F. Livet

Influence of predeformation on ageing in an Al–Zn–Mg alloy—I. Microstructure evolution and mechanical properties

Abstract We investigate the ageing kinetics of a ternary Al–Zn–Mg alloy, and the coupling between prestraining and subsequent ageing. The precipitation sequence is investigated using DSC. The microstructure is followed using TEM and Small-Angle X-ray Scattering, and the hardening resulting from precipitation studied by hardness measurements. The effect of prestraining depends significantly on the parameters of the thermomechanical treatment, and has to be considered in terms of competition between homogeneous precipitation and heterogeneous precipitation on dislocations. The effect of prestraining on peak hardening is controlled by the nucleation of the hardening η′ phase, and therefore depends critically on the heating rate to the ageing temperature. The effect of prestraining on overageing is an increase in overall coarsening kinetics due to the combination of faster precipitate coarsening on dislocations and dissolution of homogeneous precipitates followed by solute diffusion towards dislocations.

Using direct illumination CCDs as high-resolution area detectors for X-ray scattering

Abstract The use of charge-coupled devices (CCDs) under direct illumination of X-rays for two-dimensional position-sensitive detectors in high-resolution diffraction experiments is discussed. Two detectors are compared: a standard CCD and a “deep depletion” CCD. These give position resolution close to the pixel size ( ∼22 μm ), and energy resolution close to the theoretical resolution of a Si detector. These detectors can be used for photon-counting and an algorithm for electronic noise suppression is presented. This algorithm is useful for experiments with frequent readouts and low intensity. Examples demonstrating the advantages of this algorithm for diffraction experiments are given.

Diffraction with a coherent X-ray beam: dynamics and imaging

Techniques for coherent X-ray scattering measurements are detailed. Applications in the study of the dynamics of fluctuations and in lensless high-resolution imaging are described.

DLS and SAXS investigations of organic gels and aerogels

Abstract Recent investigations have shown that the structure of organic aerogels can be significantly modified by changing the precursors, the solvent and the nature of the catalyst involved in the sol–gel reaction. It is therefore highly desirable to investigate the sol–gel mechanism. For this purpose, dynamic light scattering (DLS) measurements have been performed at different stages of the reaction for base- or acid-catalyzed gelation of resorcinol–formaldehyde (RF) using water or acetone as solvents. The structure of aged gels was investigated by small-angle X-ray scattering (SAXS) and compared to that of the aerogels obtained after exchange of solvent by supercritical CO2 and drying of the aged gels. It is shown that acid-catalyzed gelation of RF in acetone can be described by percolation, which explains that this series of aerogels consists of mass fractal aggregates (Dm=2.5). The partial collapse of this polymeric gel yielding colloidal particles in the aerogel can be attributed to deswelling in supercritical CO2. DLS indicates that gelation of RF with a base catalyst yields a colloidal gel whose structure remains practically unchanged in the aerogel, as shown by SAXS.

In-situ small-angle X-ray scattering study of dynamic precipitation in an Al-Zn-Mg-Cu alloy

Dynamic precipitation during room-temperature deformation of an Al-Zn-Mg-Cu alloy in solid solution has been investigated using in-situ small-angle X-ray scattering measurements during tensile tests, performed at the European Synchrotron Radiation Facility. Guinier-Preston (GP) zones are observed to form continuously during the deformation process, and the quantitative measurement of their size and volume fraction shows that their precipitation kinetics are much faster than those of static precipitation. A strong negative strain-rate sensitivity of dynamic precipitation has been observed. A model for the hardening effect of the GP zones is deduced from the evolution of yield stress during static ageing at room temperature. This model is applied to the dynamic precipitation kinetics in order to describe the anomalously high strain-hardening rate observed in these deformation conditions. The kinetics of dynamic precipitation are discussed in terms of semiphenomenological models based on the dynamic strain-ageing theory.

X-ray intensity fluctuation spectroscopy by heterodyne detection.

A straightforward way of measuring X-ray intensity fluctuation spectroscopy in a small-angle X-ray scattering configuration is demonstrated using heterodyne techniques. Two examples are presented: the Brownian motion of latex spheres in glycerol, and a Doppler velocity experiment demonstrating the motion and the relaxation of carbon-black-filled elastomers after uniaxial stretching. In the latter case the effects of mechanical relaxation can be separated from those of aggregate diffusion. The results suggest that the dynamics of these filled elastomers are similar to the universal features observed in disordered jammed systems.

