Illustration d’une systématique des phases électroniques binaires ou ternaires de densité électronique inférieure ou égale à 1,5 électrons libres par atome par des exemples particuliers tirés du diagramme ternaire or-cuivre-gallium

Un enrichissement en électrons de valence des solutions solides à base de métaux nobles or et cuivre est obtenu par des substitutions progressives de gallium. De nombreuses phases ordonnées complexes apparaissent dont les structures sont interprétées en utilisant la notion d’empilement de plans denses ordonnés. Deux familles de phases sont distinguées : les phases αn compactes de densité électronique inférieure à 1,36 électrons par atome et les phases ßn de densité électronique 1,50. Les phases ßn subissent à basse température des cisaillements bainitiques ou martensitiques

Tarcie wewnętrzne i moduł dynamiczny w bardzo wysoko temperaturowych funkcjonalnych związkach międzymetalicznych z układu Ru-Nb

In the present work we have studied the high-temperature shape memory alloys based on the Ru-Nb system by using two mechanical spectrometers working in temperature ranges from 200 to 1450ºC and -150 to 900ºC. We have studied internal friction peaks linked to the martensitic transformations in the range from 300 to 1200ºC. In addition, we have evidenced another internal friction peak at lower temperature than the transformations peaks, which apparently exhibits the behaviour of a thermally activated relaxation peak, but in fact is a strongly time-dependent peak. We have carefully studied this peak and discussed its microscopic origin, concluding that it is related to the interaction of some structural defects with martensite interfaces. Finally, we perform a complete analysis of the whole internal friction spectrum, taking into account the possible relationship between the time-dependent peak and the martensitic transformation behaviour.W niniejszej pracy badano wysokotemperaturowe stopy z pamięcią kształtu z układu Ru-Nb, przy użyciu dwóch spektrometrów mechanicznych pracujących w zakresach temperatur od 200 do 1450ºC i -150 do 900ºC. Zbadano piki tarcia wewnętrznego związane z przemianami martenzytycznymi w zakresie od 300 do 1200ºC. Ponadto, wykazano występowanie innego piku tarcia wewnętrznego przy niższej temperaturze niż pików przemiany, który ma cechy aktywowanego termicznie piku relaksacji, ale w rzeczywistości jest to pik silnie zależny od czasu. Dokładnie zbadano ten pik i omówiono jego pochodzenie w mikro skali, stwierdzając, że jest to związane z oddziaływaniem niektórych wad strukturalnych z granicami martenzytu. Na koniec, wykonano kompletną analizę całego spektrum wewnętrznego tarcia, biorąc pod uwagę możliwość związku między pikiem zależnym od czasu i zachowaniem przemiany martenzytycznej

Diffraction profile, strain distribution and dislocation densities during stage II creep of a superalloy

Abstract. One of the major ingredients of modelling the mechanical behaviour of superalloys is the knowledge of dislocation densities and strain distribution. Both can be measured using post mortem BF TEM and CBED, but such methods do not allow following their variations during a test. The aim of the present work is to investigate the usefulness of in situ X-Ray Three Crystal Diffractometry (TCD) to measure the density and distribution of dislocations within a rafted superalloy, i.e. during stage II of high temperature creep. As the instrument contribution is very low, the two-peaked experimental profiles are representative of the lattice parameter distribution within the material. The profiles were measured within bulk specimens at the BW5 high energy beamline Hasylab (DESY), during high temperature (1050°C to 1180°C) tests under loads between 0 MPa and 300 MPa. The peak shapes were observed to change with varying experimental conditions. The peak width follows different patterns under low and high stress, i.e. with low and high strain rates. The distribution of elastic strains was calculated by assuming two main contributions: dislocation segments trapped at the γ/γ' interfaces in a more or less regular network, and dislocations moving within the γ' rafts. A comparison between experimental and simulated peaks shows that several features of their behaviour can be explained: the absolute magnitude of the peak width, the observed decrease of the peak width under low loads with increasing interfacial dislocation densities. The larger increase in the width of the γ' peak under high load (and strain rate) may be attributed to a dislocation density within the 10 13 m -2 range within the rafts. The present results are presently being cross-checked by post mortem TEM observations

Strains and dislocation densities within a rafted superalloy: In situ investigation during mechanical testing at 1080°C

During in situ investigation of the distribution of lattice parameters within a superalloy with a rafted microstructure by Three Crystal Diffractometry, both the γ and γ’ peak positions and shapes are shown to change. While the peak positions can be used to measure the average stresses and strains within both phases, the changes in shape are related to these in the dislocation distribution within the material. It is shown that the density and order of the dislocation array at the γ/γ’ interface and within the γ’ phase give different contributions to the peak width, and that the widening of both peaks under high stresses is due to an increase of the dislocation density within the rafts

Phase-specific high temperature creep behaviour of a pre-rafted Ni-based superalloy studied by X-ray synchrotron diffraction

International audienceThe phase-specific high temperature creep behaviours of the and phases of a rafted Ni-based single crystal superalloy were investigated by a combination of in situ creep experiments and diffraction of high-energy X-ray synchrotron radiation. In situ experiments were performed at constant temperatures in a 9301125 degrees C temperature range and under variable applied stress in order to study the material's response (plastic strain, load transfer) to stress jumps. Using three crystal diffractometry in transmission (Laue) geometry, it was possible to measure the average lattice parameters of both the matrix and the rafts in the [100] direction at intervals shorter than 300s. The absolute precision on the measurement of the constrained transverse mismatch (in the rafts' plane) is better than 10(5). Plastic strain occurs within the corridors as soon as the Von Mises stress exceeds the Orowan stress. The plasticity of the rafts apparently depends on the transverse stress (i.e. perpendicular to the tensile axis) exceeding a threshold value of 60MPa

In situ measurement of internal stresses and strain rates by high energy X-Ray diffraction during high temperature mechanical testing

Abstract. The combination of high temperature (1050°C -1150°C) testing and in situ high energy X-Ray diffraction measurements using synchrotron Three Crystal Diffractometry may give various insights into the mechanical behaviour of superalloys: measurement of the lattice mismatch, order within the γ' phase, elastic constants, and dynamic response to changes in the experimental conditions. Several examples are given on the rafted AM1 superalloy, resulting from experiments at the ID15A (ESRF) and BW5 (DESY) high energy beamlines. Introduction During the second stage of their high temperature tensile creep curve ([001] tensile axis), rafted single crystalline nickel-based superalloys may be seen as very simple two-phased materials: they are formed of alternate, semi coherent, layers perpendicular to the tensile axis of a L1 2 (γ') phase (so called rafts) and of a disordered fcc γ phase (γ corridors). This simplicity might make superalloys obvious candidates for physically-based modelling of their plastic behaviour, taking into account dislocation velocities, densities…, and internal stresses. However, in spite of very sophisticated attempt

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