Robert D. Field

Microtextural investigation of hydrided α-uranium

The local orientation features in as-cast α-uranium were investigated in order to ascertain the influence of texture and heterogeneity in the microstructure on hydride initiation and growth. Several samples were interrogated via automated electron back-scattered diffraction measurements in the post-hydride state. Characterization included orientation mapping, deformation twin identification, and grain boundary analysis. It was found that preferential hydride initiation sites were associated with general high-angle grain boundaries, low-angle boundaries, and twin boundaries of two systems. Linear segregation or precipitation features suggest the existence of unresolved twins that could be correlated with many of the remaining hydrides. The tendency of certain orientations to exhibit differing hydride potentials based on atomic density in the habit plane was also considered. The results suggest the dominant role of enhanced sub-surface hydrogen diffusion at misoriented regions in controlling hydride nucleation and growth.

DISLOCATIONS IN MO5SIB2 T2 PHASE

Abstract Dislocation structures in a nearly single phase annealed Mo5SiB2 T2 alloy have been investigated by transmission electron microscopy (TEM). The dislocations have been subjected to Burgers vector and trace analyses to determine the slip directions and planes with the aid of image simulations generated using single crystal elastic constants derived from first principles calculations. The experimental results are compared to predicted slip directions and planes from anisotropic elasticity calculations. Thermal expansion coefficients have been measured by dilatometry and are compared to both calculated and previous experimental values measured using diffraction techniques. Lastly, preliminary compression testing has been performed on the single phase material at 1200°C.

Fabrication of Beryllium Capsules with Copper-Doped Layers for NIF Targets: A Progress Report

Abstract The sputtering of beryllium (Be) has been used at LLNL for nearly 30 years in the fabrication of laser targets. Several years ago the prospect of using sputtering to fabricate spherical Be capsules for National Ignition Facility (NIF) targets began to be explored and a basic strategy was developed that involved sputtering down onto plastic mandrels bouncing in a pan. While this appears to be very straightforward in principle, in practice sputtering has been used almost exclusively to make thin films (< 1 micron) on flat substrates. Thick films pose a significant challenge for sputtering while materials on spherical substrates are essentially unexplored. More recently, based on computational results, the point design for the first NIF ignition target capsule was specified as a Be capsule with Cu-doped layers of specific thickness, each layer with a different concentration of copper. While the work described here was motivated by the need to make these layered capsules, progress has been made in developing a more complete metallurgical understanding of the materials that are fabricated and the relationship between the sputter processing and microstructure of these spherical samples.

Texture development and deformation mechanisms during uniaxial straining of U–Nb shape-memory alloys

The shape–memory effect is well documented in uranium–niobium alloys near the α″–γo metastable phase boundary. In situ neutron diffraction measurements during uniaxial loading indicate that U–14 at.% Nb (in the α″ monoclinic phase field) deforms by stress–induced twin reorientation. Alternatively, U–16 at.% Nb (initially γo tetragonal) undergoes a stress–induced phase transformation to the α″ monoclinic phase. The crystallographic texture of the monoclinic phase of both compositions has been measured and qualitatively interpreted by considering the orientation relationship between the most favoured α′′ variant and the parent phase. In addition, previously published observations of deformation structures within the shape–memory regime of a U–13 at.% Nb alloy are discussed within the context of the same model.

Proton Radiography Peers into Metal Solidification

Historically, metals are cut up and polished to see the structure and to infer how processing influences the evolution. We can now peer into a metal during processing without destroying it using proton radiography. Understanding the link between processing and structure is important because structure profoundly affects the properties of engineering materials. Synchrotron x-ray radiography has enabled real-time glimpses into metal solidification. However, x-ray energies favor the examination of small volumes and low density metals. Here we use high energy proton radiography for the first time to image a large metal volume (>10,000 mm3) during melting and solidification. We also show complementary x-ray results from a small volume (<1 mm3), bridging four orders of magnitude. Real-time imaging will enable efficient process development and the control of structure evolution to make materials with intended properties; it will also permit the development of experimentally informed, predictive structure and process models.

