Polysynthetically twinned (PST) TiAl, which has a two phase lamellar structure composed of multiply oriented lamellae of the TiAl phase (γ, with ordered tetragonal L10 structure) with a low density of interspersed layers of the Ti3Al phase (α2, with ordered D019 hexagonal structure), has attracted a large amount of interest in the past decade for its potential as a high temperature structural material. The strongly crystallographic twins and pseudo-twins between the lamellae of the PST TiAl crystal provide an ideal system to study interface diffusion. In this paper, we describe early results on the interrelationships between phase and interface structure, bulk and interface diffusion and phase equilibrium in the diffusion- bonded system PST TiAl and vanadium.

A PST TiAl “single crystal” was grown in an optical floating zone furnace by zone-melting and directional solidification of a polycrystalline as-cast TiAl rod. Within the “single crystal”, all the lamellar interfaces are parallel.

Intermetallics are playing an important role in the development of new materials able of sustaining the escalating demands of the aerospace industry. A significant improvement in weight, operating temperatures or mechanical performance is required for materials to be considered as replacements in the most demanding applications. Molybdenum disilicide is one such compound that has potential for high temperature applications. Its most attractive properties are its high melting point (2020°C), reasonable density (6.24 g/cm3), and excellent high temperature oxidation and corrosion resistance. However, low ambient fracture toughness and loss of strength at elevated temperatures have been the most significant limitations to the use of MoSi2 in structural applications.

The more promising solutions for improving the mechanical properties of brittle intermetallics such as MoSi2 are based on the incorporation and control of secondary phases. To date, the artificial introduction of reinforcing phases or their generation via in-situ reactions have been explored.

Directionally solidified (DS) in-situ composites based on (Nb) and Nb silicides, such as Nb5Si3 and Nb3Si, are being investigated for high-temperature structural applications. The use of alloying additions, such as Hf, Ti and Mo, to these silicides is required to enhance their properties. The present paper describes the microstructural response of a DS Nb-silicide based composite to creep testing.

The composites investigated were directionally solidified from a molten alloy using the Czochralski method as described previously. Creep tests were conducted at 1200°C to strains of up 50%. Microstructure and microtexture characterizations were performed using scanning electron microscopy, electron microprobe analysis (EMPA), and electron backscatter diffraction pattern analysis (EBSP).

Microstructures of the longitudinal section of a DS composite generated from a Nb-12.5Hf-33Ti- 16Si alloy are shown in Figure 1 in the as-DS (left hand side) and the DS+creep tested conditions (right hand side).

The ability to understand and control the microstructure of materials under “real life” conditions is of great interest to both the scientific and industrial communities. In recent years, a variety of “ex-situ” TEM methods have been developed to improve this knowledge base.

ExxonMobil's dedicated reactor system developed for “ex-situ” TEM was used to study the oxidation and reduction of copper on a carbon film and Cu on a Si support. By following the oxidation of specific Cu particles, it was possible to illustrate the oxidation process. A standard, commercially available, 200 mesh Cu grid covered with a holey C support film was used for the Cu on C study. Small Cu islands were vapor deposited from the grid onto the C film during a high temperature reduction in the reactor (Figure 1). The grid was transferred under inert conditions into the TEM where the Cu metal islands were imaged (Figure 2a).

Much effort has recently been employed in modeling steel behavior as a function of chemical composition and thermo-mechanical processing. In order for the models to be predictive, experimental data are needed to first calibrate and then validate the calculated results. Since many steel properties are conditioned in part by the state of precipitation, this study was undertaken to provide experimental measurements of precipitates in terms of their size distributions. Due to the size range (2-100 nm) of particles which exert the greatest influence on the mechanical properties, electron microscopy is practically the only technique available for obtaining images of the precipitates.

Two types of micrographs were used for image analysis in order to determine the size distribution of NbC particles present in a ternary alloy (Fe-Nb-C) heat treated at 800°C for 1000 mn. Polished surfaces lightly etched with a solution of 2% HNO3 and 98% ethanol were prepared for observations in a LEO 1550 scanning electron microscope equipped with a field emission gun.

