Louisette Priester

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...

On the accommodation of extrinsic dislocations in grain boundaries

The relaxation of the extrinsic grain boundary dislocation (EGBD) stresses, created by interaction of lattice dislocations with grain boundaries (GBs), is a phenomenon which plays an important role in recrystallization and high temperature deformation of materials. The kinetics of this phenomenon, controlled by GB diffusion, are relatively well established. On the contrary, the processes which operate in order the GBs return to equilibrium are still controversed in vicinal and general GBs.The decomposition in discrete products and the rapid motion of the glissile components observed by High Resolution Electron Microscopy (HREM) in symmetrical tilt GBs support the recent incorporation model of EGBD accommodation. But, until now, these observations and this model are restricted to GBs described by the Structural Unit/Grain Boundary Dislocation (SU/GBD) model. Otherwise, the “Spreading” phenomenon generally observed by Transmission Electron Microscopy (TEM) in vicinal and general GBs is not clearly understood.This paper is an attempt to review the different EGBD accommodation models and to raise up the question of their relevance to account for the stress relaxation in any grain boundary.

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.

Precipitation at Grain Boundaries

Several reasons explain why precipitation at grain boundaries occurs preferentially compared to precipitation within the crystals.

Image force on a dislocation in a Ni-based superalloy due to the γ-γ' interface

Abstract The image force due to the γ—γ′ interfaces on dislocations situated in the γ channels or in the γ′ precipitates in Ni-based superalloys is investigated on the basis of the anisotropic theory of elasticity. The results are affected by the choice of the elastic constants available in the literature but, in most cases, the values of the image forces are comparable with those of the other forces acting on the dislocations. At room temperature, the sense of the image forces does not depend on the choice of the elastic constants; the dislocations in γ are always attracted to the interfaces and those in γ′ are always repelled from them. At 1050°C, however, opposite conclusions may be reached depending on the constants used. These different effects are discussed by comparison with the configurations observed by in situ transmission electron microscopy.

Heterogeneities of grain boundary arrangement in polycrystals

Data on mutual arrangements of different types of grain boundaries in polycrystals are presented. The heterogeneity in grain boundary distribution, namely, the effect of gathering low-angle or special tilt grain boundaries is found in pure aluminum thin films, in sheets of Fe-3% Si alloy and in Al2O3 doped with MgO or MgO and Y2O3. The local texture, i.e., formation of colonies or clusters of close-oriented grains is considered as a reason of this heterogeneity. The influences of grain boundary gathering on the transport properties of polycrystals and on the crack propagation are discussed. A new concept of “effective” grain size is suggested to analyze the relationship between material microstructures and material properties.

Interactions Between Dislocations and Grain Boundaries

Two types of stresses act on a lattice dislocation at vicinity of a grain boundary (Fig. 8.1) The long-range elastic stresses: applied stress and internal stresses caused by other dislocations present within the grains (\(F_{d})\) and/or in the boundary. In case of an elastically anisotropic material, (or in case of an interface between two different media),an additional stress must be considered: the interface causes the self-stress exerted by a straight dislocation on itself to be non-zero. The effect of this stress is named image force \(F_{\mathrm{{I}}}\)