The ability to investigate a particular feature or region of a specimen using differing kinds of analytic instrumentation can be a very valuable technique. For example, an object viewed under an optical microscope provides color, polarization, and reflectance information which is not available via SEM viewing. Similarly, the SEM with its large depth of field and ability to utilize secondary and backscattered electron contrast mechanisms, and its capability to perform compositional analysis via EDS, provides information which the optical microscope cannot. Thus, the information provided by the two instruments is complementary.

Desireable though it may be to view the same feature of a specimen via differing instruments, this is often difficult to execute in practice. The difference in contrast mechanisms which makes the excercise valuable may also make it difficult to relate the distinguishing features viewed under one instrument with those viewed in the other. Further, the issue of scale creates a large complication in many practical instances.

The ability to investigate a particular feature or region of a specimen using differing kinds of analytic instrumentation can be a very valuable technique. For example, an object viewed under an optical microscope provides color, polarization, and reflectance information which is not available via SEM viewing. Similarly, the SEM with its large depth of field and ability to utilize secondary and backscattered electron contrast mechanisms, and its capability to perform compositional analysis via EDS, provides information which the optical microscope cannot. Thus, the information provided by the two instruments is complementary.

Desireable though it may be to view the same feature of a specimen via differing instruments, this is often difficult to execute in practice. The difference in contrast mechanisms which makes the excercise valuable may also make it difficult to relate the distinguishing features viewed under one instrument with those viewed in the other. Further, the issue of scale creates a large complication in many practical instances.

Ni-base alloy welds such as EN82H weld metal are frequently employed in nuclear power applications where resistance to corrosion is required. Results of a recently reported study of the mechanical properties of EN82H welds show that this alloy is susceptible to low-temperature (∼100°C) environmental embrittlement (LTEE) in hydrogenated water. LTEE is a manifestation of hydrogen embrittlement in these alloys.1 Recent LTEE tests have demonstrated a beneficial effect of a high-temperature (∼1100°C) anneal and furnace-cool in alleviating the material's susceptibility to LTEE. Understanding the reason for the reduction in LTEE susceptibility requires detailed characterization of the microstructure so that the specific structural and compositional changes that have been induced by the solution-anneal can be identified. This study reports the results of light optical and analytical electron microscopy (AEM) characterization of the microstructures of as-fabricated and as-solution-annealed EN82H welds with the objective of providing insight into the observed LTEE behavior.

Although friction-stir welding has been developing as a viable industrial joining process over the past decade, only little attention has been given to the elucidation of associated microstructures. We have recently produced welds of copper to 6061 aluminum alloy using the technique illustrated in Fig. 1. In this process, a steel tool rod (0.6 cm diameter) or head-pin (HP) traverses the seam of 0.64 cm thick plates of copper butted against 6061-T6 aluminum at a rate (T in Fig. 1) of 1 mm/s; and rotating at a speed (R in Fig. 1) of 650 rpm (Fig. 1). A rather remarkable welding of these two materials results at temperatures measured to be around 400°C for 6061-T6 aluminum welded to itself. Consequently, the metals are stirred into one another by extreme plastic deformation which universally seems to involve dynamic recrystallization in the actual weld zone. There is no melting.

The EELS Core-Loss Atlas, like a road map, serves as an essential guide for practitioners of electron energy loss spectroscopy (EELS). Even the best maps will be incomplete, however, and one may need to plot a course by other means. Our work with materials from the nuclear fuel cycle has required new spectral lines, and even new elements, to be charted. The actinides and technetium, for instance, have important spectral features above 2 keV. The EELS technique has not often been used for detecting lines above about 2 keV because of the rapidly diminishing differential cross sections for these higher energy transitions. However, as instrumentation has progressed from scanning to parallel to CCD-array detection, statistical limitations on the high energy data have been improved by orders of magnitude. Also, in some cases, the notion of higher energy lines as “weak” is excessively pessimistic.

The binary ReSi2 and ternary (Mo,Re)Si2 alloys are of interest due to potential applications as infrared detectors [1]. Also, Re may be a beneficial alloying element to MoSi2 which is a potential high temperature structural material [2]. A recent study of the structure of ReSi2 by X-ray diffraction (XRD) revealed an orthorhombic distortion (a= 0.3121 nm, b= 0.3139 nm and c= 0.7670 nm) from the tetragonal CI lb MoSi2 structure and a Si-deficient composition of Re4Si7 [3]. Further, a weak monoclinic distortion (β=89.87°) was also inferred from high-resolution XRD [3]. In the present investigation, the orthorhombic distortion and the Si-deficient stoichiometry were confirmed by XRD and the structure of arc-melted Re4Si7 studied by transmission electron microscopy (TEM).

