High-Entropy Alloys (HEAs) research at the J. Stefan Institute and the Faculty of Mathematics and Physics of the Ljubljana University
In 2004, a new approach to metallic alloys design with multiple principal elements in near-equimolar concentrations, termed high-entropy alloys (HEAs), has been launched. According to this concept, a high entropy of mixing can stabilize disordered solid solution phases with simple structures like a body-centered cubic (bcc), a face-centered cubic (fcc) and a hexagonal close-packed (hcp) with small unit cells, in competition with ordered crystalline intermetallic phases that often contain structurally complex giant unit cells. A HEA structure is characterized by a topologically ordered lattice with an exceedingly high chemical (substitutional) disorder, so that a HEA can be conveniently termed as a “metallic glass on an ordered lattice”. In order to achieve a high entropy of mixing, the alloys must be composed of five or more chemical elements in similar concentrations, ranging from 5 to 35 atomic % for each element, but do not contain any element whose concentration exceeds 50 at. % (i.e., there is no principal element). Examples of HEAs are alloys derived within the systems Al-Si-Co-Cr-Cu-Fe-Mn-Ni, W-Ta-Nb-Hf-Zr-Ti-Mo-V, and Gd-Tb-Dy-Ho-Er-Tm-Lu-Y.
Most existing studies of HEAs focus on the relationship between the phase, the microstructure and the mechanical properties, where it was demonstrated that HEAs exhibit enhanced mechanical properties like a high hardness and a solid-solution strengthening. Physical properties of the HEAs remain largely unexplored. In 2014, our group at the Jožef Stefan Institute and the Faculty of Mathematics and Physics of the Ljubljana University has made a breakthrough in the field of physical properties of HEAs, by discovering the first superconducting HEA within the Ta-Nb-Hf-Zr-Ti system, showing a superconducting transition temperature of 7.3 K and the upper critical field of 8.2 T. The resulting paper (P. Koželj et al., Phys. Rev. Lett. 113, 107001 (2014)) is highly cited in literature. Our group has also contributed pioneering work on magnetic properties of the transition-metals-based and rare-earths-based HEAs, where rich and complex magnetic field-temperature phase diagrams were found, including speromagnetism, asperomagnetism, a helical antiferromagnetic structure, a ferromagnetic structure, a field-induced metamagnetic structure and a spin-glass structure specific to the HEA systems.
HEAs are generally magnetically soft materials with a low coercivity. There exists a realistic possibility to develop a practically ideally magnetically soft HEA (with a vanishing coercive magnetic field) by a proper selection of the constituent chemical elements (magnetic and nonmagnetic) according to their binary mixing enthalpies, by varying the elements’ concentrations and the thermal annealing conditions. Magnetic softness originates from the phenomenon that under certain conditions, the HEA material can develop a bulk nanocomposite structure, where ferromagnetic nanodomains are diluted by nonmagnetic “nano-spacers”. In such a ferromagnetic-nonmagnetic nanocomposite, the magnetic anisotropy is averaged to zero, resulting in a zero coercivity and practically perfect magnetic softness. Materials of this kind prompt for the use in transformers, motors, generators and other electromagnetic machinery, where energy losses in alternating magnetization-demagnetization cycling must be brought to minimum. Another application is in magnetocaloric refrigeration, where hexagonal HEAs show the largest refrigerant capacity among the ternary and quaternary rare-earth alloys and other typical magnetocaloric materials. An important property of the described nanocomposite structure is also negligible magnetostriction, which is prerequisite for a “supersilent” magnetically soft material in AC applications, where e.g. the annoying humming noise of a transformer can be brought to minimum.
Our research is directed towards the experimental and theoretical studies of physical properties of HEAs composed of 3d, 4d and 5d transition elements and HEAs composed of the rare-earth elements. We study magnetic, electrical and thermal properties of these alloys as a function of concentration of the chosen chemical elements, temperature and magnetic field.
Janez Dolinšek, Head of the HEA group Ljubljana, 3rd February, 2021