The understanding of detailed structures and properties of enamel is essentia for designing new implant materials that faithfully resemble enamel structure or to regenerate enamel through biomimetics using proteins as templates. As part of a greater effort in enamel biomimetics, this work focused on nanostructures and mechanical properties of individual enamel rods using the mause as a test animal.

A mature mouse incisor consists of two layers - dentin and enamel. In order to perform its normal masticatory function, the incisor must have sufficiently high flexural strength, toughness, and wear resistance. Enamel, which is highly mineralized (∼100%) with a knitted micro-architecture, satisfies the mechanical property requirements. It is known that elongated hydroxyapatite (HAP) crystals mineralize in bundles, i.e., enamel rods, and organize in a 3-D cross pattern. [1,2] However the organizations of HAP crystals within and between rods are not well understood.

Significant improvements in the fracture resistance, fracture toughness and thermal properties of silicon nitride ceramics are obtained by tailoring the microstructure. Combined use of seeding and tape casting techniques allowed the production of highly anisotropic microstructures. The seeded silicon nitrides exhibited a distinct bimodal microstructure, with large elongated β-Si3N4 grains, grown from seeds, dispersed within a fine-grained matrix. These large grains in the seeded silicon nitrides lie in the casting planes and self-align along the casting direction during tape forming process. It is here, when due to the high degree of alignment that “special boundaries” without the, otherwise, ubiquitous amorphous phase occurs. These “special” boundaries, hardly seen in three dimensionally random microstructures, are the object of the present study.

Silicon nitride with high thermal conductivity of up to 120 W/mK (ref. 3) is produced by hot-pressing at 1800 °C for 2 h. powders with the following nominal composition: α-Si3N4 ;5 wt% Y203; 5 vol.% (β-Si3N4 seeds.

The migration of grain boundaries in polycrystalline materials can occur under a variety of driving forces. Grain growth in a single-phase material and Ostwald ripening of a second phase are two common processes involving boundary migration. The mass transport in each of these cases can be related to a chemical potential difference across the grains; due to curvature in the former case and due to a difference in the chemistry in the latter case. The mass transport across grains controls the densification process during sintering. In the case of liquid-phase sintering (LPS), a liquid film may be present at the grain boundaries which results in an enhanced mass transport between grains leading to faster densification. Hence, in LPS, it is important to understand mass transport across and along a boundary containing a liquid film. The use of bicrystals and tricrystals with glass layers in the boundary can provide a controlled geometry by which to study this phenomenon.

Ceramic materials are processed at high temperatures where different mechanisms of mass transport are operative. The liquid-phase sintering (LPS) process involves the formation of a silicate liquid at the sintering temperature. In this case, mass transport takes place at the solidliquid, solid-vapor and liquid-solid interfaces. The mechanism and kinetics of these processes controls the microstructure. The silicate phase, which is usually the sintering additive, may influence the evaporation from and of the ceramic. The importance of the evaporation process has been clearly brought out in previous studies on the dewetting of silicates on a ceramic substrate. The present study considers the influence of a silicate phase (celsian) on evaporation from a ceramic (sapphire) substrate.

Pulsed-laser deposition (PLD) has been used to deposit thin films (∼100 nm thick) of celsian (BaAl2Si2O8) on sapphire single-crystals of basal orientation. In the present study, the film/substrate assembly has been annealed at high temperatures (1750°C to 1850°C) in a vacuum furnace.

Understanding electron beam induced damage in glasses, especially in multi-component glasses, is very important, since the interaction of electron probes with glass is a very common approach to determine glass composition and structure. For example, the decay of characteristic X-ray and Auger electron intensities, using electron beams as probes, of alkalis in glasses have been known for years. In addition, both phase separation and formation of gas bubbles in the glasses have also been reported. Many irradiation effects are strongly dependent on the structure, bonding and composition of matter. In general, three types of mechanisms, knock-on damage, ionization and field-induced migration have been introduced to describe the damage induced by electron irradiation. Here, we demonstrate electron irradiation induced phase decomposition in a multi-component oxide glass, and introduce a modified model to interpret the damage process.

