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International audience; Local order is evidenced in nodules of the duplex microstructure of a Ti-6Al-4V alloy using in situ straining experiments in a transmission electron microscope (TEM). This local order is identified to be short range order (SRO) because of the absence of superlattice diffraction spots, which are associated with alpha(2) (Ti3Al) precipitates and because of the formation of single pairs of mobile dislocations, which are a signature of SRO. The strengthening effect of this SRO is quantitatively evaluated. Qualitatively, the presence of SRO inhibits strongly the cross-slip in nodules in comparison with dislocations gliding in lamellar colonies where no SRO is present. The well-known strengthening effect of the core structure of dislocation in Ti-alloy is revisited here in the presence of SRO to determine its possible influence.

International audience; The effect of thermal cycling creep on the dislocation networks at the γ/γ′ interfaces in the MC2 superalloy is investigated. Tensile creep tests were performed under thermal cycling and isothermal conditions at low stress (80 MPa) and high temperature (1150 °C). In these conditions γ′ rafts may dissolve and reprecipitate during thermal cycling creep. The difference between the effects of isothermal and thermal cycling conditions on the γ/γ′ interface dislocation networks, characterized by transmission electron microscopy, is exposed, as well as their evolution during the cycle.

En deformant in situ dans un microscope electronique en transmission des micro eprouvettes de l’alliage d’aluminium 6056 T6, les micromecanismes responsables de sa resistance mecanique ont ete identifies. Il a ete montre que les dislocations s’ancrent sur des lignes nommees lignes d’ancrage et qu’elles se propagent par bonds entre ces lignes. Les grandeurs physiques qui decrivent ces lignes ont pu etre mesurees. Il apparait que le glissement devie des dislocations est tres actif des la temperature ambiante. Ce glissement devie se produit souvent entre plans octaedriques mais aussi entre plans octaedriques et plans cubiques. L’incidence de ce mecanisme sur les proprietes mecaniques a ete analysee.

The mechanical behavior of the polycrystalline NR3 Ni-based superalloy has been investigated at the microscopic scale. The elementary deformation mechanisms have been analyzed using transmission electron microscope observations as well as in situ straining experiments. Under low stress and relatively low strain rate conditions, a large variety of shearing micromechanisms has been observed depending on the local microstructure and the local effective stress. The influence of the smallest precipitates on the creep behavior has been enlightened: they induce narrow channels which act as obstacle for the movement of the dislocations. In the case of the narrowest channel, the deformation can operate by the propagation of Shockley dislocations or else, by the only propagation of the leading partial resulting from the partial dislocation decorrelation. The occurrence of the observed micromechanisms has been quantitatively analyzed using a nodal dislocation dynamics simulation.

In situ straining of microsamples have been carried out at different temperatures in a transmission electron microscope (TEM) to provide information about the elementary micromechanisms, which control the deformation in Ni-based superalloys at the nano and microscopic scales. The obstacles to the propagation of the dislocations have been identified and quantified when possible. The strengthening effect in the -matrix channels and the shearing of ' precipitates have been chosen to illustrate some results deduced from moving dislocation analysis. A quantitative evaluation of the stresses acting on dislocations has been performed. The shearing process of the ' precipitates at 850°C has been precisely analysed.

In situ transmission electron microscopy (TEM) experiments have been carried out on the Ti–6Al–4V alloy. This work focuses on the dislocation motion close to the α/β interfaces. The interfaces influence the alloy strengthening by inhibiting the dislocation propagation but also by favouring the plastic deformation transmission. Results of deformation micromechanism dynamic are reported for the first time in this alloy at room temperature and compared to post-mortem TEM results.

The influence of the dissociation of dislocations when plasticity is localized in small volumes is illustrated for the case of γ/γ′ superalloys. Dissociated dislocations constrained to move through channels present three different types of behavior. Dislocation dynamic simulations in a face-centered cubic crystal show these three types of behavior and give the conditions under which a dissociated dislocation can move through a channel bordered by impenetrable obstacles. We show that a partial dislocation needs a lower stress for its motion than a perfect one in a significant domain of stress orientations. The consequences of the uncorrelated motion of one of the partials, leading to the formation of large stacking faults, are also examined.

