  We report the results of a zero-field muon spin relaxation (ZF-muSR) study of
superconducting Ba1-xKxFe2As2 (0.5 < x < 0.9) in search of weak spontaneous
internal magnetic fields associated with proposed time-reversal-symmetry
breaking mixed pairing states. The measurements were performed on
polycrystalline samples, which do not exhibit the mesoscopic phase separation
previously observed in single crystals of Ba1-xKxFe2As2. No evidence of
spontaneous internal magnetic fields is found in any of the samples at
temperatures down to T ~ 0.02 K.
