment to establish ZrC coating capability. This report summarizes the accomplishments and progression toward these goals. It is important to note that the results and analyses presented here are in the early stages of research (TRL 3) and should be considered preliminary.

M3FT-15OR0202237: Submit Report on Results From Initial Coating Layer Development For UN TRISO Particles

In support of fully ceramic matrix (FCM) fuel development, coating development work has begun at the Oak Ridge National Laboratory (ORNL) to produce tri-isotropic (TRISO) coated fuel particles with UN kernels. The nitride kernels are used to increase heavy metal density in these SiC-matrix fuel pellets with details described elsewhere. The advanced gas reactor (AGR) program at ORNL used fluidized bed chemical vapor deposition (FBCVD) techniques for TRISO coating of UCO (two phase mixture of UO2 and UCx) kernels. Similar techniques were employed for coating of the UN kernels, however significant changes in processing conditions were required to maintain acceptable coating properties due to physical property and dimensional differences between the UCO and UN kernels.

Design Study for a Low-Enriched Uranium Core for the High Flux Isotope Reactor, Annual Report for FY 2008

This report documents progress made during FY 2008 in studies of converting the High Flux Isotope Reactor (HFIR) from highly enriched uranium (HEU) fuel to low-enriched uranium (LEU) fuel. Conversion from HEU to LEU will require a change in fuel form from uranium oxide to a uranium-molybdenum alloy. With axial and radial grading of the fuel foil and an increase in reactor power to 100 MW, calculations indicate that the HFIR can be operated with LEU fuel with no degradation in reactor performance from the current level. Results of selected benchmark studies imply that calculations of LEU performance are accurate. Scoping experiments with various manufacturing methods for forming the LEU alloy profile are presented.