Zirconolite (CaZrTi2O7) is an important phase proposed for immobilization of plutonium. Radiation effects in zirconolite were studied by 1 MeV Kr+ and 1.5 MeV Xe+ irradiation at various temperatures. Zirconolite became amorphous at temperatures below a critical temperature, Tc. The critical temperature was found to be a function of ion species: Tc - 654 K for 1 MeV Kr+ and 710 K for 1.5 MeV Xe+. The temperature dependence of amorphization dose is shown in FIG. 1. Above Tc, the specimen remained crystalline after prolonged irradiation (up to 3.6×l015 ions/cm2). However, the high-temperature irradiated zirconolite was transformed into the fluorite structure (as shown by the strong diffraction maxima in FIG. 2). In addition to the maxima from the fluorite structure, strong diffuse maxima were observed surrounding the Bragg position of pyrochlore superlattice (FIG. 2).

Studies of defects, such as grain boundaries, in high temperature superconductors (HTS) are important due to the interaction of the defects with flux-bearing vortices. The benefit of in-plane grain alignment has been documented in YBCO thin film bicrystals, in which the high critical current density (Jc) observed across small angle grain boundaries deteriorates exponentially with grain boundary angles beyond ∼ 7°. In addition to the weak coupling effect, a grain boundary may also influence the transport properties via the grain boundary dislocations (GBDs) serving as pinning centers to increase the critical current density. There have been a number of studies on grain boundary structures in YBCO. Despite many differences in structure among the different types of boundaries, it has been established that the low angle [001] tilt boundary in YBCO consists of aperiodic array of edge type GBDs with [100] type Burgers vector that accommodate the lattice mismatch, and the regions between the GBDs are channels of relatively undisturbed lattices [1].

Transparent conductive oxide materials (TCO) are currently under development for use as electrodes in optical systems such as photovoltaics and flat panel displays. Indium-tin based oxides and cadmium-tin based oxides have been investigated in the past but have received more attention recently due to their high electron mobilities. The mechanisms for conduction in these systems are not fully understood, but the distribution of cations within the lattice is thought to be of importance. We are currently studying the Cd1+xIn2-2xSnxO4 system for use as a TCO material. In particular, we are interested in the effect of the crystallography and cation distributions in this material upon its electro-optical properties.

The investigation of cation distributions in this spinel system presents special challenges to purely diffraction based methods. X-ray diffraction cannot be used to assess the cation distribution for this system due to the similar x-ray atomic scattering factors for the cations involved.

Ferroelectrics are now being used as non-volatile memories, infrared detectors, phased array radar and optical switches. Although the development of commercial ferroelectric devices has advanced in recent years, we still do not have a clear understanding of the basic physics underpinning the behavior of actual ferroelectric devices. Trapped charge and oxygen vacancies are believed to strongly influence domain motion. In order to study these issues, we have designed an in situ TEM holder that can subject ferroelectric crystals to voltage, heat and UV irradiation. Preliminary results on BaTiO3 have been presented in previous reports.

In this study, bulk KNbO3 crystals were mechanically polished and thinned in hot orthophosphoric acid. The resulting thin flakes were attached to copper rings with conductive carbon paint, and electrical wires were glued to the copper rings which act as electrodes. The sample was placed in the cradle of the in situ TEM holder and examined in a Hitachi H9000NAR TEM.

Due to their unique dielectric and ferroelectric properties, ferroelectric superlattices have attracted increasing interest for fundamental and applied research. Such superlattices with a small layer thickness (a few unit cells) provide a model system to investigate the effects of crystal size, internal strain, interfacial structure and chemistry on physical properties. Recent development in oxide thin film epitaxial growth using reactive molecular beam epitaxy (MBE) make it possible to grow high quality metastable oxide superlattice films. BaTiO3 / SrTiO3 system has been selected for this study.

[(BaTiO3)5 / (SrTiO3)5]20 superlattice films were grown on (001) SrTiO3 substrates by reactive MBE. The atomic structure of these films and the related heterostructural interfaces have been investigated by a combination of high-resolution transmission electron microscopy (HRTEM) and computer image simulations. HRTEM studies were conducted within JEOL-4000EX electron microscope equipped with Gatan Image Filter (GIF), which is operated at 400 kV.

