  High-entropy alloys (HEAs) are a new class of materials which are being
energetically studied around the world. HEAs are characterized by a
multi-component alloy in which five or more elements randomly occupy a
crystallographic site. The conventional HEA concept has developed into simple
crystal structures such as face-centered-cubic (fcc), body-centered-cubic (bcc)
and hexagonal-closed packing (hcp) structures. The highly atomic-disordered
state produces many superior mechanical or thermal properties.
Superconductivity has been one of the topics of focus in the field of HEAs
since the discovery of the bcc HEA superconductor in 2014. A characteristic of
superconductivity is robustness against atomic disorder or extremely high
pressure. The materials research on HEA superconductors has just begun, and
there are open possibilities for unexpectedly finding new phenomena. The
present review updates the research status of HEA superconductors. We survey
bcc and hcp HEA superconductors and discuss the simple material design. The
concept of HEA is extended to materials possessing multiple crystallographic
sites; thus, we also introduce multi-site HEA superconductors with the
CsCl-type, {\alpha}-Mn-type, A15, NaCl-type, {\sigma}-phase and layered
structures and discuss the materials research on multi-site HEA
superconductors. Finally, we present the new perspectives of eutectic HEA
superconductors and gum metal HEA superconductors.
