White-beam X-ray radioscopy and tomography with simultaneous diffraction at the EDDI beamline

A set-up for simultaneous imaging and diffraction that yields radiograms with up to 200 frames per second and 5.6 µm effective pixel size is presented. Tomograms and diffractograms are acquired together in 10 s. Two examples illustrate the attractiveness of combining these methods at the EDDI beamline for in situ studies.

EDDI (from Energy Dispersive DIffraction) is a multi-purpose beamline for materials research operated at the synchrotron facility BESSY II in Berlin, Germany (Genzel et al., 2007). White beam (about 6–120 keV) is provided by a superconducting 7 T multipole wiggler. The energy-dispersive diffraction mode and the high photon flux make EDDI well suited for time-resolved in situ experiments. A high demand for both imaging and diffraction experiments motivated the construction of a set-up that offers both simultaneously (Fig. 1). Our first results using this set-up yielded radiograms with up to 200 frames per second (fps) and 5.6 µm effective pixel size, resulting in the acquisition of whole tomograms and respective entire diffractograms in 10 s.

Figure 1 Set-up for combined X-ray radioscopy or tomography with simultaneous energy-dispersive diffraction at the EDDI beamline. This example shows the simultaneous acquisition of the tomogram and diffractogram of an open-cell Ni-based foam (Alantum Europe) in 10 s and the three-dimensional reconstruction. The dashed lines in the radiogram indicate the location and height of the projected region hd analysed by diffraction.

The cross section of the incident beam and thus the field of view for imaging at the sample are adjusted by opening the slit system S2 (see Fig. 1). For these experiments, S2 was opened to 4 mm × 2 mm (axial × equatorial = horizontal × vertical in the radiograms) but the largest possible cross section is 4 mm × 4 mm (Genzel et al., 2007). After being partially absorbed by the sample, the direct transmitted X-ray beam is converted into visible light by a 200 µm-thick LuAG scintillator situated inside a cavity enclosed by a lateral Al frame wrapped by a 30 µm-thick Al foil and a bellow that makes it light-tight but almost X-ray transparent for the diffracted beam (in Fig. 1 the Al foil is indicated but not drawn in order to show the beam paths). After the scintillator, the light is mirrored by 90° and the image is magnified 2.3 times by a macro-lens and extension tubes, after which it is recorded by a fast PCO1200 CMOS camera (1280 × 1024 pixels, 12 µm × 12 µm pixel size). The rotation table for tomography is a Huber one-circle goniometer (model 408) that allows 2000 projected radiograms to be acquired in 10 s for one full rotation. Without being attenuated by any component of the set-up except twice by the Al foil, the cross section and angular divergence of the diffracted X-ray beam are adjusted by opening the slit systems S3 and S4, usually within the range 5–10 mm × 0.01–0.03 mm (axial × equatorial) and for these experiments 5 mm × 0.01 mm. After being collimated by S3 and S4, the diffracted beam is recorded by a multi-channel germanium analysing detector (Canberra model GL0110) at a fixed 2θ angle aligned in the vertical plane of the incident beam axis [for further details see Genzel et al. (2007)]. In these experiments, 2θ was 6° but for the present set-up it can be arbitrarily selected between 2° and 14°. A hexapod and a linear (vertical) motor connected to each other by a metallic support (designed to be beside the diffracted beam path) are used for positioning and fine-focusing, respectively. Both tomogram and diffractogram of the open-cell Ni-based foam shown in Fig. 1 were acquired at ambient temperature in 10 s. The dashed lines above and below the half-height of the radiogram indicate the projected height analysed by diffraction h_d, where h_d ≅ ttan(2θ) + eq_S3/cos(2θ), with t the sample thickness and eq_S3 the equatorial opening of S3.

Diffraction data can be analysed on-line using self-developed tools based on Mathematica (Genzel et al., 2007). Radiograms are background-corrected using the software ImageJ or Fiji. Tomographic reconstructions are obtained with Octopus 8.6. The program VGStudio Max is used for visualization of three-dimensional data, Avizo and Mavi for three-dimensional image analyses. Quantitative two-dimensional image analyses are performed with self-developed Matlab codes and the software Axim (García-Moreno et al., 2004).

Radioscopy and diffraction at EDDI were applied simultaneously to study in situ the dynamic process of metal foaming (Jiménez et al., 2013). For this, the rotation table shown in Fig. 1 was exchanged for a heating plate on which the sample does not rotate but experiences structural changes and phase transformations induced by heating, which are monitored in situ by radioscopy and diffraction, respectively. In Fig. 2 the foaming process of AlSi6Cu4 compacted alloy powder containing 5 wt% of the blowing agent TiH_2–x is presented. The sequence of selected radiograms in Fig. 2(a) reveals the internal evolution of the foam structure and yields the relative density of the foam [calculated according to García-Moreno et al. (2009, 2011)] during the thermal cycle T_S(t) measured at the sample bottom. Fig. 2(b) shows respective binary images of the entire foam, which were recorded by a video camera from which the overall area foam expansion was calculated. The synchronized map of diffracted intensities in Fig. 2(c) exhibits the principal lines of all crystalline phases and discloses individual phase transformations. There is liquid metal above 799 K on heating and 778 K on cooling denoted by the diffuse scattering that increases the background level as the diffraction lines of Si, Al, Cu and Al₂Cu fade. The time and temperature dependence of the lattice parameter a_TiH2–x of the blowing agent was calculated by sequential fitting of the principal line (111) of TiH_2–x that allowed d₁₁₁ ∝ constant/E₁₁₁ to be determined and then a_TiH2–x through the formula a_TiH2–x = 3^1/2d₁₁₁ (Jiménez et al., 2011). Lattice parameter contraction above 860 K during heating indicates that TiH₂ decomposes while releasing H₂ gas that expands the liquid metal (Jiménez et al., 2011, 2013).

Figure 2 (a) Synchronized radiograms, (b) corresponding binary images and (c) time–temperature-dependent map of diffracted intensities acquired in situ during foaming of the powder alloy AlSi6Cu4 with 5 wt% of the blowing agent TiH2–x.

Application of these combined methods appears especially attractive for any in situ study of dynamic processes in materials that simultaneously undergo phase transformations and localized structural transformations, such us metal foaming. For other in situ experiments (not included here), heating stages under controlled atmosphere and a tension–compression mechanical testing device are also available at the beamline. Possible applications include sintering, welding, density or volume changes, diffusion and material transport, grain growth, crack growth, fracture, etc. Therefore, this option is readily offered to all EDDI users and in the near future we aim to improve the time and the effective pixel size to between 500–1000 fps and 2–3 µm, respectively.