Evolution of Bulk Hardness on Cold Rolled Niobium with Heat Treatment

Superconducting radio frequency cavities have become highly prominent in modern particle accelerators due to their high quality factor capabilities. Niobium in particular has proven highly effective as the primary material in fabricating complex structures due to its ductility and superior superconducting properties. One of the primary factors that affects the quality factor is magnetic flux trapping, which can occur at locations of defects and impurities. High-temperature heat treatments are used to reduce defects, minimizing the trapping centers. Recently, cavities made from cold rolled niobium sheet performed better than those made from traditional niobium. The increased performance was due to the minimized flux trapping resulting from better recrystallization. We use Vickers micro-hardness tests to measure the hardness of cold rolled niobium as a function of heat treatment temperature. This work showed a marked decrease in Vickers micro-hardness on niobium sheet cross-section with increasing temperature. The decrease in hardness is more pronounced when the heat treatment temperature is greater than 600 °C. The sharp decrease in hardness corresponds to the start of recrystallization. The procedures and results in this paper provide researchers and manufacturers with a considerably simpler and relatively inexpensive method of analyzing cold rolled niobium.