By in situ TEM deformation experiments, we studied in detail the deformation micromechanisms of a Ti–6Al–4V alloy at room temperature. All dislocations have an a-type Burgers vector and glide essentially in prismatic or basal planes. They are first emitted from α/β interfaces and take a preferential orientation along their screw direction. The motion of screw dislocations controls the strain rate. Our experiments allow the microscopic parameters of plasticity for this alloy to be determined for the first time. The results concerning the screw dislocation motion, its core structure and the influence of interfaces are then discussed in comparison with previously published results.

The fine structure of slip lines in the γ-phase of Ni-based superalloy single crystals has been investigated by in situ atomic force microscopy observations of the specimen surface during deformation. For the first time, the dislocation structure is discussed in terms of short range order heterogeneity. A stress-driven unlocking mechanism is observed of dislocation pile-ups scanning the short range order in the bulk material. This is strongly enhanced by the propagation of other pile-ups a few hundreds of nanometers apart. A modelling based on elasticity is proposed and discussed.

An in situ study of the plastic deformation of single crystals of an industrial superalloy has been performed at 850 °C in a TEM to observe directly the micromechanisms which control the deformation under the actual temperature and stress conditions experienced by this material in aeroengines. A comparison between the creation and propagation modes of moving dislocations in the standard microstructure after annealing and the rafted microstructure after 20 h of creep at 1050°C evidence the important role of the width of the γ channels as well as the strength of the γ/γ’ interface in controlling the shearing events of γ’ channels.

The analysis of the distribution of moving dislocations is used to obtain the energy associated with the short-range order (SRO) present in the γ-phase of a nickel-base superalloy. This energy is evaluated at different spatial points within the sample when the dislocations propagate at 25 °C and at 350 °C. An accurate calculation reveals a clear spatial fluctuation of this energy over several micrometers. The SRO thus acts as a non-uniform obstacle with (i) soft areas extending over several micrometers where the nucleation of dislocation sources is enabled and (ii) hard ones where dislocation pile-ups need to be crossed through.

Polycrystalline NR3 nickel-based superalloy is a new-generation alloys produced by powder metallurgy for application as turbine disks of a future supersonic airplane. A special heat treatment is used to form a bimodal size distribution of γ′ precipitates. Post mortem transmission electron microscopy is used to observe dislocation structures obtained after creep around 700 °C and 650 MPa, up to 0.2% of plastic deformation. The deformation within a grain is heterogeneous and is principally due to the shearing of both types of precipitates by two different processes which where analysed. The smaller precipitates, called tertiary, are shown to play a major role in the hardening processes. In the proximity of serrated grain boundaries, where perfect dislocations are located, the deformation is higher, which is attributed to an enhanced internal stress in the middle of the grain.

Pile-ups observed in short-range ordered γ-phase of nickel-base superalloys have been investigated using both post-mortem and in situ transmission electron microscopy experiments. The influence of the foil thickness on the pile-up length, on the distances between the dislocations and on the elastic interaction stresses experienced by them, is analyzed. The results using the approximation of an infinite medium or accurate calculations taking into account the elastic relaxation of the stress by the free surface are compared. The experimental positions of the dislocations are shown to be determined by these latter calculations with a good accuracy so that precise information about short-range order friction forces can be deduced.

International audience; The precipitate microstructure in the last-generation aluminium alloy 6056 T6 [AlMgSiCu] is investigated using three complementary techniques of transmission electron microscopy (TEM) with a special focus on the density and volume fraction of strengthening particles. High-resolution TEM allows the identification of the precipitates and the measurement of the precipitate sizes to be performed. Conventional TEM is used to evaluate the number of precipitates in the investigated area as well as their distribution in the matrix. In situ TEM straining, via the analysis of the dislocation slip traces, permits to determine precisely the thickness and the volume of the foil in the region where the precipitates are analysed. Taking into account the shape and the dimensions of precipitates with respect to the foil thickness, a novel methodology for measuring the volume density and the volume fraction of precipitates is proposed.

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.

The weak-beam technique of transmission electron microscopy has been used to analyse a new shearing configuration of γ′ precipitates after creep at 700°C of a Ni-based superalloy for gas turbine discs. The shearing configurations are made up of superlattice extrinsic stacking faults, matrix stacking faults and individual (a/6)⟨112⟩ Shockley dislocations. This mechanism is initiated by the decorrelated movement of the two Shockley partials of a single (a/2)⟨110⟩ matrix dislocation. The propagation of the leading partial creates this shearing process. This phenomenon that occurs in small γ channels owing to the flexibility of dislocations can be used to evaluate microstructural evolutions during ageing in the alloy.

