Hassina Z. Bilheux

Site specific control of crystallographic grain orientation through electron beam additive manufacturing

Abstract Site specific control of the crystallographic orientation of grains within metal components has been unachievable before the advent of metals additive manufacturing (AM) technologies. To demonstrate the capability, the growth of highly misoriented micron scale grains outlining the letters D, O and E, through the thickness of a 25·4 mm tall bulk block comprised of primarily columnar [001] oriented grains made of the nickel base superalloy Inconel 718 was promoted. To accomplish this, electron beam scan strategies were developed based on principles of columnar to equiaxed transitions during solidification. Through changes in scan strategy, the electron beam heat source can rapidly change between point and line heat source modes to promote steady state and/or transient thermal gradients and liquid/solid interface velocity. With this approach, an equiaxed solidification in the regions bounding the letters D, O and E was achieved. The through thickness existence of the equiaxed grain structure outlining the letters within a highly columnar [001] oriented bulk was confirmed through characterizing the bulk specimen with energy selective neutron radiography and confirming with an electron backscatter detection. Ultimately, this demonstration promotes the ability to build metal components with site specific control on crystallographic orientation of grains using the electron beam melting process.

Neutron Imaging and Applications

Neutron Imaging and Applications offers an introduction to the basics of neutron beam production and instrumentation in addition to the wide scope of techniques that provide unique imaging capabilities over a broad and diverse range of applications. An instructional overview of neutron sources, optics and detectors, allows readers to delve more deeply into the discussions of radiography, tomography, phase contrast imaging and prospective applications using advanced neutron holography techniques and polarized beams. A section devoted to overviews in a growing range of applications describes imaging of fuel cells and hydrogen storage devices for a robust hydrogen economy; new directions in material science and engineering; the investigation of precious artifacts of cultural heritage importance; determination of plant physiology and growth processes; imaging of biological tissues and macromolecules, and the practical elements of neutron imaging for homeland security and contraband detection. Written by key experts in the field, researchers and engineers involved with imaging technologies will find Neutron Imaging and Applications a valuable reference.

Transport of Ions in Mesoporous Carbon Electrodes during Capacitive Deionization of High-Salinity Solutions

Desalination of high-salinity solutions has been studied using a novel experimental technique and a theoretical model. Neutron imaging has been employed to visualize lithium ions in mesoporous carbon materials, which are used as electrodes in capacitive deionization (CDI) for water desalination. Experiments were conducted with a flow-through CDI cell designed for neutron imaging and with lithium-6 chloride ((6)LiCl) as the electrolyte. Sequences of neutron images have been obtained at a relatively high concentration of (6)LiCl solution to provide information on the transport of ions within the electrodes. A new model that computes the individual ionic concentration profiles inside mesoporous carbon electrodes has been used to simulate the CDI process. Modifications have also been introduced into the simulation model to calculate results at high electrolyte concentrations. Experimental data and simulation results provide insight into why CDI is not effective for desalination of high ionic-strength solutions. The combination of experimental information, obtained through neutron imaging, with the theoretical model will help in the design of CDI devices, which can improve the process for high ionic-strength solutions.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ~55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ~30 mA energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mA level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ~few hundreds μistrain for 300s of data acquisition time.

Neutron imaging of ion transport in mesoporous carbon materials.

Neutron imaging is presented as a tool for quantifying the diffusion of ions inside porous materials, such as carbon electrodes used in the desalination process via capacitive deionization and in electrochemical energy-storage devices. Monolithic mesoporous carbon electrodes of ∼10 nm pore size were synthesized based on a soft-template method. The electrodes were used with an aqueous solution of gadolinium nitrate in an electrochemical flow-through cell designed for neutron imaging studies. Sequences of neutron images were obtained under various conditions of applied potential between the electrodes. The images revealed information on the direction and magnitude of ion transport within the electrodes. From the time-dependent concentration profiles inside the electrodes, the average value of the effective diffusion coefficient for gadolinium ions was estimated to be 2.09 ± 0.17 × 10(-11) m(2) s(-1) at 0 V and 1.42 ± 0.06 × 10(-10) m(2) s(-1) at 1.2 V. The values of the effective diffusion coefficient obtained from neutron imaging experiments can be used to evaluate model predictions of the ion transport rate in capacitive deionization and electrochemical energy-storage devices.

