Abstract
This study investigates the superconductivity of as-cast and as-heat-treated ternary to 6-element multi-component alloys (hereafter abbreviated as the alloys) that are made of non-equal molar Nb-Zr-Ti by addition of minor Hf, V, Ta and Ge. The alloys are principally single random BCC Nb-rich solid solutions. Difference in formation enthalpies between elements, especially for pairs with Ta and Ge, and the driving forces by heat treatments induce a Zr-bearing precipitation from the Nb-rich BCC phase to form a Zr(Ge)-rich phase. This results in a change of Nb/Zr ratio in the Nb-rich BCC phase, and thus affects the superconductivity of the alloys. The critical temperature of the alloys, Tc, ranges from 8 K to 11 K. The room-temperature resistivity of the as-cast alloys varies from 21 μΩ–cm to 35 μΩ–cm. Compared with the electrical resistivity of other multi-component ones, the alloys have a lower electrical resistivity. The residual resistivity ratio RRR value is from 1.2 to 1.3, which mentions that the resistivity is principally controlled by the impurity atoms in the alloys. If one simply emphasizes the e/a ratio from the content of Nb and Zr in the alloys, the Tc of the alloys approximately follows the Matthias empirical rule. However, factors affecting Tc, besides the e/a, include lattice distortion due to multiple element addition, and the characteristics of individual elements in the alloys. The alloys are typically type II superconductors. The upper critical magnetic field Hc2 is estimated to be in the range of 5 T to 9 T. At 2 K & 5 T, the critical current density Jc has the value of approximately 105 A/cm2. This property has something to do with the precipitates and has yet nothing to do with the lattice distortion of the alloys.