Alain Jacques

Direct evidence of dislocation transmission through ∑= 9 grain boundaries in germanium and silicon by in situ high-voltage electron microscopy observations

Abstract In situ straining experiments of Si and Ge bicrystals in the high-voltage electron microscope have clearly revealed that dislocations can be transmitted across ∊ = 9 coincidence tilt boundaries even when their Burgers vector is not a lattice vector of the second grain. Observed transmitted dislocations are either perfect dislocations with ½‘110’ Burgers vector or, more often, Shockley partial dislocations trailing behind them extended stacking faults linked to the grain boundary. Dissociation of dislocations into DSC dislocations glissile in the boundary was also observed. Observations of repeated transmission events at a given area of the grain boundary suggest that residual dislocations can be removed from the impact point. The main-but not all-observed features are explained by a simple model based on energy calculations of the possible dislocation configurations formed at successive steps of the transmission process.

The γ/γ′ mismatch in Ni based superalloys: In situ measurements during a creep test

Abstract The lattice mismatch between the fcc γ matrix and the ordered γ ′ cuboidal precipitates in superalloys induces large internal stresses within the material. These stresses have a major effect on its mechanical behaviour and on the anisotropic evolution of the microstructure (rafting…) during its lifetime. The evolution of the effective lattice mismatch of the AM1 and MCNG superalloys was measured continuously during high temperature creep tests (1100 °C, 150 MPa) at the ID 15 (high energy) beamline of the ESRF. The bulk profiles of the 200 reflection (parallel to the tensile axis) were recorded using the triple crystal diffractometer. Both materials have a negative mismatch, and exhibit a transition between the initial wide γ ′ peak to a two peaks profile during stage I of the creep curve, as rafting takes place. During stage II, the 200 mismatch decreases in magnitude. During stage III, as a microstructural transition (coalescence) occurs and the strain rate increases, the mismatch changes in the same direction, but at a faster rate, while the thickness of both peaks increases. Evolution of the MCNG specimen was the same as AM1 ones, but rafting and transition to stage II take place at a different rate.

Structure and characterization of the dislocations in tilt grain boundaries between Σ = 1 and Σ = 3; a high resolution electron microscopy study

Abstract It is shown that the displacement shift complete (DSC) lattice is no longer necessary to define the Burgers vector associated with an interfacial dislocation. In Si and Ge [011] perfect or imperfect bicrystals, the grain boundary atomic structure is made of a limited number of structural units (SUs). Using high resolution electron microscopy (HREM), it is shown that one SU can be considered as the core of a grain boundary dislocation whose Burgers vector can be easily defined on the HREM image. The limited number of SUs strongly restricts the possible Burgers vectors compared with the DSC lattice vectors. These experimental results provide a link between the geometrical approach developed by Pond and the energetic approach developed by Sutton and Vitek.

In-situ transmission electron microscopy observation of slip propagation in Σ3 bicrystals

Abstract In-situ straining experiments have been performed to study the mechanisms of propagation of deformation at grain boundaries (GB) in symmetrical Σ3 Fe–4 at.% Si bicrystals with common primary slip system in both grains. Three different orientations of tensile axis with respect to the {112} GB plane were studied and three different effects were observed. In no case was the propagation of primary-slip dislocations across the GB observed. When the tensile axis lies in the GB plane, the slip dislocations of the tertiary slip system enter the GB, interact with GB dislocations, and the dislocations of the secondary slip system are generated in the second grain. When the angle between the primary slip plane and the GB is about 20°, the primary-slip dislocations whose Burgers vectors belong to the GB plane, cross slip and follow the GB plane. Primary-slip dislocations are formed in one grain and a new sub-grain boundary is formed in the other grain when the angle between the primary slip plane and the GB is about 49°.

Mechanical testing device for in situ experiments on reversibility of dislocation motion in silicon

Abstract We present a general-purpose mechanical testing device built and tested for in situ experiments using synchrotron radiation. A new setup was used in order to investigate the reversibility of dislocation motion under stress reversal. The observations were done by in situ transmission X-ray topography, at the ID 19 beamline of the ESRF. Specimens of FZ silicon single crystals, with a [ 1 1 4 ] axis (double slip orientation), were submitted to load cycles in tension and compression (τ=10–20 MPa) at high temperature (650–750 °C). Dislocations were created at surfaces by Vickers indents. They were observed using a monochromatic beam (λ=0.035 nm) and a 220 diffraction vector. The images were recorded both on X-ray films and using the FRELON X-ray camera. The first observations show that dislocations may behave in different ways within the same specimen, and that their motion can be partially reversible.

Temperature dependence of lattice mismatch and gamma" volume fraction of a fourth-generation monocrystalline nickel-based superalloy

Abstract The strain distribution in a new generation nickel-based single crystal superalloy has been determined in the temperature range 293K–1598K. Measurements have been performed using diffraction of high energy synchrotron radiation (150keV, λ = 0.008 nm) to determine the variations with temperature of the γ/γ′ lattice mismatch and of the γ′ volume fraction. The chemical segregation at the dendrite scale and the internal stress induce a large range of values for the lattice mismatch. The measurements of the γ′ volume fraction determined by X-ray diffraction are in agreement with those obtained by atom probe tomography, image analysis or computation.

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

The phase-specific high temperature creep behaviours of the γ and γ′ 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 930–1125 °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 [1 0 0] direction at intervals shorter than 300 s. 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 γ corridors as soon as the Von Mises stress exceeds the Orowan stress. The plasticity of the γ′ rafts apparently depends on the transverse stress (i.e. perpendicular to the tensile axis) exceeding a threshold value of 60 MPa.

In situ observation of grain boundary migration by synchrotron radiation topography

The application of in situ synchrotron radiation (SR) transmission topography to the study of grain boundary migration in bicrystals of an Fe–6 at.%Si alloy is described. The details on experimental arrangement of the high-temperature in situ observations of dynamic processes in material are given. The pertinence of this method is documented by comparison of the data on migration of {130} grain boundary measured this way and by optical method after heating and cooling cycles.

Transformation of Slip Dislocations in Σ3 Grain Boundary

Grain boundary processes during plastic deformation of bicrystals were studied by TEM. Two methods were used. In situ straining in the electron microscope followed by post mortem examination and post mortem observation of specimens previously deformed by in situ synchrotron radiation X-ray topography. Two mechanisms governing slip propagation across a coherent twin boundary in a Fe-Si alloy bicrystal were identified. The first mechanism is a dissociation of a slip dislocation with the Burgers vector lying parallel to the boundary into three equal grain boundary dislocations. The second mechanism is a decomposition of a slip dislocation with Burgers vector inclined to the boundary into a dislocation mobile in the other grain and two screw grain boundary dislocations.

Dislocation multiplication during the very first stages of plastic deformation in silicon observed by X-ray topography

Abstract The first stages of plastic deformation of FZ silicon single crystals were investigated by in situ X-ray topography at the ESRF, in creep conditions at temperatures between 975 and 1075 K, and for applied stresses from 22 to 44 MPa. A special attention was given to dislocation multiplication, and several features relevant for this phenomenon were observed: cross slip at the surface or in the bulk of the specimen, instabilities in slip bands resulting from a non planar development of dislocations, creation of new sources by moving dislocations.