Abstract
This study aims to explore if the high-entropy alloys (HEAs) also possess superconducting behaviors as conventional metals and alloys possess. By doing this, one may be able to understand further the properties of HEAs. Before systematic investigation, we first selected some known Nb-containing alloy and HEAs, such as MoNbTaW, MoNbTaVW, and NbSiTaTiZr, to check the R(T) behavior down to 5 K; and found the superconducting-like behavior of NbSiTaTiZr near that temperature. Therefore, we continued to design and to prepare NbTaTiZr, HfNbTaTiZr, NbGeTaTiZr, NbSiTaTiZrV, NbGeTatiZrV, NbSiTaTiZrGe, and NbSiTaTiZrGeV alloys; and performed the electrical and magnetic properties, and Hall measurements of these ten 4- to 7-component alloys. Alloy samples were vacuum-arc-remelted, cut, and ground, and then they were performed SEM, EDS, XRD, and 5-300 K 4-probe resistivity (T), 2-300 K magnetization M(T), and 5 K- and 300 K-hysteresis M(H) measurements, as well as varied-temperature and -magnetic field Hall measurement and analysis. Microstructure of as-cast MoNbTaW, NbTaTiZr, MoNbTaVW, and HfNbTaTiZr are of single, simple, solid-solution, pseudo-unitary BCC with dendrite-interdendrite substructure. Other 6 as-cast alloys contain a structure with multi-structure and multi-phase in them. Of the most important is that all structures that contain Nb-Ta-riched, pseudo-unitary BCC, solid-solution phase possess superconductivity. Alloys with zero electrical resistance and their corresponding critical temperatures are NbTaTiZr (8.98 K), GeNbTaTiZr (9.16 K), HfNbTaTiZr (7.93 K), NbSiTaTiVZr (4.99 K), GeNbSiTaTiZr (8.10 K), and GeNbTaTiVZr (9.10 K). As to NbSiTaTiZr, it has a drastic drop in resistance near 5 K. Owing to the limitation of measurement that experimental temperature cannot be lowered further; it therefore shows zero-like resistance in the electrical resistance measurements. Single BCC-structured MoNbTaW and MoNbTaVW alloys are composed of all BCC elements, their electrical resistivity ranges 22~40 μΩ-cm, which is lower than that (100~200 μΩ-cm) of other non-all-BCC multi-component alloys measured as before. The 7-component NbSiTaTiZrGeV alloy has the highest resistivity of ~200 μΩ-cm among the ten alloys in this study. The value of residual resistivity ratio, RRR 290K/10K, is in 1.05 ~ 1.36, manifesting that the non-thermal effect, i.e., lattice defect, is greater than the thermal factor. The latter conclusion is in consistent with the one ever made in the similar experiments. From M(T) curves at 1 kOe, alloys that have superconductivity or the like in resistivity measurements show definite diamagnetism. These alloys with their corresponding critical temperatures are NbTaTiZr (7.98 K), NbSiTaTiZr (4.92 K), NbSiTaTiVZr (4.73 K), GeNbTaTiZr (8.61 K), GeNbTaTiVZr (6.34 K), GeNbSiTaTiZr (5.94 K), and HfNbTaTiZr (6.30 K). These M(T) experiments demonstrate the existence of superconductivity in these alloys. On the other hand, the alloys without zero resistivity show no diamagnetic behavior in M(T). Measurements of M(H) hysteresis at 5 K with superconducting alloys show a loop extended in 4 quadrants, demonstrating that these alloys are of the type II superconductors. By M(H), one is able to determine the critical magnetic fields of superconducting alloys. The lower critical magnetic field, Hc1, in alloys and their corresponding values are NbTaTiZr (400 Oe), NbSiTaTiZr (400 Oe), GeNbTaTiZr (300 Oe), GeNbTaTiVZr (300 Oe), GeNbSiTaTiZr (100 Oe), and HfNbTaTiZr (< 100 Oe). The higher critical magnetic field, Hc2, in almost alloys is exceeding 1 T (104 G). Only GeNbSiTaTiZr show small Hc2 (6 kOe). M(H) measurements at 300 K for all ten alloys show superparamagnetism or soft ferromagnetism. Hall measurements at 5 and 300 K and at 1 T to 9 T for these multi-component alloys demonstrate that most of the carriers are of hole-like, with concentration of 1022 cm-3 that is the same value measured as before. The mobility of the alloys is one or two orders of magnitude less than that of the pure metals. As temperature rises, the Hall resistivity increases. The summary in Hall measurements elucidates that there is a large amount of point defects in the pseudo-unitary lattice of the multi-component alloys. Alloy that has the lowest hardness (322 Hv) is NbTaTiZr. The hardness value increases after the individual addition of Hf, Si, and Ge in NbTaTiZr, while the V addition in it decreases the hardness of the alloy. The NbSiTaTiZrGeV alloy has the largest hardness value (760 Hv) that is ascribed to its largest number of multi-phases and a significant effect of the solid solution strengthening.