When deforming bulk material, micromechanisms involving moving defects result in mechanical characteristics observed at a macroscopic scale.In situ straining of microsamples in a Transmission Electron Microscope, provides the unique advantage of observing the dislocation dynamics involved in such micro-deformation processes under the combined effects of stress and temperature. Here the efficiency of this technique is illustrated by describing the different obstacles controlling the movement of dislocations in a two-phase industrial single crystal superalloy. At 25‡ and 850‡C, different core structures of the moving dislocations as well as several ways of crossing obstacles are described, which concern the movement of dislocations inγ channels, at γ/γ′ interfaces and while shearing γ′ precipitates. From these observations, a quantitative analysis is developed leading to the evaluation of the critical propagation stresses involved in the channels of the matrix and when crossing the interfaces. This allows to discuss the various sites of resistance opposed to the dislocation movements and controlling the macroscopic deformation.

International audience; TEM in-situ straining tests, performed in precipitation hardened aluminium alloy 6056-T6, show that precipitates are sheared or by-passed by dislocations. By-passing, assisted by dislocation cross-slip, results in the formation and stress-induced spreading of non-planar loops. The relationship between those elementary mechanisms and the mechanical properties of the alloy is discussed.

In-situ straining experiments were performed in a transmission electron microscope at temperatures ranging from 800 to 850 °C on a MC2 γ/γ′ nickel-based superalloy single crystal with a rafted microstructure. Perfect and partial dislocations were observed in motion. It is shown that deformation starts in the γ phase, which is the weaker phase at this temperature, although interaction of dislocations with point defects is still effective. The γ/γ′ interfaces play a different role depending on the propagation direction of the dislocations. From γ to γ′, interface crossing often acts as an additional obstacle that can be overcome by pile-ups of dislocations. From γ′ to γ, an increase of the dislocation speed is almost always observed, with no noticeable waiting time at the interface. Some dislocation multiplication processes are observed when interfaces are crossed from γ′ to γ. Unusual dissociation of superdislocations involving super stacking faults is observed in γ′, along with microtwinning. This may be taken as an indication of the low stacking fault energy of the MC2 alloy.

Dislocation motion in short-range-ordered solid solutions under stress are investigated in situ by transmission electron microscopy. For the first time, local reordering created by the back slip of mobile dislocations is proven to exist. Dislocation movement within pile-ups produces local disordering by forward slip during loading, and local reordering by back slip during unloading. Within a pile-up, the driving force for this mechanism is the diffuse antiphase-boundary energy created by the movement of the leading dislocation.

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%. © 2002 Elsevier Science B.V. All rights reserved.

Different γ phase model alloys of nickel base superalloys are investigated as a function of temperature using elastic diffuse neutron scattering, macroscopic tensile tests, TEM post mortem observations and in situ straining experiments. Short Range Order (SRO) of (1 1/2 0) type resulting from nano-domains with correlation lengths of 0.5–1 nm is observed in all alloys. The dislocation distribution confirms the presence of short-range ordered nano-domains with a size of about 0.7 nm. In all the alloys, the influence of SRO on the deformation mechanisms is clear at low and medium temperature and thus explains the similarities of the deformation behaviour observed in this temperature range. From 750°C to 1000°C, the deformation microstructure becomes progressively homogeneous which suggests the decrease of SRO as illustrated by a decrease of diffuse scattering intensities at the (1 1/2 0) positions on heating. The various techniques applied prove to be complementary in the investigation of complex solid solutions.

Le but de ce travail est d’etudier l’influence de trois solutes (W, Re et Ru) sur le comportement en traction d’alliages modeles de phase γ de superalliages a base de nickel, et de comparer leurs proprietes a celles de la phase γ du superalliage industriel MC2. Des essais de traction effectues a differentes temperatures ainsi que des observations post mortem realisees dans un M.E.T. font apparaitre des caracteristiques de la deformation similaires. De petites differences apparaissent toutefois selon la nature des solutes. Les raisons physiques controlant l’existence et la disparition des empilements et des paires de dislocations en fonction de la temperature sont discutees.

