Robert P. Harrison

Microstructure and texture analysis of δ-hydride precipitation in Zircaloy-4 materials by electron microscopy and neutron diffraction

This work presents a detailed microstructure and texture study of various hydrided Zircaloy-4 materials by neutron diffraction and microscopy. The results show that the precipitated δ-ZrH1.66 generally follows the δ(111)//α(0001) and δ[1{\overline 1}0]//α[11{\overline 2}0] orientation relationship with the α-Zr matrix. The δ-hydride displays a weak texture that is determined by the texture of the α-Zr matrix, and this dependence essentially originates from the observed orientation correlation between α-Zr and δ-hydride. Neutron diffraction line profile analysis and high-resolution transmission electron microscopy observations reveal a significant number of dislocations present in the δ-hydride, with an estimated average density one order of magnitude higher than that in the α-Zr matrix, which contributes to the accommodation of the substantial misfit strains associated with hydride precipitation in the α-Zr matrix. The present observations provide an insight into the behaviour of δ-hydride precipitation in zirconium alloys and may help with understanding the induced embrittling effect of hydrides.

Maximum entropy image reconstruction from sparsely sampled coherent field data

There are many practical problems in which it is required to detect and characterize hidden structures or remote objects by virtue of the scattered acoustic or electromagnetic fields they generate. It remains an open question, however, as to which reconstruction algorithms offer the most informative images for a given set of field measurements. Commonly used time-domain beamforming techniques, and their equivalent frequency-domain implementations, are conceptually simple and stable in the presence of noise, however, large proportions of missing measurements can quickly degrade the image quality. We apply a new algorithm based on the maximum entropy method (MEM) to the reconstruction of images from sparsely sampled coherent field data. The general principles and limitations of the new method are discussed in the framework of regularization theory, and the results of monostatic imaging experiments confirm that superior resolution and artifact suppression are obtained relative to a commonly used linear inverse filtering approach.

Defect dynamics in polycrystalline zirconium alloy probed in situ by primary extinction of neutron diffraction

After α + β-zirconium has fully transformed into β-phase upon heating, the intensities of all β-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 μm and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating α-Zr induces strain into the perfect β-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature.

Surveillance of the Fracture Behavior of Zircaloy-4 Welds Using the Small Punch Test

Zircaloy-4 (Zr-4) is a zirconium alloy that has been used extensively within the nuclear industry in both power generation and research reactors. The potential for welded reactor core components to undergo embrittlement during neutron irradiation is an important materials property issue to be considered throughout the service life of the reactor. Of particular interest is the performance of welded joints which depend on the properties of both the weld heat-affected zone (HAZ) and the parent metal. For this work Zr-4 HAZ was simulated using a Gleeble weld simulator to rapidly bring the material into the β-Zr zone at peak temperatures of 1200 and 1400°C. The small punch (SP) test was then used to investigate the variation in fracture behavior of these narrow zones of the weld and compare them with the parent metal. A finite element analysis (FEA) model of the SP test was developed which can be used to predict the deformation and fracture behavior of Zr-4 welds and is compared with the experimental data obtained.

Development of Oxide Dispersion Strengthened Steels for High Temperature Nuclear Structural Applications

Oxide dispersion strengthened (ODS) steels are the most promising candidate materials for high temperature nuclear applications. Mechanical alloying and subsequent thermomechanical treatments are applied to manufacture the ODS steels. Recently improved chemical composition and manufacturing processes have been developed to produce ultrafine grain size with high number-density of nanoscale oxide particles and high dislocation density in the microstructure. Usually, fine grains degrade creep resistance at elevated temperatures. However, the fine-grained ODS steels exhibit not only good radiation resistance, but also superior creep properties. The present paper reviews the chemical compositions, manufacturing processing, microstructural features, thermal creep properties and radiation resistance of recently developed ODS steels. Special attention is paid to the effects of the fine-scale microstructural features on thermal creep and radiation resistance.

The Observation of Slip Phenomena in Single Crystal Fe Samples During In Situ Micro-Mechanical Testing Through Orientation Imaging

This paper reports a study of local orientation change occurring within micro-scale tensile samples as a function of strain. These samples were fabricated from a thin film of single crystal bcc Fe and deformed in tension using an in situ micro-mechanical testing device inside a scanning electron microscope. Samples were loaded along the <110> direction parallel to the specimen axis, strained to different levels, and then subjected to electron backscatter diffraction scans over the entire area of the gauge section. Analysis of the surface orientation data shows that, within a necked zone of the micro-sample gauge section, there are two distinct regions of significant orientation change, in which local crystal rotations occur in opposite directions. These two regions are separated by an intermediate band that shows minimal misorientation from the original state. Crystal rotations within the two regions that develop opposite orientations are found to be consistent with classic single crystal slip, where the slip direction rotates toward the tensile axis. It is shown that increasing tensile strain causes an increasing degree of rotation away from the starting orientation. The tests also illustrate the occurrence of slip on at least two different slip systems, based on the slip traces and orientation change.

Maximum entropy methods in ultrasonic NDE

This paper explores the application of the maximum entropy method (MaxEnt) to reconstructing images from sparsely sampled ultrasonic field data, with particular emphasis on three-dimensional applications in non-destructive evaluation (NDE). MaxEnt has not previously been applied to this class of problem, and yet in comparison with conventional approaches to image reconstruction, such as matched filtering, it displays superior robustness against side-lobe formation and improved lateral and range resolution. The paper concludes with a description of the application of MaxEnt image reconstruction to a novel polyvinylidene fluoride (PVDF) array transducer that offers advantages over conventional point-sampled arrays in terms of simplified construction and improved SNR. This ultra-sparse orthogonal array is believed to be the first of its kind yet demonstrated, and was made possible by MaxEnt signal processing.