The Influence of Strain Path and Heat Treatment Variations on Recrystallization in Cold-Rolled High-Purity Niobium Polycrystals

Abstract

With the standard material and cavity production guidelines, the properties and performance of industrially produced high-purity niobium superconducting radiofrequency (SRF) cavities are unpredictably variable. Currently, assembled niobium SRF cavities undergo post-purification vacuum heat treatment at 800°C for three hours to remove hydrogen; however, recent work has shown significant improvements in cavity performance when annealed between 900-1000°C, causing recrystallization (Rx). Rx is a thermally activated process, so increasing the annealing temperature and/or the heating rate should facilitate the onset of Rx, resulting in a greater reduction in the density of geometrically necessary dislocations (GNDs) that are strongly associated with the degradation of cavity performance via trapped magnetic flux. Improving the SRF community's understanding of the influence of both heat treatment and deformation history on the resulting microstructure and Rx behavior in high-purity niobium is needed. This fundamental metallurgy is critical for identifying improved specifications for the consistent production of high-performing niobium SRF cavities. This research investigates the effects of heat treatment variations with different deformation paths, heating rates, and annealing temperatures on deformed samples of polycrystalline high-purity niobium. To assess the effects of strain path, samples were cold-rolled to ∼30% reduction prior to annealing, with half of the sample set rolled parallel to the original rolling direction of the as-received niobium sheet and the other half in the transverse direction.