In-situ deformation experiments were performed on single crystals of the short-range-order-containing γ phase of a nickel-based superalloy. The dislocation pile-ups moving under stress are used as a probe exploring this complex solid solution and giving precise information on the local glide resistance encountered. Their analysis gives access for the first time to several contributions to this glide resistance. The threshold friction stress, which pile-ups must overcome in order to move is equal to the dynamic friction stress, which the pile-up experiences when propagating at a constant velocity. In comparison, the glide resistances deduced from the analysis of relaxed configurations are shown to be significantly underestimated. The softening effect due to the destruction of short-range order is also quantitatively determined.

Pile-up interactions in the γ phase of MC2 nickel-based superalloy have been studied by both atomic force microscopy and transmission electron microscopy. The examinations of surface slip-line patterns and the fine structure of dislocation pile-ups enable a minimum distance between pile-up slip planes to be derived. The behaviour of two pile-ups gliding in opposite directions has been simulated. The critical applied stress required for these pile-ups to cross over in the bulk crystal has been determined in good agreement with the microscopy observations.

A method is proposed to evaluate the glide plane resistance τf opposing the propagation of dislocation groups in a concentrated solid solution. The method is based on the measurement of the positio...

Both ‘in situ’ experiments and ‘post mortem’ observations have been performed on single crystals from MC2 superalloy γ matrix. Based on the measurement of the dislocation positions within the observed pile ups, a method is proposed that allows the evaluation of friction stresses from the head to the tail of the moving or relaxed pile ups. It appears that friction stresses must be measured on mobile pile ups, when they are deduced from relaxed pile ups they are considerably underestimated.

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.

The internal stresses arising from the misfit between the {gamma} and {gamma}{prime} phases of singlecrystal superalloy have a great influence on the mechanical properties of such materials at high temperatures. Several authors have determined these stresses by finite element calculations. These authors used a two dimension model (2D) to describe the approximately cubic shape of the {gamma}{prime} precipitates. Two questions arise from the use of 2D models for 3D structures. The first concerns the uncertainty of such models and this has been evaluated by Muller in a recent paper. The second involves the Von Mises (VM) stress. The authors use this stress as defined for 3D to describe the state of stress that can bring about plastic 2D strain. This stress is directly related to the distortion energy and is appropriate for single crystal material undergoing plastic deformation through dislocation gliding; however careful consideration is required for 2D analyses. In this paper, a VM stress in 2D is defined having the same physical significance as for 3D. Then, the difference is evaluated between these two stresses (VM for 2D and VM for 3D) for the case of a spherical precipitate.

International audience; A new generation of high performance martensitic steel hardened by FeNiAl intermetallic nanoprecipitates has been investigated using conventional and advanced Transmission Electron Microscopy (TEM). Cs-corrected high resolution TEM has been used to provide a fine and accurate microstructural characterization: the shape, size and crystallographic characteristics of the nanoprecipitates have been identified. TEM in situ straining tests have been performed to determine the relevant parameters, which control the deformation. The nanoprecipitates are observed to create anchor lines acting against the propagation of the mobile dislocations and to be crossed by shearing process. In addition, in situ TEM experiments have allowed to measure directly the average distance between these nanoprecipitates along the dislocation lines. This value is of great interest as it is the more accurate data to be used in the calculation of the stress associated with the structural hardening. The strength of the alloy due to these nanoprecipitates has been also determined. It is in good agreement with the value deduced from macroscopic mechanical tests.

By deforming the Mc2 nickel-based superalloy at room temperature in a TEM, we were able to study the succession of mechanisms that control the movement of the dislocation through the channels of they phase, at the interfaces and on shearing of the γ′ precipitates. We demonstrated the occurrence of small pile-ups that locally multiply the stress and adapt it to the various sites of resistance within the material. The results led us to reconsider the relationships from the literature giving the critical propagation stresses in the channels of the matrix and crossing the interfaces. We were thus able to account for the dynamic observations made and, in particular, to predict the number of dislocations in the small pile-ups that allow various obstacles to be crossed.

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.

[001] oriented nickel-based γ-γ′ superalloy MC2 was deformed in situ in a transmission electron microscope at room temperature. The arrangement of moving dislocations was observed in detail. Two complementary deformation modes were analysed, namely the propagation of pile-ups of bowed dislocations in the γ matrix, and the shearing of γ′ precipitates. The corresponding stresses involved in these processes were estimated and compared with the stresses needed to cross the interfaces and with the yield strength of the material.

