Weak-beam Dark-field Microscopy Research Articles

Transmission electron microscopy characterization of CrN films on MgO(001)

Two CrN(001) films with different thickness were grown on MgO(001) substrates using unbalanced d.c. magnetron sputtering. The morphology and interfacial structure of the films are characterized by using conventional transmission electron microscopy, weak-beam dark-field microscopy and spherical aberration (CS)-corrected high-resolution transmission electron microscopy. The microscopy studies revealed the well-known cube-on-cube orientation relationship. While an interface dislocation network with b→=½aCrN<100> edge dislocations was identified, only part of the lattice mismatch is relaxed. The misfit dislocation structure and growth defects are analyzed and discussed based on the weak-beam dark-field and high-resolution transmission electron microscopy results.

{103}⟨331⟩ slip in MoSi2

Abstract {103}⟨331⟩ slip is found to be activated in MoSi2 single crystals by deformation along [001] in uniaxial compression at 1000°C. The {103} slip plane has been determined by slip trace analysis. Observations by transmission electron microscopy reveal the presence of screw dislocations with a Burgers vector of ½⟨331⟩. However, ½⟨331⟩ dislocations of mixed character lying along the ⟨ 010 ⟩ direction are observed to have decomposed into two other perfect dislocations by the energetically favourable reaction ½⟨ 331 ⟩→½⟨111⟩ + ⟨110⟩. The occurrence of such dislocation decomposition explains the difficulty of observing ½⟨331⟩ dislocations in previous work. The ½⟨331⟩ screw dislocations have been examined by weak-beam dark-field microscopy but dissociation into three ½⟨331⟩ collinear partials could not be detected.

The compositional dependence of antiphase-boundary energies and the mechanism of anomalous flow in Ni3Al alloys

Abstract The theory of the anomalous flow behaviour of Ll2 compounds has developed over the last 30 years. This theory has a foundation in the early estimates of the crystallographic anisotropy of antiphase-boundary (APB) energy in these compounds. In spite of this critical aspect of the theory, it is only in the last 5 years that electron microscopy has been employed to quantify the APB energies and to determine the detailed nature of dislocation structures at each stage of deformation. The present study examined binary Ni3Al single crystals having compositions which span the single-phase region, and selected ternary compositions expected to have a dominant influence on either {001} or {111} plane APB energies. Crystals were deformed in compression at an orientation near the [001] direction over the temperature range from — 196 to 700°C. Weak-beam dark-field microscopy was employed to quantify the APB energies and the APB energy anisotropy over the composition range. Experimental uncertainties associated...

Dissociation of Misfit Dislocations in GeSi/{111}Si Interfaces

AbstractThe accommodation of lattice mismatch is studied in Ge0.15Si0.85 layers grown epitaxially on {111}-oriented Si substrates by chemical vapor deposition (CVD) at 1100°C. Weak beam dark field microscopy reveals a regular misfit dislocation network, which resembles the honeycomb network of edge-type dislocations anticipated by the O-lattice theory. In contrast to the latter, however, the real network exhibits extended nodes where the misfit dislocations dissociate into misfit partial dislocations. Between the partials, high resolution transmission electron microscopy (HRTEM) reveals intrinsic and extrinsic stacking faults. Owing to the presence of these stacking faults, three different atomistic structures of the GeSi/Si interface coexist and compete for the interfacial area according to their energy. The observed configuration is shown to minimize the total energy of the interface.

Weak-Beam Imaging of Coherent Twin Boundaries After High-Temperature Deformation

Weak beam dark field microscopy has been used extensively to study deformation induced intragranular defects in materials. However as noted by Tones et al weak beam imaging is also an asset in examining intergranular defects. The localisation of strain contrast close to the defect core by weak beam imaging allows the interaction of matrix dislocations with the boundary (to form extrinsic dislocations) to be studied more easily. Also, the reduction in extinction distance with weak beam microscopy produces a higher density of extinction contours, which allows topographical discontinuities associated with long range strain fields to be more readily observed.Thus in the present study, where we were interested in interfacial phenomena during high temperature deformation processing of a cobalt base alloy (Co-Cr-Mo), weak beam imaging was used to obtain a better understanding of the role of intergranular defects.