Transmission Electron Microscope Contrast Research Articles

Microstructural characterization of the M23C6 carbide in a long-term aged Ni-based superalloy

The microstructural features of M23C6 carbide in a long-term aged heat- and corrosion-resistant Ni-based superalloy have been investigated in detail using various kinds of transmission electron microscope (TEM) techniques. It is found that TEM contrast, which is related to structural and chemical inhomogeneities inside the grains, always exists in the interior of grains in the alloy. The structure of these inhomogeneous regions has been determined to be the same as that of the γ′ and t-M23C6 phases, where t-M23C6 indicates a transitional and metastable phase. Although possessing the same structure as the M23C6 phase, the chemical composition of the t-M23C6 is different from that of the M23C6 phase. Compared with M23C6, t-M23C6 is richer in Ni, Co, Al and Ti but poorer in W, Mo and Cr. This phenomenon of structural and chemical inhomogeneity demonstrates that pristine M23C6 carbide (p-M23C6) precipitated in standard heat-treated samples is unstable. Therefore, upon long-term ageing treatment, Ni, Co, Al and Ti may locally enrich inside the p-M23C6 phase, finally forming the γ′ phase, which can be described by the decomposition reaction p-M23C6 → M23C6 + γ′.

In situ transmission electron microscopy of resistive switching in thin silicon oxide layers

Silicon oxide-based resistive switching devices show great potential for applications in nonvolatile random access memories. We expose a device to voltages above hard breakdown and show that hard oxide breakdown results in mixing of the SiOx layer and the TiN lower contact layers. We switch a similar device at sub-breakdown fields in situ in the transmission electron microscope (TEM) using a movable probe and study the diffusion mechanism that leads to resistance switching. By recording bright-field (BF) TEM movies while switching the device, we observe the creation of a filament that is correlated with a change in conductivity of the SiOx layer. We also examine a device prepared on a microfabricated chip and show that variations in electrostatic potential in the SiOx layer can be recorded using off-axis electron holography as the sample is switched in situ in the TEM. Taken together, the visualization of compositional changes in ex situ stressed samples and the simultaneous observation of BF TEM contrast...

Polycrystal orientation maps from TEM

Determination of topography of crystallite orientations is an important technique of investigation of polycrystalline materials. A system for creating orientation maps using transmission electron microscope (TEM) Kikuchi patterns and Convergent beam electron diffraction patterns is presented. The orientation maps are obtained using a step-by-step beam scan on a computer-controlled TEM equipped with a CCD camera. At each step, acquired diffraction patterns are indexed and orientations are determined. Although, the approach used is similar to that applied in SEM/electron back scattered diffraction (EBSD) orientation imaging setups, the TEM-based system considerably differs from its SEM counterpart. The main differences appear due to specific features of TEM and SEM diffraction patterns. Also, the resulting maps are not equivalent. On these generated by TEM, the accuracy of orientation determination can be better than 0.1°. The spatial resolution is estimated to be about 10 nm. The latter feature makes the TEM orientation mapping system an important tool for studies at fine scale unreachable by SEM/EBSD systems. The automatic orientation mapping is expected to be a useful complement of the conventional TEM contrast images. The new technique will be essential for characterization of fine structure materials. To illustrate that, example maps of an aluminum sample produced by severe plastic deformation are included.

Differential phase contrast in TEM

We present the first results using a novel transmission electron microscope (TEM) technique that combines electron holography and Fresnel imaging to give simultaneous information about magnetization and domain wall position in magnetic materials. This technique provides differential phase contrast through electron holography in a manner similar to the established differential mode in scanning transmission electron microscopy. It has been implemented by placement of an electrostatic biprism in the condenser aperture plane of a field emission TEM. Changes of one in-plane component of the magnetic field of a Co specimen have been imaged. With rotation of either the biprism or the specimen, it should be possible to map the entire in-plane magnetic field.

Surface Waves and Strain Modulations in Si1-xGex Alloy Layers on Si

ABSTRACTThe growth of strained, continuous Si1-xGex epitaxial alloy layers on Si can, under certain conditions, result in the occurrence of marked, small-scale layer thickness fluctuations in the form of crystallographically-aligned, interlocking ripple arrays. In the present work, combined transmission electron microscope (TEM) and atomic force microscope studies are employed to reveal the detailed nature of these surface ripples. TEM contrast studies demonstrate that well-defined, oscillatory strain variations accompany these ripple structures, the presence of which is shown to be associated with partial elastic strain-relief and lowering of the energy of die strained-layer system.