Equal-channel angular extrusion of beryllium

The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions have been successfully completed on Ni-canned billets of Be at approximately 425 °C. No cracking was observed in the billets, and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed for a single-pass extrusion, including evidence of 〈c〉 and 〈c+a〉 dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation processing technique and the underlying physical processes which sustain the deformation.

Examination of the α-ω two-phase shock-induced microstructure in zirconium and titanium

Omega (ω) phase is formed in alpha (α) zirconium during dynamic loading and can be retained in the recovered material. The pathway for the α to ω change (or the reverse transformation) is not well understood. Zirconium was shock-loaded and the resulting two-phase microstructure was examined. Electron backscatter diffraction (EBSD) was used to characterize the orientation relationships and habit planes between phases to understand the pathways between α and ω phases and compare to Molecular Dynamics (MD) simulations. Based on key microstructural features, a significant amount of α phase appears to have originated from the reverse transformation from ω-Zr on unloading. Results of microstructural analysis will be discussed, along with implications toward phase transformation pathways.

Microstructure and Properties of Laser Deposited and Wrought Alloy K-500 (UNS N05500)

Feasibility tests were performed by manufacturing transition joints between Nickel-200 (Ni-200) and Alloy K-500 (K-500) with direct laser deposition. Both sharp and functionally graded interfaces were manufactured with no apparent issues. In addition, Alloy K-500 specimens were manufactured with direct laser deposition to analyze the isothermal hardening response of Alloy K-500. The laser deposited and wrought materials were precipitation hardened at three different temperatures (600 °C, 650 °C, and 700 °C) and various times. A relationship between the hardness and the volume fraction of Ni3(Ti,Al) was developed, and the subsequent analysis showed that the corresponding Johnson-Mehl-Avrami time constants (or n-values) ranged from 1.1 to 1.3 at an aging temperature 600 °C, and decreased to values ranging from 0.6 to 0.66 at an aging temperature of 700 °C. For site saturation and spherical precipitates, the expected n-value is 1.5. This analysis showed that the mechanism for precipitation changed as a function of temperature. The results of the analysis at higher temperatures were rationalized by the possibility of the decay of quenched-in vacancies, precipitation along the grain boundaries, and precipitation along the dislocations. It was, however, difficult to confirm these possibilities with transmission electron microscopy (TEM), since imaging the precipites is difficult at very early aging times. Finally, it was determined that the heat treatment schedule of laser welded or laser clad Alloy K-500 should be similar to that of wrought Alloy K-500.

Ferrite Scanning Microscope Based on Magnetic Tunnel Junction Sensor

We have developed a scanning magnetic microscope (SMM) based on a magnetic tunneling junction (MTJ) magnetoresistive (MR) sensor. The microscope is based on commercially available components employing two sets of scanning stages and a MTJ sensor. Spatial resolution and noise sensitivity were investigated using two MTJ sensors, one having high spatial resolution and the other low noise but coarser spatial resolution. We present measurements of magnetic field images from ferrite concentration calibration standards and a stainless steel welded specimen both imaged using a magnetoresistive scanning microscope. A sensitivity of ~ 10 μT/FN was obtained from standards with defined ferrite numbers (FN). This microscope represents a new powerful tool for the characterization and investigations of delta ferrite concentrations in stainless steel welded samples.

The Presence of Higher-Order Laue Zone Intensities and the Relrod Effect in Cubic Metals in the Transmission Electron Microscope

The presence of 1/3{422} reflections in <111> selected area diffraction patterns (SADP) has been attributed to short-range order (SRO) leading to diminished stress corrosion cracking resistance in IN690 after extended aging. In this study, it is shown that these diffuse intensities in IN690 are likely the result of scattering from higher-order Laue zone (HOLZ) reflections into the positions observed in the zero order Laue zone (ZOLZ). To support this hypothesis, it is shown that several fcc structures, such as IN600, IN945, commercially pure (CP) nickel, Co-28.5Cr-6Mo and Ni-33at%Cr as well as diamond cubic structures such as Si and zinc-blende structures (CdTe) also give rise to intensities in these positions. This hypothesis explains the presence of diffuse scattering down the <111>, in addition to several other higher index zone axes.