Jany Thibault

Structure and role of the interfacial layers in VC-rich WC-Co cermets

The microstructure of WC-Co composites sintered with VC or with a mixture of VC and Cr3C2 is investigated by several techniques in order to understand the grain growth inhibition process. In this work, using the high-resolution transmission electron microscopy and high-resolution energy-filtering electron microscopy, we are able to study on the atomic scale the microstructure and composition of the phases present in the alloys. A thin (VW)C x layer less than 1 nm thick covering all surfaces of WC grains and thin (VW)C x platelets embedded in the WC grains are evidenced. Microsteps are observed at the interface between Co and WC along the ⟨11amp;2macr;0⟩WC directions. Small (VW)C x precipitates lie on the (0001)WC and {10amp;1macr;0} facets of these steps. On the (0001) surface of WC grains, other stacking sequences of the metal planes are sometimes observed and, in particular, the occurrence of the compound (VW)2C is shown. Owing to these observations a grain growth inhibition mechanism is proposed.

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.

Structure of copper precipitates in a symmetrical silicon tilt bicrystal : high-resolution electron microscopy and energy-dispersive X-ray analysis

Abstract Precipitates have been found in silicon bicrystals after heat treatments. Most of them are localized in the grain-boundary (GB) plane where they form disc colonies. The GB precipitates have been examined using conventional electron microscopy, high-resolution electron microscopy and energy-dispersive X-ray analysis techniques. They contain a large amount of copper. The structure of the precipitates in the GB is b.c.c. and is likely to be disorderd. The grain-precipitate epitaxy relationship has been determined. A mechanism for the copper precipitation in disc colonies along the GB is proposed.

Structural evolution of Au[001]/Ni molecular-beam epitaxy multilayers with Ni thickness: Comparison with Ag/Ni and Au/Co systems

Abstract Strain relaxation in metallic multilayers with a large misfit (15%) has been investigated by high-resolution electron microscopy. For this purpose, model systems, namely Au[001]/Ni multilayers and Ni thin films on Au[001], have been grown by molecular-beam epitaxy. The strain relaxation involves two regimes depending on the thickness of the Ni layer: pseudomorphic growth for thin layers (less than five monolayers of Ni) which is possible because of interdiffusion at the interfaces, and a structure and orientation change for thicker layers. These two modes of relaxation will be emphasized and discussed in terms of energy and symmetry considerations. Two complementary systems (Ag/Ni and Au/Co) have been studied for comparison. It will be shown that, despite the same geometrical features, the early stages of the growth are different in these three systems and that the consideration of only lattice misfit is not sufficient to predict the growth.

Platelet copper precipitates in silicon: A high-resolution electron microscopy study

Abstract A new shape of copper precipitates in silicon has been observed to form after a heat treatment with rapid cooling. Its structure and epitaxial relationship have been determined by high-resolution electron microscopy. The influence of the experimental conditions (cooling rate and dislocation density) on the appearance of this kind of precipitate is discussed.

The atomic structure of Σ=33{144}〈011〉 (θ=20.05°) tilt grain boundary in germanium

Abstract By combining high resolution transmission electron microscopy images with energy calculations of relaxed structures described using semi-empirical potentials, we show that the atomic structure of the Σ=33{1 4 4}〈0 1 1〉 ( θ =20.05°) tilt grain boundary in germanium contains a glide mirror symmetry and not a mirror plane symmetry as was proposed earlier in literature.

Structural evolution of Au[001]/Ni MBE thin films and Au1-cNic, solid solutions with temperature : a HREM study

The evolution with temperature of Ni MBE (molecular beam epitaxy) ultra-thin films embedded between Au layers is described. Strong oscillations in the contrast, with a period of 4 atomic planes in the growth direction, appear on high-resolution electron microscopy (HREM) images. These oscillations are related to chemical modulations. For comparison, the thermal stability of solid solutions has been studied, where the same feature is found. The main conclusion is that anisotropic decomposition occurs only when the film or the solid solution is under an in-plane stress.

Correlation between electrical activity and various structures of Ge grain boundaries

The links between the electrical activity and the atomic structure of various Ge grain boundaries (GBs) are investigated. The atomic structure is studied using high resolution electron microscopy, while the electrical activity is evaluated thanks to the measurement of minority carrier lifetime by means of the contactless microwave phase shift technique. Results show that in the Σ = 51 GB the electrical activity depends on the atomic structure connected to the configuration of the grain boundary, i.e. tilt, twist or mixed. Lower energy structures such as Σ = 3 and 9 GBs appear not to be recombinant.

Structural change induced on an atomie scale by equilibrium sulphur segregation in tilt germanium grain boundaries

Abstract In the present study, structural modifications induced by eauilibrium sulphur segregation in pure tilt germanium {710}<001>, ∑=25 (θ=16.26°) and {551}<011>, ∑=51 (θ=16.10°) grain boundaries (GBs) were investigated using high-resolution electron microscopy coupled to electron-energy-loss spectroscopy and supported by structural modelling and image simulations. Our results showed that the as-grown ∑=25 GB is composed of two parts: a stable structural region and a variable perturbed core. On the basis of our simulations, it is shown that this boundary can only be formed by a multiplicity of configurations which are energetically close to each other but differently configured along the boundary plane. When sulphurized, drastic changes in the structure of the GB were observed. Energy-filtered electron microscopy imaging revealed a sulphur enrichment at the perturbed part of the boundary. Although sulphur segregation at the boundary is detected, no information can at the present stage be extracted on segregation sites and bonding configurations because of the complexity of the boundary structure. To simplify this aspect, a simpler GB, that is germanium ∑=51, was studied. The structure of such a GB is a well-known configuration, that is a Lomer dislocation, which is basically a fivefold ring adjacent to a sevenfold ring. After sulphur treatment, high-resolution electron microscopy imaging also shows significant contrast modifications apparently concentrated on the dislocation core. Chemical imaging indicates again the presence of sulphur enrichment along the boundary plane strongly sustaining that eauilibrium sulphur segregation in the Ge(S) system oceurs into the GB and therefore confirms our previous results on the ∑= 25 GB. One can therefore argue that it is the presence of those odd-membered rings at the boundary, which should possess a specific crystallographic and electronic nature, coupled to the electronic properties of sulphur, that are responsible for the preferential segregation into the boundary.