Variation of mechanical properties of high RRR and reactor grade niobium with heat treatments

Abstract

Superconducting rf cavities used as accelerating structures in particle accelerators are made from high purity niobium with residual resistance ratios greater than 250. Reactor grade niobium is also used to make wave-guide and/or end group components for these accelerating structures. The major impurities in this type of niobium are interstitially dissolved gases such as hydrogen, nitrogen, and oxygen in addition to carbon. After fabricating the niobium accelerating structures, they are subjected to heat treatments for several hours in vacuum at temperatures of up to 900°C for degassing hydrogen or up to 1400°C for improving the thermal conductivity of niobium considerably. These heat treatments are affecting the mechanical properties of niobium drastically. In this paper the variation of the mechanical properties of high purity and reactor grade niobium with heat treatments in a vacuum of ~ 10–6 Torr and temperatures from 600°C to 1250°C for periods of 10 to 6 hours are presented. High purity niobium starts exhibiting upper and lower yield stress points while reactor grade niobium, on the other hand, stops exhibiting this characteristic BCC material behavior of upper and lower yield stress points after heat treatment at 1250°C in the presence of Ti as a solid state getter material.