  With the aim of the discovery of face-centered-cubic (fcc) high-entropy alloy
(HEA) superconductor, we have carried out materials research on Nb or
Pb-containing multi-component alloys. Although the X-ray diffraction (XRD)
patterns of some Nb-containing samples exhibited the dominant fcc phases, no
superconducting signals were observed down to 3 K. Examination with an energy
dispersive X-ray spectrometer revealed that all samples were multi-phase, but
the existence of several new Nb-containing HEA phases was found in them. It was
confirmed that the synthesis of Pb-containing an HEA or quaternary alloy would
be difficult, probably due to the large differences in the crystal structure
and atomic radius among constituent elements, low reaction temperature and the
lack of a rapid cooling process in the synthesis. Despite the negative results
in this research, some hints for an improved strategy for the search for an fcc
HEA superconductor are provided. Moreover, our results are useful as
fundamental data for future HEA predictions or for studies of phase relations
in Nb or Pb-containing multi-component alloys based on the CALPHAD (calculation
of phase diagram) method.