Neutron imaging at the Oak Ridge National Laboratory: Application to biological research

The Oak Ridge National Laboratory Neutron Sciences Directorate (NScD) has recently installed a neutron imaging beamline at the High Flux Isotope Reactor (HFIR) cold guide hall. The CG-1D beamline supports a broad range of user research spanning from engineering to material research, energy storage, additive manufacturing, vehicle technologies, archaeology, biology, and plant physiology. The beamline performance (spatial resolution, field of view, etc.) and its utilization for biological research are presented. The NScD is also considering a proposal to build the VENUS imaging beamline (beam port 10) at the Spallation Neutron Source (SNS). Unlike CG-1D which provides cold neutrons, VENUS will offer a broad range of neutron wavelengths, from epithermal to cold, and enhanced contrast mechanisms. This new capability will also enable the imaging of thicker biological samples than is currently available at CG-1D. A brief overview of the VENUS capability for biological research is discussed.

Neutron Imaging of Diesel Particulate Filters

This article presents nondestructive neutron computed tomography (nCT) measurements of Diesel Particulate Filters (DPFs) as a method to measure ash and soot loading in the filters. Uncatalyzed and unwashcoated 200cpsi cordierite DPFs exposed to 100% biodiesel (B100) exhaust and conventional ultra low sulfur 2007 certification diesel (ULSD) exhaust at one speed-load point (1500rpm, 2.6bar BMEP) are compared to a brand new (never exposed) filter. Precise structural information about the substrate as well as an attempt to quantify soot and ash loading in the channel of the DPF illustrates the potential strength of the neutron imaging technique.

Flexible sample environment for high resolution neutron imaging at high temperatures in controlled atmosphere

High material penetration by neutrons allows for experiments using sophisticated sample environments providing complex conditions. Thus, neutron imaging holds potential for performing in situ nondestructive measurements on large samples or even full technological systems, which are not possible with any other technique. This paper presents a new sample environment for in situ high resolution neutron imaging experiments at temperatures from room temperature up to 1100 °C and/or using controllable flow of reactive atmospheres. The design also offers the possibility to directly combine imaging with diffraction measurements. Design, special features, and specification of the furnace are described. In addition, examples of experiments successfully performed at various neutron facilities with the furnace, as well as examples of possible applications are presented. This covers a broad field of research from fundamental to technological investigations of various types of materials and components.

Investigation of coded source neutron imaging at the north carolina state university PULSTAR reactor

A neutron imaging facility is located on beam-tube #5 of the 1-MWth PULSTAR reactor at the North Carolina State University. An investigation has been initiated to explore the application of coded imaging techniques at the facility. Coded imaging uses a mosaic of pinholes to encode an aperture, thus generating an encoded image of the object at the detector. To reconstruct the image recorded by the detector, corresponding decoding patterns are used. The optimized design of coded masks is critical for the performance of this technique and will depend on the characteristics of the imaging beam. In this work, Monte Carlo (MCNP) simulations were utilized to explore the needed modifications to the PULSTAR thermal neutron beam to support coded imaging techniques. In addition, an assessment of coded mask design has been performed. The simulations indicated that a 12 inch single crystal sapphire filter is suited for such an application at the PULSTAR beam in terms of maximizing flux with good neutron-to-gamma ratio. Computational simulations demonstrate the feasibility of correlation reconstruction methods on neutron transmission imaging. A gadolinium aperture with thickness of 500 μm was used to construct the mask using a 38 × 34 URA pattern. A test experiment using such a URA design has been conducted and the point spread function of the system has been measured.

Time-Resolved High Resolution Neutron Imaging Studies at the ORNL Spallation Neutron Source

Our evaluation of a high-resolution digital imaging detector capable of providing millisecond time resolution and high sensitivity for neutrons is presented. This detector is a modified version of the high-resolution gamma-ray imager developed previously at Radiation Monitoring Devices, Inc. (RMD, Inc.), and consists of an Electron Multiplying Charge Coupled Device (EMCCD) attached to a neutron-sensitive scintillator via a fiberoptic taper. By virtue of its internal gain, the EMCCD permits high speed readout without introducing additional noise, thereby enabling high frame rate operation with an enhanced signal-to-noise ratio (SNR). Detector sensitivity is enhanced through the use of a back-thinned EMCCD, which provides high quantum efficiency over a typical emission range for many neutron-sensitive scintillators. Preliminary evaluations conducted at the Liquids Reflectometer beam port of the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) demonstrate that this new detector has the sensitivity to detect individual neutrons and the acquisition speed to perform energy-selective imaging with a temporal resolution of milliseconds. While substantial improvements in timing and imaging performance are planned, this prototype detector has already generated the first ever images of the SNS Liquids Reflectometer beam profile and was also used to demonstrate a technique for obtaining Bragg edge transmission imaging using energy-selective neutrons. The preliminary data, along with the detector design, evaluation, and planned developments are discussed in this paper.