Isoelectronic substitutions and aluminium alloying in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor

Abstract

High-entropy alloys (HEAs) are a new class of materials constructed from multiple principal elements statistically arranged on simple crystallographic lattices.Due to the large amount of disorder present, they are excellent model systems for investigating the properties of materials intermediate between crystalline and amorphous states.Here we report the effects of systematic isoelectronic replacements, using Mo-Y, Mo-Sc, and Cr-Sc mixtures, for the valence electron count 4 and 5 elements in the BCC Ta-Nb-Zr-Hf-Ti high entropy alloy (HEA) superconductor. We find that the superconducting transition temperature T c strongly depends on the elemental make-up of the alloy, and not exclusively its electron count. The replacement of niobium or tantalum by an isoelectronic mixture lowers the transition temperature by more than 60 %, while the isoelectronic replacement of hafnium, zirconium, or titanium has a limited impact on T c . We further explore the alloying of aluminium into the nearly optimal electron count [TaNb] 0.67 (ZrHfTi) 0.33 HEA superconductor. The electron count dependence of the superconducting T c for (HEA)Al x is found to be more crystalline-like than for the [TaNb] 1-x (ZrHfTi) x HEA solid solution. For an aluminum content of x = 0.4 the high-entropy stabilization of the simple BCC lattice breaks down. This material crystallizes in the tetragonal β-uranium structure type and superconductivity is not observed above 1.8 K.

Pressure effects on the electronic structure and superconductivity of(TaNb)0.67(HfZrTi)0.33high entropy alloy

“…It is a type-II superconductor with the transition temperature of T c = 7.3 K. Experimental data as well as theoretical calculations [10] suggest conventional mechanism of superconductivity with a relatively strong electron phonon coupling parameter λ ∼ 1. Several other examples of * wiendlocha@fis.agh.edu.pl superconducting HEAs were later reported [11][12][13], however, the TaNbHfZrTi family is still the most investigated one [9,10,14,15]. When the atomic concentration is slightly changed to Ta 33.5 Nb 33.5 Hf 11 Zr 11 Ti 11 [15] [denoted as (TaNb) 0.67 (HfZrTi) 0.33 or TNHZT in short], superconducting transition temperature slightly increases to 7.7 K. This alloy also hosts a cubic body-centered crystal structure, with the lattice parameter of 3.34Å.…”

Structure and Superconductivity of Tin-Containing HfTiZrSnM (M = Cu, Fe, Nb, Ni) Medium-Entropy and High-Entropy Alloys

“…According to the chemical/structural classification by Sun and Cava [28], the HEA superconductors are divided into four classes. Type-A HEA superconductors consist of the early transition metals with the bcc structure and small unit cells, with the representative examples Ta-Nb-Hf-Zr-Ti [22,[29][30][31][32][33] and Nb-Hf-Zr-Ti-V [34]. Type-B HEA superconductors are composed of the 4d and 5d transition metals (early and late), with the representatives (HfTaWIr) 1−x Re x , (ZrNb) 1−x (MoReRu) x and (HfTaWPt) 1−x Re x and x < 0.6.…”

Cutting Edge of High-Entropy Alloy Superconductors from the Perspective of Materials Research

“…It should be noted here that, according to the well-known relation between VEC and the phase stability for fcc and bcc solid solutions in HEAs, a single bcc phase HEA would not be obtained at a VEC larger than 6.87 [1]. When we design a bcc HEA superconductor, the elemental makeup is an important factor to be considered [17]. To elucidate the trend of the elemental makeup, the frequency with which elements are used in the bcc HEA superconductors reported to date (Nos.…”