T. Buslaps

Fast microtomography using high energy synchrotron radiation

At the High Energy Beamline ID15A at the European Synchrotron Radiation Facility we have developed a fast three-dimensional x-ray microtomography system, which acquires a complete dataset in typically less than 10s. This unprecedented speed is achieved by combining a high efficiency phosphor screen, a reflecting microscope objective and a fast charge coupled device detector with the very intense high-energy white beam radiation provided by a wiggler source. The achieved spatial resolution is 2μm. The available x-ray energy spectrum spans from 20to250keV and can therefore be used for low and high Z materials. The spectrum can be modified by inserting different filters into the x-ray beam in order to optimize the signal-to-noise ratio and to avoid beam-hardening artifacts. Different phosphors with different energy sensitivity can be used. The very high speed allows in situ studies of systems evolving on the time scale of a few seconds or minutes. Three examples are given on sintering of metallic powders, so...

High-resolution strain mapping in bulk samples using full-profile analysis of energy-dispersive synchrotron X-ray diffraction data

The feasibility of both high spatial and strain resolution is demonstrated using high-energy X-rays between 100 and 300 keV on beamline ID15A at the ESRF. The data analysis was performed using a multiple-peak Pawley-type refinement on the recorded spectra. An asymmetric peak profile was necessary in order to obtain a point-to-point uncertainty of 10−5. The measurements have been validated with complementary techniques or reference data.

Experimental studies and thermodynamic simulation of phase transformations in high Nb containing γ-TiAl based alloys

Abstract Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.

High diffraction efficiency at hard X-ray energy in a silicon crystal bent by indentation

The diffraction properties of a crystalline silicon plate of which one face was mechanically indented have been studied. This treatment induced a permanent curvature in the sample, which allowed a diffraction efficiency of 88% for 150 keV photons, i.e. a reflectivity of 64% including the absorption. This efficiency is constant over 14 arcseconds and is very close to the theoretical expectation, meaning that the curvature is highly homogeneous. The technique enables the fabrication, in a very reproducible fashion, of crystals for the realization of an astronomical hard X-ray concentrator (Laue lens).

Phase transformation kinetics during continuous heating of a β-quenched Ti–10V–2Fe–3Al alloy

The effect of heating rate on the phase transformation kinetics of a Ti–10V–2Fe–3Al metastable β titanium alloy quenched from the β field is investigated by fast in situ high energy synchrotron X-ray diffraction and differential scanning calorimetry. The initial microstructure is formed by α″ martensite and fine ωath particles distributed in the retained β-phase matrix. The phase transformation sequence varies with the heating rate as revealed by analysis of the continuous evolution of crystallographic relationships between phases. At low temperatures an athermal reversion of α″ martensite into β takes place. This reversion occurs to a larger extent with increasing heating rate. On the other hand, diffusion–driven precipitation and growth of the ω phase is observed for lower heating rates accompanying the reverse martensitic transformation. Furthermore, the results show that the stable α phase can form through three different paths: (a) from the ω phase, (b) from α″ martensite, and (c) from the β phase.

HOLE DEPLETION AND LOCALIZATION DUE TO DISORDER IN INSULATING PRBA2CU3O7-DELTA : A COMPTON SCATTERING STUDY

The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7-d is nominally iso-structural and always superconducting, we have measured the electron momentum density in these materials. We find that they differ in a striking way, the wavefunction coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr on Ba-site substitution disorder is responsible for the metal-insulator transition. Preliminary efforts at growth with a method to prevent disorder yield 90K superconducting PrBa2Cu3O7-d crystallites.

Isotope quantum effects in the electron momentum density of water

The isotope quantum effects in the ground-state electron momentum density of water are studied at temperatures ranging from 5 to 90 degrees C by combining Compton scattering experiments utilizing synchrotron radiation and computational analysis within density functional theory. We observe clear differences in the momentum density between normal and heavy water at room temperature, which are interpreted as predominantly reflecting intramolecular structural differences. The changes in the momentum density upon increasing the temperature are found to be larger for heavy than for normal water, which is attributed primarily to temperature-induced intramolecular structural effects. Both model computations and an ab initio approach qualitatively reproduce the changes in the momentum density as a function of temperature.

Stress relaxation due to ultrasonic impact treatment on multi-pass welds

Abstract Ultrasonic impact treatment (UIT) is a relatively novel technique applied to the toe of welded joints to improve the fatigue life by changing the weld geometry and the residual stress state. In this study, the stress relaxation due to ultrasonic impact treatment is investigated on a six pass welded high strength quenched and tempered steel section. Stress measurements in two orthogonal directions were conducted by energy dispersive synchrotron X-ray diffraction. Results show that the application of only ultrasound to a welded component re-distributes the residual stresses more uniformly, while mechanical impacts in combination with ultrasound is an effective way to release the residual stresses. After welding, diffraction peak broadening due to the lattice distortion, characterised by the full width at half maximum (FWHM), is observed in the region of the weld toes. Ultrasonic impact treatment reduces the FWHM at these locations.

Experimental validation of multiple scattering calculations with high energy X-ray photons

Abstract Experimental data and results of numerical simulations of multiple scattering of polarized X-ray photons are presented and compared. The simulations were performed with a Monte Carlo code developed by the authors optimized to simulate Compton scattering experiments. The ultimate goal is to validate a procedure for correction of experimental Compton profiles. Measurements were done on Al samples with synchrotron radiation and the contribution of photons suffering several scattering events was enhanced by minimizing or canceling out the single scattering part. This was achieved by either taking advantage of the polarization properties of the beam or by making the difference of spectra with the same single scattering component. Samples of different sizes and thickness were measured in transmission and reflection geometry. Good agreement with simulations was observed in all cases and even detailed spectral structure of the multiple scattering component was properly reproduced by the calculations.

Residual stresses, microstructure and tensile properties in Ti-6Al-4V friction stir welds

Abstract This paper reports the results of a systematic investigation of residual stresses as a function of welding speed in a set of Ti–6Al–4V friction stir welds. The investigation focuses on residual stress but links these data with microstructural information derived from micrographs and hardness measurements as well as mechanical testing results. Residual stresses were determined using energy dispersive synchrotron X-ray diffraction, which allows phase specific stresses to be distinguished. The data presented in this paper demonstrate that welds with high tensile properties can be obtained, which also have relatively low peak tensile residual stress values of ∼30% of the tensile strength. The data also show a clear correlation between heat input and the width of the residual stress profile.