Thin films of doped perovskite manganites of the type R1-X AxMnO3-y are currently become the focus of intense theoretical and experimental research due to their promising technological implications. However, the microstructure of the film and the interface structure at the atomic level remains unclear. In this report, we will present the atomic structure of the film/substrate interface and the microstructure of the film using a combination of transmission electron microscopy (TEM), Z-contrast scanning transmission microscopy (STEM), and electron energy loss spectroscopy (EELS).

The La0.67Ca0.33MnO3 (LCMO) samples were grown on (001) LaA1O3 (LAO) using pulsed laser deposition. X-ray diffraction indicated the films to be oriented with the [001] directions parallel to the <001> directions of the pseudo-cubic perovskite LAO. Rocking curve measurements using the (002) reflection from the film indicated a full width half maximum of less than 0.3°. A Philips EM-400 electron microscope at 100 kV and a VG HB603 STEM at 300 kV were employed for sample analysis.

Charge ordering (C.O.) in the colossal magnetoresistive (CMR) manganites gives rise to an insulating, high-resistance state. This charge ordered state can be melted into a low-resistance metallic-like state by the application of magnetic field. Thus, the potential to attain high values of magnetoresistance with the application of small magnetic fields may be aided by a better understanding of the charge-ordering phenomenon. This study focused on microstructural characterization in Nd1/2Sr1/2MnO3. In Nd1/2Sr1/2MnO3, the nominal valence of Mn is 3.5+. On cooling, charge can localize and lead to a charge ordering between Mn 3+ and Mn 4+. The ordering of charge results in a superlattice structure and a reduction in symmetry.

Thin foil specimens were prepared from bulk samples by conventional thinning and ion milling (at LN2 temperature) methods. The TEM work was carried out in a Philips CM30 at 300 kV with a Gatan LN2 cold stage, and a JEM 4000EX with a resolution of 0.17 nm at 400 kV for HREM observation.

High resolution X-ray mapping and dating of monazite (Th, REE-phosphate) using the electron microprobe is an exceptionally powerful technique for structural, metamorphic, and tectonic analysis in geology. Age determination of geologic materials has been conventionally accomplished by mass spectrometry-based analysis of radioisotopic ratios in minerals from hand-picked mineral separates, careful sampling of individual grains out of petrographic thin sections, or by detailed ion probe analysis. Recently, use of the electron microprobe for dating purposes has been attempted. In principal, the concentrations of Th, U and Pb uniquely define the age if non-radiogenic Pb is either not initially present or can be subtracted from the total Pb. Monazite contains high concentrations of Th and U and does not appear to incorporate significant non-radiogenic Pb during mineral growth. Furthermore, monazite is a ubiquitous accessory phase in many metamorphic and igneous rocks, making it ideal for microprobe dating.

Xenotime is an igneous mineral commonly present in pegmatite and fractionated granite. Recent studies reveal that it also forms as a diagenetic mineral. Minute (0.1-5 μm) xenotime overgrowths typically crystallise on the surfaces of detrital zircon shortly after sedimentation, in a wide range of siliciclastic sedimentary units. For example, in backscattered electron (BSE) imaging using a scanning electron microscope (SEM), two minute, euhedral, pyramidal, xenotime overgrowths on an oscillatory-zoned detrital Ambergate zircon are evident (figure 1).

Electron microprobe analysis (EMPA) geochronology is a chemical dating method that uses precisely measured concentrations of U, Th, and Pb, and the decay rates of U238, U235, and Th232, to calculate an age for a mineral. The EMPA dating method used in this study to date igneous xenotime and igneous-metamorphic monazite is the chemical isochron method (CHIME). EMPA geochronology is not a widely used technique because of the higher precision of isotopic geochronology.

Quantitative analysis of Na in minerals like feldspar in an Electron Probe Micro Analyser (EPMA) confronts two problems - the first one arising at the time of data acquisiton of Xrays of Na (say Kα) and the second one during conversion of measured xray intensity ratios into true concentration while using certain correction procedure The fall in the count rate of Na xray signals as a function of time under stationary electron beam in EPMA is quite well known. This was attributed to volatilization of Na during electron bombardment. Another explanation for this phenomenon has been that Na+ ions made mobile in the crystal lattice of feldspar by the electron beam, move away from the point of its impact. This results in the fall in the count rate of Na with a corresponding increase in me count rate of matrix element like Si, Al.