Brigitte Décamps

On the mechanism of shear of γ′ precipitates by single (a/2)⟨110⟩ dissociated matrix dislocations in Ni-based superalloys

Abstract The weak-beam technique of electron microscopy combined with bright-field image matching has been used to study the elementary shearing configurations of γ′ precipitates in single crystals of Ni-based superalloys after creep and dynamical experiments. The shearing configurations encountered involve superlattice intrinsic and extrinsic stacking faults and a single (a/2) ⟨110⟩ matrix dislocation as proposed by Condat and Decamps. The observations show changes in configuration between tension and compression that may be explained by considering the interplay between the crossing of the interface by elementary Shockley partials and the effect of the applied stress on the dissociation width of matrix dislocations. An original model for the shearing of the γ′ precipitates by a single (a/2) ⟨110⟩ dissociated matrix dislocation is proposed; this model is based upon the detailed mechanism of crossing of the interface by the matrix dislocation. The model provides an understanding of the dependence of the s...

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...

Stacking‐fault energy at room temperature of the γ matrix of the MC2 Ni‐based superalloy

Abstract The γ-phase of the MC2 Ni-based superalloy has been characterized using both in situ experiments and post-mortem observations (bright field and weak beam) in transmission electron microscopes. A mean value of the stacking-fault energy (about 31 mJ m−2) has been obtained through the measurement of dissociated nodes and of the dissociated width of dislocations. Furthermore, the plastic deformation has been shown to be controlled by the collective movement of dense pile-ups, and the existence of a local order has been deduced.

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.

High resolution imaging of shearing configurations of γ precipitates in ni-based superalloys

It has been shown that shearing of [gamma][prime] precipitates involving superlattice stacking faults is one of the important mode of deformation in Ni-based superalloys containing a high volume fraction of precipitates. The purpose of this paper is to show how high resolution observations can give pertinent and precise information about shearing configurations of precipitates involving superlattice stacking faults. As a result, together with previous weak-beam studies, a shearing mechanisms connected to the atomic structure will be proposed.

On the atomic structure of an asymmetrical near Sigma=27 grain boundary in Copper

The atomic structure of an asymmetrical near Σ = 27 {525} tilt grain boundary (GB) in copper is determined by coupling high-resolution transmission electron microscopy and molecular dynamics simulation. The average GB plane is parallel to {414} in crystal (1) and {343} in crystal (2). The detailed GB structure shows that it is composed of facets always parallel to {101} and {111} in crystals (1) and (2), respectively. The atomic structure of one facet is described using the structural units model. Each facet is displaced with respect to its neighbours by a pure step, giving rise to the asymmetry of the GB plane orientation. The energy of this asymmetrical GB is significantly lower than that of both the {525} symmetrical and the {11,1,11}/{111} asymmetrical Σ = 27 GBs. One GB region displays another atomic structure with a dislocation that accounts for the misfit between interatomic distances in the {414} and {343} GB planes.

Interactions between dislocations and interfaces – consequences for metal and ceramic plasticity

Abstract Interactions between dislocations and grain boundaries contribute significantly to plastic behaviour in polycrystalline metals. But a full understanding of the processes and of their influence on plastic response has yet to be achieved. In the present paper, the elementary interaction mechanisms, from the entrance of dislocations in grain boundaries to the relaxation of the resulting intergranular stresses, are briefly reviewed. They are examined for two types of deformation tests and two types of materials: low temperature deformation and relaxation in copper and creep behaviour of alumina. Only the comparison between the response at different scales (nanoscopic, microscopic and mesoscopic) allows us to analyse the response of grain boundaries to an applied stress and to propose an interpretation of their role in macroscopic behaviour. This work may be considered as a preliminary step towards “Interface Engineering”.

On the presence of fine matrix channels in Ni-based superalloys and their effect on shearing processes of ý precipitates

Abstract The bright field image matching technique has been used to study a shearing configuration of ý precipitates involving superlattice stacking faults situated on three different levels. The presence of fine matrix channels is reported and a shearing mechanism involving a climb process is proposed.

Extended interfacial structure between two asymmetrical facets of a Σ = 9 grain boundary in copper

Abstract The presence of an extended structure in a Σ = 9 grain boundary of copper has been highlighted by high-resolution transmission electron microscopy. The phase, located in a grain boundary region between two different asymmetrical facets, may be described as a three dimensional phase with a hexagonal structure. A detailed analysis of the defects at the limits between the phase and the two copper crystals allows us to propose an interpretation for the occurrence of this local phase transformation.

Structure/Property Relationships in a Rapidly Solidified and Annealed Ternary Iron Aluminide

Rapidly solidified strips of Fe66.5-A128.5-Cr5 alloy with the addition of 0.5wt.% TiB 2 , were produced by planar flow strip casting. Correlations of the microstructure and room temperature mechanical properties were made for strips in the as-cast condition and after annealing at 1273K for periods up to 4hrs. The results showed that grain size, tensile strength, ductility, hardness and Youngs modulus were very stable for the times investigated. Studies of tensile fracture surfaces revealed essentially 100% transgranular cleavage in the ascast strip with a greater tendency for intergranular failure after prolonged exposure to high temperatures. TEM studies of this alloy revealed diffraction patterns characteristic of DO 3 ordering but the dislocations observed in both ascast and deformed specimens were those typical of the B2 structure without any extended APBs. This is attributed to the very fine DO 3 , domain size . At room temperature slip is predominant.