High-entropy alloys (HEAs) represent a novel class of metallic materials. They are composed of at least five elements in equiatomic or near-equiatomic proportion, and show crystalline long-range order while maintaining chemical disorder throughout their lattice. The simultaneous presence of topological order and chemical disorder is unique in metals and poses fundamental questions in materials science. Numerous alloy systems have been reported to form HEAs, crystallising in face-centered cubic, body-centered cubic and hexagonal structure.
Investigating the salient structure-property relationships of HEAs requires high-quality single-phase, preferably single-crystalline samples. Single-crystalline samples allow the determination of intrinsic materials properties without the influence of secondary phases or grain boundaries. We report on our advancements in the production of single-crystalline equiatomic fcc CrMnFeCoNi and bcc TiVZrNbHf HEAs. The techniques employed include master-alloy production from high-purity elements by arc-melting and in an inductively-coupled levitation crucible, and single-crystal growth by the Bridgman, Czochralski and zone-melting technique.
CrMnFeCoNi single-crystals produced by the Bridgman technique have a volume of several cubic centimeters and a maximum diameter of 20 mm, making the high-quality material accessible to most physical-property measurements. X-ray Laue back-scattering images taken across the surface are sharp and mutually consistent. SEM imaging additionally reveals a grain-free microstructure, with EDS measurements confirming the equiatomic composition of the crystal. Specimens prepared from the large single crystals are currently used to perform mechanical compression testing along different crystallographic directions.
A growth route for equiatomic TiVZrNbHf single crystals is currently under development and will be presented additionally. Our preliminary experiments reveal that this material is a homogeneous single-phase HEA with bcc structure. Single-crystal growth of TiVZrNbHf HEA will be addressed by means of the Czochralski and the zone-melting technique.