Energy dispersive diffractometer

The technical development of the energy-dispersive diffraction (EDD) setup is strongly correlated to the continuously increasing interest of the engineering materials science (EMS) community to use non-destructive diffraction techniques such as high energy x-rays. One of the main interests is to understand the relationship between the micro-structure and the macroscopic properties of materials after they have been processed by a thermo mechanical treatment. Therefore, diffraction techniques play an important role and historically neutron diffraction has been applied to characterise the stress state in bulky engineering components due to their high penetration power for matter. With the construction of 3rd generation synchrotron radiation sources powerful high-energy x-rays became available and the EMS community started to exploit the opportunities of this new source which offered a very high flux in small beams of few tens µm2 not available with neutrons. The EDD using white beam from a high energy wiggler source opened up a whole new field of research: real engineering components could be studied in the bulk and up to the surface with very high spatial resolution, down to the intra- and sub granular level. In EDD experiments a multitude of Bragg reflections are collected simultaneously with a solid state Germanium detector. The scattering gauge volume is defined by an entrance slit in front of the sample and by two slits in front of the detector. The slit size is typically 100µm defining an elongated gauge volume parallel to the incoming x-ray beam. Motorised sample positioning enables a full 3D analysis of the sample. Acquisition times range from 10s to several minutes depending on the attenuation of the x-rays in the sample. With the detectors available today the resolution is about 2x10–4.

Energy dispersive diffraction setup.

In order to liberate space in the experimental hutch for new experimental setups the concept of the 3-axis instrument was revised in 2004 and the EDD experiment was now fully integrated into the standard operation of this instrument. A fixed 2nd detector in vertical scattering geometry was added with an immediate doubling of the performance. In order to handle the FaME38 stress rigs with sufficient mechanical precision a new sample positioning for loads up to 400 kg was installed in 2005. Most recently the former analogical data acquisition system was replaced by modern digital electronics and old RC-type detectors were replaced by TRP-type Germanium detectors optimised for high count-rates. This system has just become operational and the full performance needs to be evaluated. The aim is to accept higher count-rates in order to perform in-situ experiments for the studies of thermo-mechanically driven phase-transitions, fast strain rate experiments, cyclic loading, etc. The basic concept of the EDD diffraction setup is well defined and today its performance is essentially given by the available photon flux from the insertion device and the count-rate capability of the detection system. Due to the specific needs of each project several pieces of equipments have been implemented into the experimental setup: an in vacuum oven (up to 1200C) with integrated tensile machine (5kN) to study relaxation process in Al metal matrix composites, a cryo-magnet (from University of Warwick) to study martensite transformation at low temperatures, a mirror based focusing lamp oven (up to 1100°C) integrated in the Eulerian circle to achieve fast heating rates to study thermal barrier coatings, and an industrial robot as an alternative way to manipulate non-regular shaped samples.