Evaluation of predictive correlation between flux expulsion and grain growth for superconducting radio frequency cavities

A series of experiments were carried out in an effort to develop a simple method for predicting magnetic flux expulsion behavior of high purity niobium used to fabricate superconducting radio frequency (SRF) cavities. Using conventional metallographic characterizations in conjunction with high spatial resolution electron backscattered diffraction-orientation imaging microscopy (EBSD-OIM), we found that the flux expulsion behavior of 1.3 GHz single cell SRF Nb cavities is significantly associated with the grain growth of the Nb material during heat treatment. Most of Nb grains rapidly grew during 900C heat treatment, and likely full-recrystallized with 1000C HT. With comparison of the magnetic flux expulsion ratio (Bsc/Bnc) at dT = 5 K, the flux expulsion efficiency of the cavities increases along with increasing of grain size. Most interestingly, 900C HT shows a roughly linear trend that suggests this criterion could be used to predict appropriate heat treatment temperature for sufficient flux expulsion behavior in SRF-grade Nb. This result would be used to see if flux expulsion can be predicted by examining the materials coming from the Nb vendor, prior to cavity fabrication.