J. Douin

Dislocation line stability in Ni3AI

Abstract The equilibrium shapes of dissociated dislocations in strained Ni3Al poly-crystals have been investigated by weak-beam electron microscopy and the results compared to the predictions derived from the theory of anisotropic elasticity. Where dislocations are dissociated exclusively by glide the agreement is good. However, it is shown that climb dissociation induces further instabilities in the shape of dislocation segments with pronounced edge characters. The results are applied to explain the repeated observation of long dislocations deviating by 25° from the exact screw character.

On the presence of super lattice intrinsic stacking faults in plastically deformed Ni3Al

Abstract Superlattice intrinsic stacking faults (SISFs) have been observed in Ni3Al polycrystals deformed in compression between room temperature and 800°C. A detailed weak-beam analysis indicates that the SISFs originate at a screw partial with a Burgers vector of 1/2〈110〉. Starting from the stable dissociation into two partials with collinear Burgers vectors and with an antiphase boundary stabilized on {001}, one of the two 1/2〈110〉 partials may split under the effect of a high local stress into an edge Shockley and a partial with a Burgers vector of 1/2〈112〉. The latter produces a SISF as it escapes from its initial position in a {111} plane. Calculations of the total energy of the threefold dissociation shows the presence of a secondary minimum when the 1/3〈112〉 partial lies between 25 and 50 nm from the Shockley partial, in good agreement with the weak-beam observations. However, the SISFs exhibit large deviations from this equilibrium distance when they result from a deformation at low temperature a...

Dependence of the antiphase boundary energy upon orientation in the L12 structure

Abstract Antiphase boundaries are examined in an L12 structure, with particular attention paid to those that result from dissociation via a glide or a climb process of a superlattice dislocation with an a〈110〉 Burgers vector. In the case of a climb dissociation process, emphasis is placed on those antiphase boundaries which could exhibit local deviations from the stoichiometric composition A3B. The antiphase boundary energies are computed for a number of plane orientations so as to leave no region of the stereographic projection unexplored. A set of empirical potentials representing mechanically stable L12 alloys with different glide antiphase boundary energies on {001} and {111} and two distinct anisotropy ratios are used. It is found that the main effect of atomic relaxation is to significantly reduce the amplitude of the antiphase boundary energy variations with orientation: given the a/2[110] displacement vector, the low antiphase boundary energy on (001) is practically unaffected, whereas the high an...

Stacking fault energy in short-range ordered γ-phases of Ni-based superalloys

Several aspects on the stacking fault energy (SFE) in short-range ordered -phase model alloys of new nickel based superalloys are discussed. Using transmission electron microscope weak beam observations and computer simulated images of dissociation widths, the SFE was determined as a function of temperature. The values for both alloys are close (31 4m J m −2 for the Re-containing alloy and 28 6m J m − 2 for the Ru-containing alloy). In both cases, the dissociation widths remain quite constant up to 350 °C and a slight decrease is observed at higher temperature. The deformation micromechanisms and the constancy of the SFE up to 750 °C are analysed in connection with short-range order. The deformation at low temperature is characterised by dislocation pile ups, the influence on the dissociation widths of the internal stress due to such configuration is analysed. The calculation reveals a strong effect on the leading dislocation of the pile-up whose dissociation distance can be reduced at most by 50%.

The dislocation microstructure in orthorhombic O Ti2AlNb deformed between room temperature and 800°C

Abstract Ti‒26Al‒21Nb (at.%) samples with the orthorhombic O structure, deformed at various temperatures, have been analysed in order to determine the sensitivity of their microstructures to the deformation temperature. The activation of [100] and ½ < 110] dislocations, and interactions between them, have been observed from room temperature to 800°C. At high temperature, the activation of dislocations with [001] component Burgers vectors has also been identified. The poor ductility in tension at room temperature is ascribed to an insufficient number of slip systems to accommodate the deformation. Attention is paid to the splitting modes and to the energies of the various planar defects involved in the dissociations.

Climb dissociation on {310} planes in nearly-stoichiometric Ni3Al

Abstract In nearly-stoichiometric Ni3Al deformed in compression above the temperature where the flow stress peaks, superdislocations may be dissociated either by glide, on their {001} slip plane, or by climb, prefentially on {310} planes. Conditions that control the choice of these specific configurations are examined. Simple rules are introduced in order to determine the equivalence between antiphase boundaries (APBs) and to identify the occurrence of composition violations. It is shown that non-conservative APBs occur principally for APB configurations where the crystal composition is conserved and at dislocation orientations with a strong edge component. The set of APB energies, which is available in this material, is inconsistent with a pair-wise model limited to second neighbours. The fact that, for a mixed dislocation, the most favourable configurations may not coincide with the plane of lowest APB energy, is explained within the frame of elastic anisotropy, according to the dislocation character an...

Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order

The chemical force acting on dislocations in a short-range-ordered concentrated Ni-based alloy is investigated by post mortem and in-situ transmission electron microscopy. For this purpose, the positions of the dislocations in a pile-up are calculated, taking into account the stress relaxation at the free surfaces. The calculation shows that the distribution of the first dislocations of the pile-up is only slightly affected by the presence of the free surfaces, while the length of the pile-up strongly depends on the thickness of the foil. Analysis of pile-ups in two short-range-ordered Ni-based alloys shows that the chemical force resulting from short-range order (SRO) is noticeable up to the sixth dislocation of the pile-up. These results indicate the presence of very small short-range-ordered clusters, rather than a homogeneously distributed SRO.

High resolution transmission electron microscopy study of interfaces

Abstract The study of interfaces by means of High Resolution Transmission Electron Microscopy (HRTEM) is discussed through selected observations conducted on the Atomic Resolution Microscope (NCEM, Berkeley, USA). The precipitation of germanium into aluminium, the study of interfacial non-crystalline films in silicon nitride, the determination of the chemical nature of twin planes in CuAlO2 and the structure of the ∑3 grain-boundary in aluminium are the examples that serve to illustrate the important problems of relevance in the atomistic study of interface with HRTEM.

An Approach of Precipitate/Dislocation Interaction in Age-Hardened Al-Mg-Si Alloys: Measurement of the Strain Field around Precipitates and Related Simulation of the Dislocation Propagation

TEM study of deformed samples complemented by a new approach combining image analysis and simulation of the dislocation motion have been carried out to study the precipitate /dislocation interaction in Al-Mg-Si alloys (AA-6XXX). The analysis of HRTEM images allows a direct measurement of the strain field around precipitates and is further introduced in the simulation of dislocation propagation. In the case studied here, the simulation indicates that for an applied stress close to the yield stress, dislocations’ motion in the matrix occurs by both the by-pass of precipitates through the activation of the Orowan mechanism and the shearing of precipitates. This is in agreement with TEM observations on deformed samples showing numerous dislocations loops along with laths shearing

Dislocation structure in L12 Mn-stabilized Al3Ti deformed between 77 and 873 K

Abstract Polycrystalline samples of L1 2 Mn-stabilized Al 3 Ti were deformed in compression from low temperature to the temperature of the flow stress plateau. The fine structure of the superdislocations was analyzed under weak-beam conditions with particular attention paid to the operating glide systems at different temperatures. The dissociation mode of the dislocations was also addressed in order to discriminate between antiphase boundary-coupled dislocations and Superlattice Intrinsic Stacking Fault (SISF)-coupled partial dislocations. The existence of a strong Peierls effect on undissociated 〈110〉 dislocations during deformation at 573 K is reported, as well as the observation at low temperature of dislocations with 〈100〉 and 〈112〉 Burgers vectors.