  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.
