Said Atieh

Bio: Said Atieh is an academic researcher from CERN. The author has contributed to research in topics: Necking & Niobium. The author has co-authored 1 publications.

Characterization of the Formability of High-Purity Polycrystalline Niobium Sheets for Superconducting Radiofrequency Applications

Abstract: The forming limit diagram of high-purity niobium sheets used for the manufacturing of superconducting radiofrequency (SRF) cavities is presented. The Marciniak (in-plane) test was used with niobium blanks with a thickness of 1 mm and blank carriers of annealed oxygen-free electronic copper. A high formability was measured, with an approximate true major strain at necking for plane-strain of 0.441. The high formability of high-purity niobium is likely caused by its high strain rate sensitivity of 0.112. Plastic strain anisotropies (r-values) of 1.66, 1.00, and 2.30 were measured in the 0°, 45°, and 90° directions. However, stress–strain curves at a nominal strain rate of ~10−3 s−1 showed similar mechanical properties in the three directions. Theoretical calculations of the forming limit curves (FLCs) were conducted using an analytical two-zone model. The obtained results indicate that the anisotropy and strain rate sensitivity of niobium affect its formability. The model was used to investigate the influence of strain rate on strains at necking. The obtained results suggest that the use of high-speed sheet forming should further increase the formability of niobium.

Reverse coating technique for the production of Nb thin films on copper for superconducting radio-frequency applications

Abstract: In the framework of the Future Circular Collider Study, the development of thin-film coated superconducting radio-frequency copper cavities capable of providing higher accelerating fields (10–20 MV m−1 against 5 MV m−1 for the Large Hadron Collider) represents a major challenge. The method investigated here for the production of seamless niobium-coated copper cavities is based on the electroforming of the copper structure around a sacrificial aluminium mandrel that is pre-coated with a niobium thin film. The first feasibility study, applied to a flat aluminium disk mandrel, is presented. Protective precautions are taken towards the functional niobium film during the production process and it is shown that this technique can deliver well performing niobium films on a seamless copper substrate. This way, the non-trivial chemical treatments foreseen by the standard procedures (e.g. SUBU, EP) for the preparation of the copper surface to achieve the proper adhesion of the niobium layer are also avoided. The only major chemical treatment involved in the reverse-coating method is represented by the chemical dissolution of the aluminium mandrel, which has the advantage of not affecting the copper substrate and therefore the copper-niobium interface.

Relation Between Tensile Strength and Annealing Temperature for High Purity Niobium

Abstract: Herein, the relationship between the annealing temperature and the tensile strength of high-purity fine-grain niobium is systematically examined. The superconducting radio frequency (SRF) cavity is housed in a helium tank to be cooled by liquid helium; therefore, it is subject to the High-Pressure Gas Safety Act, and its strength must be guaranteed; for example, whether its wall possesses sufficient strength to endure the outer pressure. It is essential that the strength of the processed material is investigated according to the actual treatment protocol of the SRF cavity. The specimens for tensile testing, residual resistivity ratio (RRR) measurement, and microscopy are cut from the same niobium sheet and annealed simultaneously. This letter is significant to specify seven properties (annealing temperature, RRR, tensile strength, 0.2% proof stress, elongation, hardness, and grain size) altogether. When vacuum annealing is performed in the range of 800 °C –1100 °C, the RRR is slightly changed. As the annealing temperature increases, the tensile strength decreases. The 0.2% proof strength, elongation, and hardness are almost constant. As the annealing temperature increases, the recrystallization of niobium is promoted, thereby resulting in coarsening of the crystal grains. Evidently, the relationship between the average grain size and tensile strength depends on the Hall–Petch relationship.