Structure and mechanism of bonding at a diffusion-bonded Al/SiC interface

Al/SiC interfaces were fabricated by diffusion bonding a pure Al foil between two blocks of SiC for temperatures ranging from 500 to 600°C. For samples bonded below 586°C, the interfacial strength was low and TEM speciments could not be fabricated due to separation of the Al and SiC pieces dring thinning. For samples bonded at and above 586°C, a strong bond was formed and conventional and high-resolution transmission electron microscopy revealed the formation of a thin amorphous phase at the interface. Compositional analysis showed that the interfacial phase contained Al, Si, C and O. Formation of the amorphous phase was demonstrated to occur by a solid state reaction and is discussed on the basis of thermodynamic and kinetic considerations. Lastly, some of the advantages of having an amorphous phase at a metal/ceramic interface are discussed.     

Observation of gold-silicon alloy formation in thin films by high resolution electron microscopy

Thin films (<30 nm) containing mixtures of gold and silicon were co-deposited on (100) surfaces of NaCl from the pure elements co-evaporated at equivalent molar rates. The resulting samples, believed to be homogeneous distributions of the elements, were used to observe alloy formation during electron beam heating in an electron microscope. The formation of the alloy was shown to accompany the displacement and subsequent crystallization of excess silicon. The crystallographic data derived from the microscopic study were found to agree within experimental error with those reported from studies of bulk samples prepared by the rapid quenching of gold/silicon melts and assigned the composition Au₂Si. Samples heated under intense electron beam irradiation resulted in the decompositon of the alloy and the formation of crystalline gold and silicon. Thicker samples (∼100 nm) were prepared by evaporation on (100) NaCl and subsequently heated at temperatures up to 250°C. These specimens examined by the “thin-film” X-ray diffraction technique confirmed the formation of Au₂Si as well as its subsequent decomposition at temperatures above 100°C.     

铝锂合金显微组织高分辨电子显微研究

以新型高强铝锂合金（Al-Cu-Li）为研究对象,合金经过520 ℃固溶淬火后分别在165 ℃时效6 h和165 ℃时效18 h,然后采用高分辨电子显微镜观察Al-Cu-Li铝锂合金显微组织中δ相和T1相的形貌和晶格像,研究了主要强化相T1相的形核机制。结果表明：单胞T1相为五层密排六方堆垛结构,T1相的化学堆垛次序为CBABC。观察发现T1相既可以单独在基体形核生长,也可以依附在原有的T1相上形核以台阶方式生长。细小的球形析出相δ相为有序结构相,并且与基体共格。     

High resolution transmission electron microscopy studies of the Ag/MgO interface

Ag layers with a thickness of 100 nm were deposited on {100} MgO substrates by molecular beam epitaxy (MBE). The substrates were produced by cleaving MgO single crystals and subsequent annealing in ultra high vacuum. X-ray texture measurements indicate a preferential orientation of Ag {100} planes parallel to the {100} surface of the MgO substrate. The atomistic structure of the Ag/MgO interface was imaged by high resolution transmission electron microscopy (HRTEM). Lattice images of Ag and MgO in parallel orientation show structural defects and lattice strain at the Ag/MgO interface, which are introduced by the lattice mismatch of 3% between Ag and MgO. The atomic resolution images are compared with the computer-simulated lattice image of a model structure. Finally our atomistic model of the Ag/MgO interface, which includes a misfit dislocation network is discussed in connection with theoretically calculated interfacial energies.     

An analytical electron microscopy study of the high temperature carbide formed in a V-5 Ti-C alloy

A finely dispersed high temperature carbide that precipitates at 1000°C in a V-5 pct Ti-C alloy has been studied using analytical electron microscopy. The crystal structure has been determined using electron diffraction as B1 (NaCl-type) with a lattice parameter of 4.29Å. The carbide develops a habit plane parallel to {001} of the matrix. The orientation relationship between the carbide and the matrix is [010] carbide ¦ | [ $$ \begin{gathered} [010] carbide || [0\bar 10] matrix \hfill \\ (001) carbide ||(101) matrix. \hfill \\ \end{gathered} $$ The elemental concentrations of the metal constituents of the carbide have been deduced using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy, through examination of core electron excitations. The composition of the carbide is found to be V₁₁Ti₃gC₅₀, assuming a stoichiometric carbide.     

