Flexible Production of Spherical Powders of Ti-V-Zr-Nb-Mo-Hf-Ta-W Refractory High-Entropy Alloys by Plasma-Assisted Centrifugal Atomization

Abstract

In this work, the capabilities of plasma-assisted centrifugal atomization for the fabrication of spherical powders of Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloys (R-HEAs) are thoroughly studied. Atomization in the transient regime by direct droplet formation is investigated through batch processing by simultaneous melting, alloying, and atomization from a blend of intimately mixed elemental powders pre-packed in the atomization crucible. In this processing approach, selective depletion of the heaviest elements is attributed to solid-solid centrifugal separation during the early stages of melting by plasma arc. Conversely, atomization in the steady-state regime by ligament formation is investigated through continuous processing by plasma-assisted centrifugal re-atomization of pre-alloyed feedstock fed into the rotating crucible from the top. In this case, diffusion-driven elemental redistribution in the liquid state occurs only within a thin molten metal film, leading to reduced selective depletion of the heaviest elements. The effect of the rotational speed of the atomizing unit on the particle sizes, the chemical composition, the crystal structure, and the microstructure of the atomized powders is investigated. The use of both inert (Ar) and reactive (Ar+N2) plasma carrier gases is considered for the in situ, one-step fabrication of purely metallic and metal-ceramic composite powders, respectively.