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 study 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 800–1100°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.