Physical properties of tin-containing high-entropy alloys

Gačnik, Darja (author), Dolinšek, Janez (mentor)

The concept of introducing a maximum amount of disorder, i.e., entropy, is realized in high-entropy alloys. At first, first generation high-entropy alloys were generated as single-phase solid solutions, but nowadays, the research has shifted towards second generation high-entropy alloys, i.e., non-equimolar multiphase alloys. In this Doctoral Thesis, we investigated the physical properties of tin-containing alloys. Low melting point (505.06 K) and high boiling point (2876 K) make tin quite different from other elements, with the vast difference between the two temperatures. Generally, tin has low solubility with other elements, so it promotes phase segregation in the alloys, being a perfect candidate for multiphase alloy structure. We specifically investigated six alloys: a ternary HfTiZr, a quaternary HfTiZrSn, and four pentary HfTiZrSnM (M = Fe, Ni, Cu, Nb) alloys. Only a non-tin alloy solidified into a single-phase solid solution, while other tin-containing alloys formed two-phase (only HfTiZrSnNb) or four-phase structures. We explored how chemical and structural disorders affect physical properties like superconductivity. For superconducting samples, we measured a relatively broad temperature range of the heat capacity peaks and the decrease in the resistivity, indicating that Cooper pairs do not form instantly and that superconducting phases have nano-regions that vary in critical temperatures. The magnetic properties of all the samples were analyzed, but a superconducting state was examined only for the HfTiZrSnNb alloy. The critical current, estimated via the Bean model, surpassed the NbTi characteristic critical current values even to four orders of magnitude. Moreover, the effect of applied mechanical pressure was evaluated with magnetization measurements up to 1.4 GPa. We performed the first-ever reported STM spectroscopy of superconducting high-entropy alloy to investigate its atomic-size inhomogeneity. We mapped the surface of the two-phase HfTiZrSnNb sample and detected the superconducting gaps that varied significantly, implying that the sample is a typical BCS-type “dirty” superconductor.

• Language: English
• Keywords: tin, high-entropy alloys, multicomponent alloys, disordered systems, superconductivity
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-145127
• COBISS.SI-ID: 148203523
• Publication date in RUL: 07.04.2023

Secondary language

• Language: Slovenian
• Title: Fizikalne lastnosti kositrovih visokoentropijskih spojin

Abstract

V visokoentropijskih spojinah je vključen koncept največjega nereda, t. i. entropije. Le-te so bile najprej, v okviru prve generacije visokoentropijskih spojin, narejene kot enofazne trdne raztopine, toda dandanes se raziskave usmerjajo v razvoj t. i. druge generacije visokoentropijskih spojin – neekvimolarnih večfaznih zlitin. V tej doktorski disertaciji bodo predstavljene fizikalne lastnosti kositrovih zlitin. Z nizkim tališčem (505,06 K) in visokim vreliščem (2876 K) se kositer s precejšnjo temperaturno razliko med obema temperaturama zelo razlikuje od preostalih elementov. Na splošno se kositer zelo slabo meša s preostalimi elementi in tako spodbuja fazno segregacijo, zato je popoln kandidat za vzpostavitev zlitin z večfazno strukturo. Posebej smo se osredotočili na raziskavo šestih vzorcev: ternarne HfTiZr, kvaterne HfTiZrSn in štiri pentarne HfTiZrSnM (M = Fe, Ni, Cu, Nb) spojine. Samo spojina, ki ni vsebovala kositra, se je uredila v enofazno trdno raztopino, medtem ko so preostale kositrove spojine formirale dvofazne (samo HfTiZrSnNb) ali štirifazne strukture. Preiskali smo, kako kemijski in strukturni nered vplivata na fizikalne lastnosti, na primer na superprevodnost. Vrhovi izmerjene toplotne kapacitete in upad upornosti so se pri superprevodnih vzorcih pojavili na precej širokem temperaturnem območju, kar nakazuje na to, da Cooperjevi pari ne nastanejo naenkrat. Superprevodne faze namreč sestavljajo nanoregije, ki se med sabo razlikujejo v kritičnih temperaturah. Analizirali smo magnetne lastnosti vseh vzorcev, toda samo magnetizem zlitine HfTiZrSnNb smo preučili v superprevodnem stanju. Kritični tok, ki smo ga določili preko Beanove metode, je presegel karakteristični kritični tok NbTi superprevodnika tudi za 4 velikostne rede. Dodatno smo ocenili učinek dodajanja mehaničnega pritiska z magnetnimi meritvami vse do 1,4 GPa. Izvedli smo prvikrat objavljeno vrstično tunelsko spektroskopijo superprevodne visokoentropijske spojine, s katero smo raziskovali nehomogenost na atomski ravni. S spektroskopskim skeniranjem površine dvofaznega HfTiZrSnNb vzorca smo odkrili superprevodne vrzeli, ki so se med sabo precej razlikovale. To nakazuje, da je vzorec tipični predstavnik »umazanih« superprevodnikov tipa BCS.

• Keywords: kositer, visokoentropijske spojine, večkomponentne zlitine, neurejeni sistemi, superprevodnost