A bright field (BF) TEM image and the corresponding [010] zone axis selected area diffraction pattern (SADP) are shown in Fig. 1. In addition to the fundamental reflections for tetragonal Cllb MoSi2 structure,

Laves-phase intermetallics are of potential use as high-temperature structural materials. NbCr2-based C15-structured alloys are of particular interest for such applications. The effect of Ti alloying on the microstucture and mechanical properties of such alloys has been investigated and it has been shown that Ti can improve the fracture toughness of the monolithic CI 5 Laves phase8 and dual-phase (bcc and CI 5) alloys. In the Nb-Cr-Ti system, there is a complete solid solution between NbCr2 and TiCr2, with a significant range of solubility of the C15 phase and a large two phase, bcc and C15, region. The initial characterization of the defect structure of an alloy of overall composition Nb10Cr75Ti15 is discussed here.

Nb10Cr75Ti15 was prepared by arc-melting the high-purity elemental metals followed by annealing at 1400°C for 120 h and then cooling at l°C/min. Specimens were prepared for observation in the TEM by cutting 3 mm discs with a coring saw, followed by dimpling and ion milling.

Massive phase transformations exhibit a change in crystal structure without a change in composition. Nucleation and growth of this difTUsionless transformation has been studied in numerous systems including Cu-Zn, Ag-Zn and Ag-Al. There is general agreement that heterogeneous grain boundary nucleation occurs with a rational orientation relationship with at least one of the parent grains, although growth of the massive product can often occur with a partially coherent interface.

The typical Ti-48at%Al microstructure is a lamellar structure of hexagonal α2 (DO19 superlattice) and the ordered tetragonal γ phase (Ll0 with c/a = 1.01). Water or ice brine quenching of the high temperature disordered α phase results in a partial massive transformation of a into γ. Electron beam melting has been used to further suppress the equilibrium transformations. However, the current study uses rapid solidification by electromagnetic levitation and twin anvil splat quenching (cooling rates > 106 K/s) to capture earlier stages of the transformation.

Magnetic properties of permanent magnets are sensitive to their microstructure. In particular, for a class of Nd(Pr)-Fe-B magnets very different coercivity and energy products may be obtained by different processing techniques. In this work we report on the preliminary results of magnetic domains (MD) structure and grain boundaries (GB) analysis of such hot-deformed Nd(Pr)-Fe-B magnets. To reveal the fine details of MD and GB interaction the experiments were carried out by the combined use of Magnetic Foucault-Fresnel Imaging and High Resolution Microscopy.

The magnets used in this study had a nominal composition Nd13.75Fe80.25B6 (I) and Pr13.75Fe80.25B6 (II) and were produced at the General Motors Research and Development Center. Buttons of the die-upset samples were prepared from overquenched ribbons using the melt-spun technique. Thin sections were cut from the buttons obtained by die-upset of the ribbons of I and II. These sections were mechanically polished and then ion milled until perforation. Electron microscopy was done using a JEM 2000FX microscope operating at 200 kV.

Fe-13Cr stainless steel is a corrosion resistant alloy (CRA) that is widely used in oil and gas production for equipment such as tubes in wells. The high temperature, high CO2 and H2S partial pressures of typical production environments and the significant amounts of Cl- containing water that is produced along with the hydrocarbons affects the corrosion processes. Fe-13Cr is used for tubes since it exhibits passivity and low corrosion rates in production environments. But, during production operations, it is often necessary to pump strong acids into the underground rock formation through the tubes to stimulate the field and increase productivity. This is done in order to dissolve deposits that clog pores in the rock formation, thus improving permeability and facilitating the flow of hydrocarbons. This is expected to significantly degrade the passivity and lead to increased corrosion. Therefore, there is a concern about the corrosion rate during the operation and the rate at which the alloy repassivates when production is resumed.

The intermetallic, stoichiometric compound NiAl is a candidate for aggressive environments such as those encountered in aircraft engines due to its excellent oxidation resistance and high melting point. However, NiAl is limited in application by its insufficient ambient temperature ductility. This may potentially be overcome via synthesis of NiAl with a nanocrystalline microstructure. Since microstructure can also affect oxidation resistance [1], the focus of this investigation is to examine the oxidation characteristics of nanocrystalline NiAl.