Prospects of three-dimensional high resolution electron microscopy of non-periodic structures

In the first part of the paper the general features of three-dimensional electron microscopy of non-periodic structures at high resolution are discussed. Such an analysis requires series of 20 or more exposures of mechanically moved specimens. It is fundamentally important that the integral radiation dose, however, need not be increased--three-dimensional analysis yields extra information without additional radiation loading. It is demonstrated that the constraints characteristic for atomic resolution can in future facilitate many difficulties like restricted tilting range (conical tilting), inaccuracies in the refinement of the origin determination etc. Data collection up to atomic resolution is possible with existing image reconstruction methods if the chromatic spread of the cathodes can be partially corrected. The relatively great radiation induced changes of "stable" specimens will require additional averaging in the experiment (quasi-simultaneous registration). The advantage of a time dependent analysis of radiation sensitive structures--especially the possibility of using redundancies in the analysis (trace structure analysis)--is discussed. The experimental part (studies of radiation damage and three-dimensional work on carbon foils) presents examples of such analyses. The main experimental difficulty consists in the collection of many exposures with no extra load of the specimen in the intervals between exposures.     

High resolution electron microscopy investigations of interface and other structure defects in some ceramics

To evaluate occurrence of oxidative stress in circulating blood, we developed standard methods to assess (1) granulocytes status as a source of reactive oxygen species (ROS) and (2) lipid peroxidation (LPO). A simplified and highly sensitive assay was developed by utilizing the chemiluminescence (CL) from luminol oxidized by ROS. 1. The CL, from 300 microliters medium containing 1% blood, 10 micrograms/ml luminol and 0.025 microgram/ml phorbol myristate acetate, well reflected the primed granulocyte status induced by in vitro contact with lipopolysaccharide (LPS). This CL was weakened slightly by superoxide dismutase and catalase, but markedly decreased by sodium azide. 2. We determined the optimal conditions for the t-butyl hydroperoxide (t-BuOOH)-stimulated CL method to evaluate plasma LPO in experiments on rat plasma added with phosphatidylethanolamine hydroperoxide (PEOOH). The CL from 300 microliters medium containing 6.67% plasma. 10 micrograms/ml luminol and 5 mumol/ml t-BuOOH was proportional to the added PEOOH amount. The integrated CL of the plasma with 0-60 nmol of PEOOH gave values of 8.280-14.213 x 10(6) counts/60 min/tube. 3. Only 100 microliters of freshly drawn blood was enough for the two CL methods to detect the generation of ROS and the occurrence of LPO. These CL methods enabled the determination of the time course of oxidative stress occurrence in circulating blood of rats treated with 5 mg/kg LPS, i.p.     

Evaluation of microstructural changes in alloy 713 LC by neutron small angle scattering and analytical electron microscopy

Small angle neutron scattering (SANS) studies in combination with transmission electron microscopy (TEM) were performed on the cast nickel base superalloy Alloy 713 LC and on a Hf-modified version of the same alloy. With the aid of the TEM results the profile of the scattering curves was correlated with the M₂₃C₆ carbide and the γ′ precipitates. A coarsening of the γ′ precipitates with increasing creep deformation to a larger plate-like shape was observed. The small axis of these precipitates averaged over the grains was parallel to the stress axis. The γ′precipitates start to become anisotropic even in the primary stage of creep. In the vicinity of the fracture surface the volume fraction of the cavities and microcracks caused by creep deformation was 10⁻³ to 10⁻² The influence of the cavities was indicated by the smaller anisotropy factor measured near the fracture surface in comparison to the rest of the cylindrical part of the specimen. The variation of the anisotropy factor as determined by the SANS-method may be used to nondestructively measure the accumulated damage in the material. formerly at Oak Ridge National Laboratory, Oak Ridge, TN     

New phases in the platinum-titanium system and their formation mechanism studied by electron diffraction and high resolution electron microscopy

In Pt₄Ti one stable and one metastable long period structure are discovered in material annealed at 900°C. These phases are investigated using electron diffraction and high resolution electron microscopy indicating a large non-uniform tetragonal distortion in the long period direction. The stable phase can be described as a stacking of ordered and disordered units while in the metastable phase the disordering affects every cube plane. The formation mechanism is described in terms of a shear mechanism along f.c.c. cube glide planes.     

Studies of superplasticity by high voltage electron microscopy

Experiments and calculations are described that enable the validity of in situ high voltage electron microscopy studies of superplasticity to be estimated. It is shown that previous in situ studies of the Zn-Al eutectoid are open to question because the diffusivity in the aluminium-rich grains is calculated to be enhanced by a factor of 10⁵ by irradiation damage from the electron beam. Direct evidence of this damage has been observed in terms of irradiation induced dislocation loops, and enhanced precipitation and coarsening rates. The results from the in situ experiment were not typical of bulk material and their conflict with previous texture measurements performed on bulk material can be explained.     

