Superconductivity and hardness of the equiatomic high-entropy alloy HfMoNbTiZr

We have found that body-centered cubic (bcc) HfMoNbTiZr is a type-II BCS high-entropy alloy (HEA) superconductor with a superconducting critical temperature of Tc = 4.1 K. By employing a Debye temperature θD of 263 K and Tc, the electron-phonon coupling constant λe-p is calculated to be 0.63. The electronic structure calculation revealed band broadening with energy uncertainties due to atomic disorders. The superconducting properties are compared among equiatomic quinary bcc HEA superconductors. Tc decreases with decreasing λe-p, which is negatively correlated with θD. The negative correlation between λe-p and θD would be caused by a shorter phonon lifetime at a higher θD, which is based on phonon broadening due to atomic disorder and the uncertainty principle. The Vickers microhardness was measured for several bcc HEA superconductors. Tc is exceptionally low in a superconductor with relatively high hardness, because the hardness generally increases with increasing θD, and θD is negatively correlated with Tc in the high-entropy state. •HfMoNbTiZr is a new high-entropy alloy (HEA) superconductor.•The superconducting properties are compared among HEA superconductors.•The higher Debye temperature suppresses superconducting critical temperature Tc.•High hardness leads to lower Tc.