Nanophased NiAl specimens were prepared by mechanical alloying followed by shock wave consolidation, which yielded consolidated material with an average grain size of 11 ±8 nm [2]. Thermogravimetric analysis (TGA) of nanosized NiAl was used to characterize the material's oxidation resistance, and performed for three different times (0.5 h, 5 h, 48 h) in compressed air at 1373 K. It was found that the thickness of the scale increased with increasing oxidation time, in accordance with parabolic oxidation kinetics.

A thorough microstructural study of common ceramics requires analysis at various length scales. Yttrium doped, polycrystalline α-Al203 was studied using a broad selection of microscopy techniques over a scale range covering seven-orders of magnitude (i.e. from mm to Å).

Yttrium doped, (1000 (Y/Al) atomic ppm) polycrystalline alumina samples were prepared by hot pressing at 1450°C at 50 MPa. Some were subsequently crept in tension. Microstructure of these samples was studied by optical microscopy, scanning electron/orientation imaging microscopy (OIM™), conventional and high resolution transmission electron microscopy. Analytical electron microscopy was performed on a VG HB501 FEG-STEM equipped with an EDS and PEELS.

On a large scale, the microstructure often consisted of bimodal sized alumina grains and second phase YAG (Y3A15012) precipitates. YAG precipitates at grain boundaries (GB) and triple points caused clusters (50μm to 1mm) of small equi-axed alumina grains (0.5-2 μm) randomly distributed over 40 area% of the sample.

Tetrahedrally co-ordinated materials can undergo stress-induced structural transformations often under invariant plane strain conditions; examples of such transformations are twinning and polytypism. The structure of a tetrahedrally co-ordinated material, XY, may be considered in terms of an assembly of “normal”, Ti’and “twinned” tetrahedra, T'I’all connected to each other at their corners. In some of these materials, e.g. SiC, GaN or CdS, these corner-sharing tetrahedra may be connected to each other in different ways, giving rise to different polytypes. In this paper, a dislocation model for polytypic transformations is discussed and a few examples of stress-induced transformations are illustrated.

In general, dislocations in XY polytypes he on basal (0001) planes and are generally dissociated into “leading’ and “trailing” partial dislocations with Burgers vectors b1 and bt’ respectively. There is also good evidence that dissociated dislocations in tetrahedrally-coordinated materials belong to the glide set and move on the slip plane in a dissociated manner.

Of the four groups of crystal lattice defects, i.e., point defects, dislocations, interfaces and particles, dislocations are often thought to be the least important for ceramic materials. However, they not only give considerable insight into interfaces (they can be thought of as the link between point defects and interfaces), but they are becoming more important as interest grows in epitactic oxide heterostructures.

The present paper provides a review of our current understanding of dislocations in ceramics. To simplify the discussion, we can separate the task into the following sections, namely

1. Simple oxides (MgO, ZnO etc.)

2. Simple non-oxides (A1N, GaN, SiC, TiC, etc.)

3. More complex binary oxides (AI2O3, Fe203, etc.)

4. Multicomponent oxides and complex non-oxides (spinels, garnets, S13N4)

5. Interfacial dislocations (misfit, grain boundary, etc.)

The features which are most often associated with ceramic materials are their complex structures and large unit cells. For example in bcc garnets, the smallest Burgers vector for a perfect lattice dislocation is ∼0.8nm long.

Lead-based cubic perovskites such as Pb(B2+1/3B5+2/3)O3 (B2+ = Mg, Co, Ni, Zn; B5+ = Nb, Ta) are relaxor ferroelectrics. Localized order and disorder often occur in materials of this type. In the Pb(Mg1/3Nb2/3)O3 (PMN) family, previous studies have proposed two models, space-charge and charge-balance models. In the first model, the ordered regions carry a net negative charge [Pb(Mg1/2Nb,/2)03], while in the second model it does not carry a net charge [Pb((Mg2/3Nb1/3)1/2Nb1/2)03]. However, no direct evidence for these two models has appeared in the literature yet. In this paper we report the first direct observations of local ordering in undoped and La-doped Pb(Mg1/3Nb2/3)03, using high-resolution Z-contrast imaging.

Because the ordered structure in Ba(Mg1/3Nb2/3)03 is well known, the Z-contrast image from an ordered domain is used as a reference for this study. Fig. 1(a) shows the projection of the supercell of fully ordered Ba(Mg1/3Nb2/3)03 along the [110] direction.