X-ray diffraction from rectangular slits

It is shown that for micrometre-sized beams the X-ray diffraction from slits is a source of strong parasitic background, even for slits of high quality. In order to illustrate this effect, the coherent diffraction from rectangular slits has been studied in detail. A large number of interference fringes with strong visibility have been observed using a single set of slits made of polished cylinders. For very small apertures, asymmetrical slits generate asymmetrical patterns. This pattern is calculated from the theory of electromagnetic field propagation and compared with experiment in the far-field regime. The use of guard slits to remove Fraunhofer diffraction from the beam-defining slits is treated theoretically. Numerical simulations yield the optimum aperture of the guard slits with respect to the distance to the primary slits. Diffraction theory is shown to be essential to understand how to reduce the background-to-signal ratio in high-resolution experiments.

Dynamics of long-wavelength phason fluctuations in the i-Al-Pd-Mn quasicrystal.

We report on the dynamics of phason modes in the i-Al–Pd–Mn icosahedral quasicrystal, measured between room temperature and 650°C, using the X-ray intensity fluctuation spectroscopy (XIFS) technique. Up to 500°C, the autocorrelation function, ℑ(q, t), displays almost no time evolution as expected for frozen-in phason fluctuations at low temperature. At higher temperatures, ℑ(q, t) follows a single exponential time decay from which the characteristic time τ c(q) is extracted. These results are compared to the expected shape of ℑ(q, t) as derived from the expressions of the eigenvalues and eigenvectors of the C ⊥ ⊥(q) phason dynamical matrix. In agreement with the hydrodynamic theory of quasicrystals, which predicts phasons with diffusive character, we find that τ c(q) varies linearly with q −2 at 650°C. The corresponding diffusion coefficient is 2.2(±0.5) × 10−18 m2 s−1 and the activation energy is estimated around 2.3(±1) eV.

Investigation of the multi-scale structure of silica aerogels by SAXS

An investigation is described into polyethoxydisiloxane (PEDS-Px) silica precursors of aerogels prepared in ethylacetoacetate (etac) under HF conditions and dried under CO2 supercritical conditions. The influence upon the internal nanostructure of aerogels of the number of water molecules (n*) used to synthesize the precursors is studied, as well as that of their volume fraction (xPrec) in solution during the second catalytic step. Correlation between structure and optical transmission (%TR) is necessary to improve optical behavior of monolithic silica aerogels for double-window applications. For this reason, small angle X-ray scattering (SAXS) was performed to characterize the nanostructural silica skeleton, and especially to investigate particle and cluster properties. Optical transmission was also measured to estimate visible optical quality of the samples (%TR). Among other results, it is shown in this study that increasing n* and xPrec contributes to the decrease of the particle and cluster sizes, which indirectly improves %TR in the visible range. These observations confirm and help to explain previously published results.

ASAXS, SAXS and SANS investigations of vulcanized elastomers filled with carbon black

Small-angle X-ray scattering (SAXS) performed down to small q values (q <or= 10(-3) A(-1)) is a powerful method for investigating the arrangement of filler aggregates in filled elastomers under uniaxial strain. Meanwhile, for vulcanized samples, zinc oxide is used as an additive. Owing to their high contrast, the ZnO particles remaining in the manufactured composite are strong X-ray scatterers. In the low-q domain, their scattering hides that of filler aggregates (carbon black, pyrogenic silica) and must be quantified in order to be suppressed. To this end, anomalous SAXS (ASAXS) and small-angle neutron scattering (SANS) have been performed. It is shown that ASAXS measurements can be performed down to small q values (q <or= 10(-3) A(-1)). Therefore ASAXS is well adapted to separate the contributions of ZnO and filler scattering. For neutron scattering the contrast of the ZnO particles is similar to that of carbon. Because the amount of ZnO is much smaller than that of filler, ZnO scattering can be neglected. Owing to multiple scattering effects, however, SANS can only be used for very thin samples (less than about 0.25 mm). It is shown that, providing multiple scattering is avoided, ASAXS and SANS yield similar scattering curves for the filler aggregates.