The Search for Single-Phase Hexagonal Close-Packed High-Entropy Alloy Superconductors with Low Valence Electron Counts

Abstract

This thesis studies the possibility for new, single-phase hexagonal close-packed (hcp) high-entropy alloy (HEA) superconductors to exist at low valence electron counts (VECs). Known hcp HEA superconductors have been reported with VECs ranging from 5.79 to 8, so there is a lack of research exploration into regions of lower VEC. To synthesize a single-phase hcp HEA superconductor with low VEC, the (Zr-Hf-Ti-Nb)100-xScx and [(Zr-Hf-Ti)65-xNbx]65Sc35 series were chosen as good candidates to study due to their constituent elements having VECs of either 3, 4, or 5. These HEAs were synthesized through three variations of arc melting—traditional arc melting, splat cooling, and heated-hearth splat cooling—and were analyzed by X-ray diffraction (XRD) and a Physical Property Measurement System (PPMS). All synthesized HEA samples had a mixed body-centered cubic (bcc) + hcp structure, though simultaneously increasing the scandium content while decreasing the niobium content helped minimize the bcc phase. The faster cooling rates attained through splat cooling also helped to decrease the presence of the bcc phase. Resistance and magnetization measurements indicated that the bcc phase was likely responsible for the observed superconductivity in the mixed-phase samples. Further studies using even faster cooling rates may help to completely subvert the bcc phase to form a single-hcp structure.