The evaluation of the two dimensional projected atom column positions around a defect or an interface in an electronic ceramic, as it has been performed in numerous examples by (quantitative) conventional high-resolution electron microscopy (HRTEM), is often not sufficient to relate the electronic properties of the material to the structure of the defect. Information about point defects (vacancies, impurity atoms), and chemistry or bonding changes associated with the defect or interface is also required. Such complete characterization is a necessity for atomic scale interfacial or defect engineering to be attained.

One instructive example where more than an image is required to understand the structure property relationships, is that of grain boundaries in Fe-doped SrTi03. Here, the different formation energies of point defects cause a charged barrier at the boundary, and a compensating space charge region around it. The sign and magnitude of the barrier depend very sensitively on the atomic scale composition and chemistry of the boundary plane.

In ion irradiation damage studies on ceramics, damage evolution is often assessed using Rutherford backscattering spectroscopy and ion channeling (RBS/C) techniques. In a typical experiment, a single crystal ceramic sample is irradiated with heavy ions and then the crystal is exposed to He ions along a low-index crystallographic orientation. Simultaneously, the backscattered He ion yield is measured as a function of ion energy loss. For He ions scattered from the heavy ion irradiated volume, the He ion yield increases in proportion to the heavy ion dose. The RBS/C yield rises because the He ion beam is dechanneled by, for instance, interstitial point defects and clusters and their associated strain fields. A quantitative measure of dechanneling is denoted by χmin, defined as the ratio of the He ion yield along a low-index crystal orientation, to the yield obtained in a random (non-channeling) orientation. The damage parameter xmin varies from 0 to 1, where 1 represents the maximum damage level that can be measured by RBS/C.

Secondary phases that form during the corrosion of nuclear waste forms may influence both the rate of waste form dissolution and the release of radionuclides [1]. The identification of these phases is critical in developing models for the corrosion behavior of nuclear waste forms. In particular, the secondary uranyl (VI) minerals that form during waste form alteration may control uranium solubility and release of radionuclides incorporated into these phases [2].

The U6+ cation in uranyl minerals is almost always present as a linear (UO2)2+ ion [3]. This uranyl (Ur) ion is coordinated by four, five, or six anions (ϕ) in the equatorial plane resulting in the formation of square (Urϕ4), pentagonal (Urϕ5), and hexagonal (Urϕ6) bipyramids, respectively [3]. These bipyramid polyhedra may polymerize to form complex infinite sheet structures. The linking of Urϕ5 is observed in a number of uranyl minerals formed during waste glass and spent fuel corrosion [2,4], such as weeksite [Na,K(UO2)2(Si205)3*4H2O] and β-uranophane [Ca[(UO2)(SiO3OH)]2*5H2O].

The white lines on the L2,3 absorption edges of the transition metals and on the M4,5 absorption edges of the rare earth elements are the most prominent feature in electron-energy-loss spectra (EELS). The white lines of these elements have been studied and used to determine valance and coordination of atoms in various systems. Fe-O compounds with valence 2, 3 and mixed valence states, for example, have been carefully examined and characterized. However, in spite of the importance of iron sulfides in geochemistry and environmental science, the EELS data of these compounds are not available. Pyrite (FeS2) is widespread in hydrothermal ores, modern sediments, and sedimentary rocks deposited throughout the geologic record. The ferromagnetic iron sulfide greigite (Fe3S4) is another important compound. It has been recognized as a carrier of magnetic remanence in young sediments. Sedimentary greigite may be precipitated inorganically or biogenically by magnetotactic bacteria.

Substrates of fused silica or other refractory glasses are of interest for the fabrication of diamondbased electronic sensors (such as thermistors) and field-emitter arrays. For temperature-sensing applications, the diamond thin film layers must be deposited on thermally insulating substrates, in order to optimize the accuracy and dynamic response time of the device. Silica and silicate glasses are good insulators, providing a barrier to the heat flow out of the device and thereby allowing for optimized performance of temperature sensors. Also, fabrication of diamond field emitters on large area glass substrates is important for manufacture of field emission flat panel displays. Recently, it has been demonstrated that significant enhancement of diamond nucleation on Si02 substrates can be achieved by applying electrical bias to the substrate during its exposure to a microwave generated CH4/H2 plasma.

To investigate the nucleation mechanism of diamond on Si02 substrate, induced during the biasing pretreatment, several TEM techniques were used to characterize the films and the film/substrate interface.