EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Contrary to the conventional alloys with one principal element, multicomponent alloys (MCAs), e.g. high-entropy alloys (HEAs) and compositional complex alloys (CCAs), contain many elements in near-equimolar ratios. We investigated whether the structural and functional properties like superconductivity would vary from three- to five-component alloys. For that purpose, we chose six alloys: a ternary TiZrHf, a quaternary TiZrHfSn, and four pentary TiZrHfSn(Fe,Ni,Cu,Nb) alloys. According to the criteria for an ideal equimolar solid solution, the 3-component TiZrHf and the 4-component TiZrHfSn alloys should classify as medium-entropy alloys (MEAs), whereas the four 5-component TiZrHfSn(Fe,Ni,Cu,Nb) alloys would be HEAs. However, these criteria apply only to the near-ideal, single-phase TiZrHf alloy. Our detailed characterization analysis showed that other alloys solidify as MCAs with TiZrHfSnNb being a two-phase mixture of a MEA and a HEA phase and the other four alloys are four-phase mixtures of MEA phases.

From heat capacity and resistivity measurements, we have determined that superconducting phases in the TiZrHf(Sn(Fe,Ni,Nb)) alloys have nano-regions that vary in critical temperatures. For the TiZrHfSnNb sample, the scanning tunneling spectroscopy (STS) measurements yielded consistent results with the nanoscale variation of the local electronic density of states (DOS). The detected superconducting gap widths ranged from zero (no SC gap) to around 2 meV.

We can conclude that the electrons in these investigated superconducting strongly-disordered systems scatter from physical and chemical defects elastically and at an extremely rapid rate. Therefore, they are typical representations of BCS-type “dirty” superconductors.

[1] D. Gačnik, et. al., Superconductivity in High-Entropy and Medium-Entropy Alloys From the Ti-Zr-Nb-Sn-Hf-Ta System, Reference Module in Materials Science and Materials Engineering, 2020 (Elsevier).