G. Molénat

Dislocation mechanisms in Ni3Al at room temperature. In situ straining experiments in TEM

Abstract Transmission electron microscopy in situ straining experiments were performed under weak beam conditions, in Ni3Al(0.25 at.% Hg) at room temperature. They revealed rectilinear screw superdislocations dissociated either in the two octahedral or in the cube cross-slip plane, and fully reversible cross-slip between these planes. Screw superdislocations have the lowest mobility. They glide in octahedral planes through series of jumps and waiting times in sessile positions for which the antiphase boundary (APB) is contained in the glide plane. The jump amplitude can either be large, or equal to the superpartial separation. In order to explain these experimental results, a new sessile configuration of screw superdislocations has been introduced, with the APB ribbon in {111}, and both superpartials cross-slipped in {100} on a short distance (about 20 A). This configuration has some common points with that already proposed by Paidar, Pope and Vitek (1984), but is different in several aspects. It is consi...

In situ straining investigation of slip transfer across α2 lamellae at room temperature in a lamellar TiAl alloy

Processes by which deformation spreads throughout a lamellar TiAl alloy have been investigated by in situ tensile experiments performed at room temperature in a transmission electron microscope. Several situations are found and analysed in which dislocations cross the γ/α2 interfaces and the α2 lamellae – the hard phase of the structure. Conditions by which strain transfer can be elastically mediated across sufficiently thin α2 lamellae are discussed.

Investigation of the evolution of hardening precipitates during thermal exposure or creep of a 2650 aluminium alloy

Abstract In a 2650-T8 aluminium, the initial bi-modal homogeneous precipitation (GPB fine needles and S ′ precipitates) evolves during a thermal exposure at 150 °C: the GPB fine needles disappear. This dissolution is accelerated by creep and leads to a loss of mechanical resistance.

In situ TEM observations of dislocation processes in iron-aluminides with 25 and 30 at.% aluminium

Abstract Thin single crystal foils of long-range ordered binary iron—aluminium alloys with 25 and 30 at.% aluminium have been tensile tested inside of a transmission electron microscope. The motion of dislocations has been recorded under full load at temperatures ranging from 300 to 970 K. These experiments yield information pertaining to: i) the activated glide planes; ii) the mode of glide (viscous versus jerky and planar versus cross-slip); iii) the dissociation of perfect dislocations into partials and the magnitude of the partial Burgers vectors; and iv) the widths and specific energies of the antiphase boundaries between the partial dislocations. The results are discussed with a view of elucidating the well-known anomalous, positive temperature dependence of the critical resolved shear stress of these intermetallic compounds between 500 and 780 K.

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al2CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.

Dynamic dissociations of >101< dislocations in L12 alloys

Abstract The development of dislocation dissociation configurations in the temperature range of the yield stress anomaly in L12 ordered alloys has been analysed by taking into account the interaction forces between superpartial dislocations within the framework of anisotropic linear elasticity and the forces due to an applied stress. The results obtained are in agreement with recent observations of dissociated dislocations by both in situ and post-mortem electron microscopy.

In situ observation of edge and screw dislocations gliding in cube planes of Ni3Al

Abstract The glide of dislocations in the cube planes of Ni3Al is studied in situ between 573 and 1013 K. Frictional forces act on both screw and edge orientations of dislocation loops. However, rectilinear or slightly curved screw dislocations move steadily, in contrast with very rectilinear edges which move more jerkily. These edge dislocations are always dissociated in two superpartials. After some relaxation time, they become fully sessile, owing to an evolution of their core structure, probably by climb. These results are compared with those obtained post mortem in similar alloys, and with those obtained in situ in a superalloy.

Investigations of dislocation mechanisms in Ni3Al at different temperatures: in situ straining experiments in a transmission electron microscope

Abstract The glide of dislocations in the octahedral planes of Ni3Al is studied in situ between 165 and 1123K. Different microscopic behaviours are observed in the low- and high-temperature ranges of the yield stress anomaly domain, and at temperatures above the peak. Local stress measurements and respective intensities of glide in cube and octahedral planes are consistent with results of other techniques. Experimental results in the temperature range of yield stress anomaly are interpreted in terms of double-cross-slip processes between octahedral and cube planes, and octahedral glide at temperatures above the peak in stress involves diffusion processes.

A Study of the Hardening Precipitation in a 2650 Aluminium Alloy for Aeronautics

This paper focus on the hardening precipitation in a 2650 Al(CuMg) aluminium alloy and on its interactions with the aluminium matrix. The study has been carried out using High Resolution Transmission Electron Microscopy (HRTEM) images analysis and simulated images. Two types of precipitation (an homogeneous one and an heterogeneous one) and two types of precipitates (fine needles and S-laths) can be distinguished. A tomographic atom probe analysis of the fine needles reveals a strong copper and magnesium atom enrichment. The lath shaped precipitates can be sorted into two types, each one presenting specific interface morphology and orientation relationship with respect to the matrix.

Quantitative measurements in in situ straining experiments in transmission electron microscopy

Several examples of recent studies by in situ straining experiments in a transmission electron microscope performed in the Toulouse group (France) are presented. In particular, quantitative measurements of the features of the dislocation motion are described. These examples deal with individual or collective propagation of dislocations, which are submitted to various types of obstacle. Different metallic materials are investigated: magnesium, intermetallics, aluminium alloys and γ phase of a superalloy.