Hardness is defined as the resistance of a material to localized plastic deformation. Owing to their non-destructive nature, static indentation hardness tests are widely used in industry. Hardness testing is particularly useful for the mechanical characterization of materials that cannot be tested otherwise, e.g. powdered materials. In this study, challenges related to Vickers microindentation hardness testing of hard brittle cast tungsten carbide (CTC) powders were extensively investigated. Test load was optimized to obtain sufficiently large crack-free indentations allowing for precise measurement of the diagonal lengths. The influence of the operator and imaging technique on the measured hardness value was evaluated. Topography of residual imprints was investigated using atomic force microscopy (AFM) and a systematic and operator bias-free method to locate the indentation vertexes was developed. Results suggested that measurement variability introduced by AFM scanning and post-processing was as low as 3.1% and 1.3% with respect to the mean hardness value, respectively. Since the variability due to the measuring system can be isolated, the homogeneity of powders can be reliably evaluated from the hardness measurements thus obtained.

Cast tungsten carbide (CTC) is a biphasic, pearlitic-like structure composed of WC lamellae in a matrix of W₂C. Besides excellent flowability, spherical CTC powders exhibit superior hardness and wear resistance. Nevertheless, the available literature generally fails to explain the physical mechanisms behind such a phenomenon. In the present work, the microstructure and the mechanical properties of the novel centrifugally-atomized spherical CTC have been extensively investigated. This material exhibited an extremely fine microstructure, with WC lamellae of 27-29 nm in thickness and bulk lattice strains of 1.0-1.4 %, resulting in a microindentation hardness of 31.4 ± 1.6 GPa. The results of this study clearly show that centrifugally-atomized CTC is mechanically superior to both spheroidized CTC and conventional cast-and-crushed CTC. In addition, the effect of a series of heat treatments on the bulk fracture toughness and the fatigue life of entire CTC particles was also investigated. The reduction of residual stresses in the bulk of particles upon annealing dramatically increased the indentation fracture toughness, whereas the bulk microindentation hardness remained essentially unaffected. Regarding the fatigue life of entire particles under uniaxial cyclic compressive loading, local phase transformation phenomena at the surface of the particles upon heat treatment were concluded to play the most critical role. Indeed, the cumulative fatigue damage was minimized in surface-carburized CTC powders, where compressive stresses were induced at the outermost surface.

In this study, plasma-assisted centrifugal atomization was used to produce spherical powders of a refractory high-entropy alloy (R-HEA) with particle sizes within 50–250 µm, a median particle size by weight of 163 µm, and average chemical composition of Ti12V15Zr5Nb15Mo17Hf5Ta16W15. The powdered feedstock material for the atomization process was produced by a combination of partial solid-state consolidation of elemental powders by spark plasma sintering and cryogenic grinding, thereby eliminating the need for complex hydrogenation-dehydrogenation pulverization operations. The resulting spherical powder with particle sizes within 150–180 µm consisted of a main BCC-structured solid solution incorporating all principal elements, and trace amounts of FCC-structured HfxZr1-xN precipitates. The microindentation hardness of the centrifugally atomized spherical powder was as high as 7.0 ± 0.3 GPa, approximately 30 % higher than that of previously reported R-HEAs with similar compositions and larger atomic size mismatch. The enhanced mechanical performance was attributed to the refined dendritic microstructure, with primary dendrite arms and interdendritic spacing of around 2 µm and 0.3 µm in thickness, respectively. These results suggested that high configurational entropy alone was not the cause for the superior mechanical properties of HEAs, and demonstrated that the grain boundary strengthening through microstructure refinement was a powerful tool to achieve outstandingly high hardness for such single-phased metallic alloys. The reported processing route showed promising potential for the pilot- and large-scale fabrication of spherical powders of R-HEAs for additive manufacturing purposes such as electron beam melting (EBM), laser metal deposition (LMD), and laser cladding (LC). 

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.