Electron microscopy of transformation dislocations at interphase boundaries

The concept of structural units (SU’s) developed in order to describe the atomic structures of twin boundary facets is also used for interphase boundary (IB) facets quasi-parallel to small near-coincident planar cells of the two adjacent lattices. These facets, which have their own SU’s, are separated by transformation dislocations (TD’s), the cores of which are often related to ledges having heights equal to several interplanar spacings. It is shown that the Somigliana dislocation (SD) concept is a good tool for the computation of elastic displacement fields of these TD’s in anisotropic elasticity. Applications are presented concerning the following IB’s observed in high-resolution transmission electron microscopy (HRTEM): Si/TiSi₂, Si/CoSi₂, and Ni₃Al/Ni₃Nb. The identification of the atomic rows around some TD’s at Si/CoSi₂ and Ni₃Al/Ni₃Nb has been obtained by careful comparisons of experimental and calculated images. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Controlled environment transmission electron microscopy

The design of a controlled environment electron microscope is described and examples are presented to demonstrate the information that can be obtained by studying gas-solid interactions in real time and at high spatial resolution.     

The use of dimethylsulfoxide for fixation of yeasts for electron microscopy

Increasing numbers of young women with ovarian failure and women of advanced reproductive age (> 40 yrs) utilize oocyte donation to treat their infertility. In both groups, women who become pregnant frequently experience multiple gestation, occurring in up to 30% of pregnancies. Advanced maternal age and high-order multiple gestations are associated with an increased risk for obstetric complications. We reviewed the pregnancies of patients with high-order multiple gestations (> or = 3 gestational sacs) with respect to their antepartum course and neonatal outcomes. Mothers were divided into two groups according to age at conception; Group I (> or = 40 yr, n = 20) and Group II (< 40 yr, n = 10). These 30 high-order multiple gestations were found among 127 successful oocyte donation cycles (23.6% of all pregnant patients). Data regarding pregnancy outcomes were gained by chart review and telephone interview. Results demonstrated spontaneous reductions in the number of implantation sites were similar between groups (Group I: 21.4% vs. Group II: 17.6%). Multifetal pregnancy reduction (MFPR) was more often chosen by older women (Group I: 45% vs. Group II: 10%; P < 0.05). Antenatal complications were commonly experienced by both groups (> 80%) as were operative deliveries (> 85%). However, neonatal outcomes were generally good, with only one death occurring in the 79 delivered infants (1.3%). We conclude transferring supernumerary embryos to women undergoing ovum donation places patients at great risk for high-order multiple gestations. These pregnancies are associated with increased antenatal and neonatal complications. Although advanced maternal age is normally an added risk factor, well-screened older patients carrying high-order multiple gestations experienced similar outcomes as younger mothers.     

Analytical electron microscopy of grain boundaries in high-strength steels

Phosphorus could be detected at prior austenite grain boundaries (PAGB) in high-strength alloy steels quenched and tempered at 500°C when using a VG's HB 501 dedicated field emission STEM but not with a conventional JEOL 4000FX STEM. No phosphorus was detected at PAGB's in the as-quenched materials or away from PAGB's in tempered materials of either type. The grain boundary coverage of phosphorus was, assuming a specimen thickness of 80 nm, 0.7 monolayers for the 3.5NiCrMoV rotor steel and 0.4 monolayers for the AISI 4340 steel. The grain boundary concentration of phosphorus, assuming a specimen thickness of 80 nm and a segregated layer thickness of 1 nm, for the 3.5NiCrMoV rotor steel was 6 wt% and for AISI 4340 4 wt%. Compared to the bulk concentration of about 0.01 wt% this means that the enrichment factor of P to the grain boundaries was several hundred times (610 respectively 370). Our measurements showed no correlation between the stress corrosion crack growth rate and the grain boundary phosphorus concentration. The yield strength, however, decreased after tempering while the phosphorus concentration at the grain boundaries increased.     

Computer-assisted enhancement permits the use of five nanometer gold probes in low magnification transmission electron microscopy

Since the red blood cells are characterized by the different resistance to action of the colloidal silica particles, this fact allows us to suppose that heterogeneity of the cell response to silica can be used as a criterion for the study of changes in the membrane systems. Flow cytometry has been proposed as one of most effective techniques for detecting a dynamic changes of cell morphology during silica-cell interaction.     

Cross-sectional transmission electron microscopy of ZnO tetrapod-like particles

ZnO tetrapod-like particles having legs as long as several tens of micrometers have been observed by cross-sectional electron microscopy. The specimen is prepared by a new method where a photo-resist suspension of the particles fills up holes of an electron microscopy grid and is solidified for the subsequent ion-milling. Electron diffraction and high-resolution electron microscopy of a section of a ZnO tetrapod reveal the orientation relationship among its four legs, which enables us to discuss on the growth mechanism of these particles.     

The structure of interphase boundaries in Al-CuAl2 curved eutectic crystals

Direction lly solidified crystals were grown in which the Al-CuAl₂ interphase boundary was uniformly misoriented by 6-8 deg with respect to the usual crystallographic relations interface . The rrays of features then found in the boundary were shown to be ledges by transmission electron microscopy. These ledges could move in the boundary plane during heating-stage experiments. The broad faces of the ledges were the low-energy orient tions mentioned above and were rotated with respect to the macroscopic ihterphase boundary plane by bout 7 deg. From these observations, it is concluded that the misoriented interphase boundaries in Al-CuAl₂ curved eutectic crystals are best described in terms of regions of good fit separated by diffusion-glide (DG) ledges.     

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.     

Geometric aspects of low energy metal-ceramic interphase boundaries

Geometric models relating the energy of heterophase boundaries including metal-ceramic interfaces to their atomic structure can be based on the static distortion wave concept. This approach allows to separate the energetic from the substrate-induced strain contribution of commensurate and incommensurate boundary structures. The lock-in energy is gained when both crystallites are properly oriented, e.g. when the reciprocal lattice vectors of both lattices are equal and the strain energy term can be neglected. However, the chemical interaction and interfacial entropy are important as well for the structural stability evaluation and can be approximated on the basis of the universal binding energy relation. These relationships are discussed and compared with experimental resutls.     

A technique for preparing thin foils of ti and ti alloys for transmission electron microscopy

Metallurgical and Materials Transactions A -