S. Poulat

Weak-beam transmission electron microscopy study of dislocation accommodation processes in nickel Σ = 3 grain boundaries

Abstract The first steps of the accommodation of extrinsic dislocations in Σ = 3 grain boundaries have been observed in nickel bicrystals. Two types of reaction have been investigated in detail using the weak-beam technique in addition to conventional transmission electron microscopy analyses and the image-matching technique. In one case, the decomposition of a trapped lattice dislocation in two products, not visible in the bright-field condition, can be seen as the necessary step preceding the emission of one product dislocation into the neighbouring crystal. This means that dislocation transmission through a grain boundary, even Σ = β, is never a direct process. In the other case, a reaction between an extrinsic dislocation and an intrinsic dislocation having different line orientations could be analysed owing to the visualization of a small dislocation segment in the weak-beam condition. This reaction has been interpreted as the first step of incorporation of an extrinsic dislocation in the intrinsic n...

High-resolution transmission electron microscopy observations and atomic simulations of the structures of exact and near Σ = 11, {332} tilt grain boundaries in nickel

Abstract A Σ = 11, {332} nickel bicrystal was grown by solidification. The aim of this paper is to show the evolution of the grain-boundary (GB) structure depending upon the position in the bicrystal of the extracted sample. Conventional transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM) observations were used to characterize the GBs on microscopic and nanoscopic scales respectively. The detailed atomic structures of the exact {332} and the asymmetrical {111}‖{331} GBs were investigated by numerical calculations and compared with the HRTEM images. There is a perfect agreement between the calculated and the experimental image for the symmetrical {332} GB at the head of the bicrystal. The high GB defect density in the asymmetrical GBs at the end of the bicrystal makes the comparison less straightforward. It is, however, noteworthy that two complete periods of the calculated structure are actually observed in the experimental image. In order to approach the atomic description of the {111}1 ‖{hkl}2 asymmetrical tilt GBs, it is proposed to use for each side the structural units which appear in the corresponding symmetrical {111} and hkl tilt GBs respectively.

In-situ transmission electron microscopy study of the dislocation accommodation in [101] tilt grain boundaries in nickel bicrystals

Abstract The relaxation of stresses associated to extrinsic grain-boundary dislocations (EGBDs) in singular ∑ = 3, {111}, in vicinal ∑ = 11, {311} and in general ∑ = 11, {332} grain boundaries (GBs) has been investigated by in-situ transmission electron microscopy (TEM) using weak-beam conditions. In a singular ∑ = 3, {111} GB, reactions of combination and annihilation occur between extrinsic and intrinsic dislocations when they initially intersect each other. These reactions yield the parallelism of the two types of dislocation which is the preliminary configuration for any of the existing incorporation models to operate, but further evolution by decomposition of the EGBDs in glissile and sessile components, which can move away, as depicted by theory, was observed once only at a very high temperature. Furthermore, the rearrangement of all the sessile dislocations, extrinsic and intrinsic, in a periodic network was not observed, even after maintaining the thin foil at 930°C = 0.6T m (T m is the melting te...

HRTEM Studies of the Structures and the Defects of Exact and Near ∑ = 11 {332} Tilt Grain Boundaries in Ni

A ∑ = 11 {332} Ni bicrystal was grown by solidification. The Grain Boundary (GB) atomic structure studied by high resolution electron microscopy (HREM) has been shown to depend upon the position in the bicrystal of the extracted sample. Strongly correlated to the atomic structure, (GB) defects (GBD) have been characterized: all their Burgers vectors belong to the Displacement Shift Complete (DSC) lattice and the height of the step associated with each GBD varies up to a few nanometers. In the symmetrical and almost symmetrical GBs, at the head of the bicrystal, the dislocation cores can be well localized, whereas in the asymmetrical GBs, at the end of the bicrystal, their cores are much more difficult to localize on the HREM image. The influence on the GB structures of the impurity content which varies along the bicrystal is also discussed.

Thermal stability of extrinsic dislocations in near Σ11 grain boundaries in nickel

Abstract Thermal stability of the extrinsic grain boundary dislocations (EGBDs) in near Σ 11 grain boundaries (GBs) has been investigated in nickel bicrystal thin foils containing different levels of impurities. The geometrical (GB misorientation and plane-GB steps) and chemical (segregation) factors which may affect the EGBD relaxation have been considered. The results show that the EGBD accommodation kinetics depends strongly on the segregation level, related to the GB plane orientation, and on the EGBD line orientation in the GB plane. They are mainly interpreted in terms of intergranular diffusion. They support the conclusion that no geometrical criterion can predict GB behavior, particularly in presence of GB segregation.

Epitaxy of Ge on sapphire

Abstract Ge was grown on sapphire (α-Al 2 O 3 single crystals) by molecular beam epitaxy. On the (0001) surface (basal plane) of sapphire as well as on the (11 2 0) surface (prism plane), Ge forms islands with a unique orientation relationship to the substrate. On the (1 1 02) surface (rhombohedral plane), however, Ge grows with a fibre texture: While the grains have a unique crystallographic direction parallel to the substrate normal, the crystal lattices of the individual grains are rotated more or less randomly about this axis. It is attempted to compare the experimentally observed orientation relationships with those of a model that considers the ‘reciprocal space overlap’ of the substrate and the overgrowth

ZrNi-H2: microstructural analysis of the thermodynamically controlled hydride phase growth, and electronic properties

Abstract The ZrNi–H 2 phase diagram indicates the presence of two stable hydrides: a triclinic monohydride ZrNiH and an orthorhombic trihydride ZrNiH 3 [1] . In this paper, we present the results of some thermodynamic, microstructural and electronic properties of the ZrNi–H 2 system. A strict control of the thermodynamic variables ( P , T ) during the hydride phase growth allowed us to prepare two samples of overall composition ZrNiH 0.70 and ZrNiH 2.5 . In a first step, the microstructure evolutions induced by hydrogen absorption are investigated during the activation process. The intermetallic and hydrided compounds are studied by energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD) methods. The nature of defects generated in ZrNi and its hydrides compounds are observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Ab-initio electronic structure calculations have been performed. They clearly show the importance of chemical effects besides geometrical factors in explaining the preferential site occupancy of hydrogen atoms in the ZrNiH phase.