Determination of carbide and matrix compositions in high-speed steels by analytical electron microscopy

Procedures for the analysis of the blocky (“primary”) carbides and of the matrix in high-speed tool steels with scanning transmission electron microscopy-energy dispersive X-ray (STEM-EDX) are described. Results for the alloys AISI Tl, M2, and M7 and for an experimental, Nballoyed material are reported and compared to determinations by quantitative metallography in scanning electron microscopy (SEM), combined with compositional analysis by SEM-EDX. With proper precautions, good accuracy can be obtained with either method, and the results obtained by the two routes are in good agreement. On leave of absence at Max-Planck-Institut für Metallforschung. formerly Research Assistant, Max-Planck-Institut für Metallforschung.     

Scanning electron microscopy of the bone-bioactive implant interface

The purpose of this study was to compare the intelligibility of two types of alaryngeal speech commonly used after total laryngectomy. Four male oesophageal speakers and four male tracheo-oesophageal speakers read a series of monosyllabic words, multisyllabic words and sentences. The monosyllabic word list consisted of several minimal pairs for each of eight phonetic contrasts; multisyllabic words and sentences were not selected on specific phonetic grounds. Audio recordings of all subjects' readings were presented to eight na?ve adult listeners who completed both an item identification task and a scaling procedure. The item identification task revealed higher intelligibility fpr tracheo-oesophageal speakers than for oesophageal speakers during the monosyllabic word condition. Results from the scaling procedure indicated that listeners' subjective intelligibility ratings were also higher for the tracheo-oesophageal speakers than for the oesophageal speakers. Moreover, a high positive correlation was found between the speakers' intelligibility scores obtained from the word identification task and the scaling procedure.     

Characterization by fluorescence and electron microscopy in situ hybridization of a double Y isochromosome

A patient with mixed gonadal dysgenesis and Y isochromosomes i(Y) is described. Lymphocyte cultures from peripheral blood contained a high proportion of 45,X cells and several other cell lines with two different marker chromosomes (mars). These markers had either a monocentric (mar1) or a dicentric appearance (mar2). Following high-resolution GTG, RBG, QFQ, and CBG bandings, five cell lines were identified; 45,X/46,X,+mar1/46,X,+mar2/47,X,+mar1x2/47,X,+mar2x 2. The percentages were 66/6/26/1/1%, respectively. Chromosome banding analyses were insufficient for characterization of the markers. In situ hybridization of specific probes for the Y centromere and its short arm showed, both in fluorescence and electron microscopy (EM), two different Y rearrangements. Mar1 is an isochromosome for the short arm i(Yp) and mar2 is a dicentric which was shown by EM to be a double isochromosome Yp, inv dup i(Yp). The breakpoint producing mar1 is within the centromere and the one producing mar2 is within one of the short arms of the Y isochromosome. The findings of different cell populations in peripheral blood lymphocytes indicate the postzygotic instability of this i(Yp).     

Transmission electron microscopy investigation of interfaces in a two-phase TiAl alloy

The atomic structures of the γ/α₂ and γ/γ_T interfaces in a TiAl alloy were investigated using conventional and high-resolution transmission electron microscopy (TEM) in order to understand the growth mechanisms and deformation behavior of the two-phase alloy. The results show that the α₂ plates grow from the γ phase by the migration of a/6〈112〉 partial dislocation ledges across the faces and that the γ/α₂ interface usually contains closely spaced arrays of interfacial dislocations. Deformation twins cut through both γ twin boundaries and α₂ plates during deformation, although slip of twinning c slocations through α₂ appears to be a difficult process. Both the γ/α₂ and γ/γ_T interfaces can be imaged and modeled at the atomic level, although slight crystal and/or beam tilt can complicate image interpretation. G.J. MAHON, formerly Research Associate with the Dep rtment of Metallurgical Engineering and Materials Science, Carnegie dellon University This paper is based on a presentation made in the syrr losium “Interfaces and Surfaces of Titanium Materials” presented at tl; 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29 1988, under the auspices of the TMS Titanium Committee.     

Evaluation of low-level asbestos exposure by transbronchial lung biopsy with analytical electron microscopy

To improve diagnostic sensitivity for detecting low-level asbestos exposure (AEx) in patients, a new method was developed using an analytical transmission electron microscope (ATEM) for specimens of transbronchial lung biopsy (TBLB). The TBLB specimens from 28 patients were examined and the results were: 1) In cases with long-term AEx, the present method detected a large amount of asbestos fibers (AF) as well as asbestos bodies (AB) showing a good agreement with the results of light microscope (LM) which detected definite amounts of ferruginous bodies (FB). 2) In cases with short-term or suspected AEx, the LM failed to detect FB in some cases, but an appreciable amount of AF was detected using the present method, and AEx was disclosed through a second close interview. 3) Neither AB nor AF were detected in most of the cases without any dust exposure. Although small amounts of chrysotile fibers were observed in some cases, this might simply reflect the exposure level of urban dwellers. These results show that the ATEM applied to the TBLB specimens promises to confirm low-level AEx in such small specimens even if the patients were unaware of their past AEx.