This new instrument, coined the toroidal slit diffractometer (TSD), transforms the state-of-the art of EDXRD, simultaneously improving both the quality and quantity of information accessed by this technique.
EDXRD is a variant of x-ray diffraction that leverages the highest x-ray energies available at CHESS, up to 200 keV, to measure metallic components that pose a challenge to other characterization methods due to their thickness or complexity. The atomic-scale distortions EDXRD measures, known as strain, are directly related to internal forces, known as residual stress, that is a key determinant of part strength and failure.
The heart of the upgrade is replacement of a single-element detector with a 23-element detector array. Each “element” is a cryogenically-cooled Germanium single-crystal diode optimized for collecting the high energy x-rays required for EDXRD. Perhaps surprisingly, the critical motivation for the new detector was not to improve scan speed, but instead to measure the atomic-level distortions along different directions simultaneously. These multi-directional strains can be used, in some cases, to determine the internal stress at each position of the part, which is one of the most important parameters determing the parts ultimate strength.
"EDXRD is crucial for measuring residual stress in large, as-manufactured metallic components, which is one of the most important design parameters in structural engineering," said Arthur Woll, Director of the Materials Solutions Network at CHESS (MSN-C). "The new detector geometry eliminates the main trade-off typically associated with EDXRD measurements compared to other diffraction measurements -- less information -- enabling new insights into how stress arises during manufacturing processes such as welding and additive manufacturing.”
What differentiates EDXRD from most x-ray diffraction setups is the use of a fixed scattering angle in conjunction with varying x-ray wavelength. Fixing this angle permits selection of a specific 3D measurement volume within a sample. This finite sample volume is the key advantage of EDXRD over other diffraction geometries, since it is why thick, complex samples can be measured without sacrificing the measurement accuracy.
Next to the detector itself, the most important aspect of the TSD is the slitting system by which this 3D measurement volume is defined. The previous EDXRD setup used two pairs of slits to define this volume, requiring four independent motors and slit blades. A simple scaling of this solution to the 23-element detector would require 92 independent motors and blades, which would be both grossly cumbersome and exorbitantly expensive. TSD gets its name from the custom solution to this challenge, namely with two pairs of so-called toroidal slits (see figures). Each slit was produced by cutting a precision, convex surface onto a ¼” thick tungsten plate. The word “toroid” refers to the fact that each slit surface is defined by revolving a short arc around a single rotation axis. Each pair of toroidal slits is mounted on a customized stage, enabling easy adjustment of the sample volume and diffraction angle.
“X-rays scatter from everything they hit, like a bowling ball rolling through pins. The slits are made to block all of the scattered X-rays except those from a specific region of the sample,” said Joe Crum, lead engineer on the project. “It's like narrowing your view of the bowling lane to only see what happens to the middle pin. The narrower the slit, the smaller the region is that each detector can "see", permitting researchers to determine the residual stress in a smaller and smaller region of the sample."
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This advancement is expected to benefit a wide range of users, but is especially valuable for researchers from the Air Force Research Laboratory (AFRL) and manufacturers focused on novel materials and manufacturing processes. The new detector system, funded by the AFRL, represents a quarter-million-dollar investment in the future of materials and mechanical engineering research.
"The implementation of this detector is unique in the world," said Kelly Nygren, a beamline scientist at the Structural Materials Beamline. "There are not many EDXRD setups globally, and even fewer with multiple detectors. Our new system, with its 23-element array and toroidal slit geometry, positions CHESS at the forefront of global research facilities with such capabilities."
The new system, which involved significant contributions from many members of the CHESS technical staff, is expected to have broader impacts on advanced mechanical engineering, potentially establishing new standards for emerging technologies such as additive manufacturing.
This work was supported by funding from the Air Force Research Laboratory, ensuring that CHESS remains a vital resource for both academic and industrial researchers.
Savan DeSouza is a Communications Assistant at the Office of the Vice President for Research and Innovation (OVPRI)