In situ deformation experiments have been performed on TiAl in order to study the twin propagation in two different cases. In the first case the deformation is due to only one twinning system, whereas in the second case both twinning and simple dislocations are activated. In the first case twin propagation has been found to be controlled by frictional forces acting on the Shockley dislocations. In the second case Frank dislocations are formed at the twin interface as the result of reactions between Shockley dislocations and simple dislocations. The twin propagation is hence controlled by the crossing of these Frank dislocations by the other Shockley dislocations. The effects of these mechanisms on the mechanical properties are discussed in relation to the specimen orientation.

In situ deformation tests in T.E.M. are currently performed in our laboratory on different types of material to study their mechanical properties. Recent developments of this technique are presented in this paper. Many examples from our recent works are demonstrated to support this presentation. The procedure of the specimen preparation and the special straining holders are described in details. The validity of the results to explain the behaviour of the bulk materials is discussed with special attention to the stress control and the possible artefacts. The qualitative and quantitative potentialities of this technique are reviewed. Microsc. Microanal. Microstruct. 4 (1993)

An in situ study of twinning in a TiAl alloy has been performed in order to determine twin properties and to discuss the processes controlling twin propagation. This study shows that twinning is due to a/6⟨112] partial dislocations gliding in octahedral planes and that twinning is an intensive mode of deformation for some specimen orientations. Twins move slowly at an approximately constant velocity by a collective movement of partial dislocations which are separated by a constant distance during the motion. Twins in the TiAl alloy are perfect. A polar twin source is analysed. The occurrence of twinning with respect to other possible deformation modes is discussed and interpreted using the Schmid law. It is shown that the twin propagation is controlled by thermally activated frictional forces acting on partial dislocations and by the dragging of a stacking fault.

A general method is presented based on transmission electron microscopy observations of the segmented shape of dislocations under stress for measuring the decrease in dislocations’ self-energy when they are locked in specific orientations. The general formulation is given for any direction of locking in any material, and the method is exemplified in the case of the hexagonal phases of an industrially important Ti-based alloy, where a significant in-core rearrangement reduces the energy of screw dislocations by ∼16%.

An original method for characterizing the γ′-phase tertiary precipitates in a Ni-based superalloy manufactured by powder metallurgy is described. This investigation is made using post mortem transmission electron microscopy (TEM). It is based on the analysis of sheared areas within crept specimens, which allows the precipitates revealed by the dislocations in their glide plane to be observed. The characteristics of these nano-precipitates, i.e. their size, their volume fraction and the channel width between them, are determined for two different heat treatments (HTs). The results show a wide distribution of the microstructural parameters for a given HT, but only slight differences between the microstructures produced by the two different HTs. This microstructural information allows a better understanding of the wide variety of the deformation micromechanisms observed during creep at high temperature.

International audience; In nickel-based superalloys with high volume fraction of γ ' precipitates, dislocations have to experience high curvatures in order to enter narrow channels by glide in the {111} planes of the f.c.c. 冏 matrix. Observation of in situ dynamic sequences performed in a Transmission Electron Microscope on several industrial superalloys have shown the occurrence of decorrelated movements of Shockley partial dislocations, originating from perfect dislocation dissociation. By evaluating the effective stress acting on each one of these partial dislocations, as well as their respective flexibility, the possible occurrence of such movements for some particular dislocation characters and channel widths is accounted for. These movements can play an important role in the creep behaviour of these materials in the low deformation rate regime.

Through in situ straining experiments performed at 850 °C, on a γ / γ′ MC2 superalloy in a transmission electron microscope (TEM), the elementary mechanisms that control deformation were identified. On the recorded dynamic sequences, it is shown that deformation preferentially localizes in channels perpendicular to the [100] traction axis. Expanding from small loops, the creation and propagation of dislocations is observed. Both cross slip and climb events are observed on the moving dislocations which promote a tridimensionnal workhardening of these channels.

International audience; In-situ straining experiments and post-mortem observations in a transmission electron microscope have been performed between 20 and 170C on Al-Mg-Si alloy 6056. Evidence is found for activation of cross-slip and occurrence of glide in the {001} planes. These mechanisms occur more readily as temperature increases. Their existence is discussed in the light of microstructural characteristics of the alloy and an attempt is made to correlate them with macroscopic properties.

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 gamma phase of a superalloy.

In this paper, the contribution of in situ straining experiments to the understanding of plasticity phenomena is illustrated by different examples taken among recent works done in the Toulouse group concerning high performance materials.