Investigating the nanoscale hardness/strength properties of high-entropy alloy particles using the nanoindentation technique

Particulate feedstock constitutes the building block in modern day additive manufacturing (AM) era. Cold spray (CS) is a leading process technology to adhere to the AM principle. Therefore, meeting feedstock qualities is of utmost interest to ensure the conformability and competitiveness of the developed industrial modules, including coatings, architectured components, and additively repaired devices. This research advances the understanding of nanoscale hardness/strength properties of particulate matters, specifically of an emerging material class, - high-entropy alloys (HEAs). The feasibility of determining the hardness of mechanically alloyed AlCoCrFeNix (x = 0, 1, 2.1) HEA particles was studied employing the nanoindentation technique. Mechanical properties of milled AlCoCrFeNix particles with varying Ni atomic ratio (x = 0, 1, 2.1) were investigated over different milling times ranging between 4 to 24 hours. The study analyzed the impact of mounting resin, pre-determined maximum load, and indentation depth on hardness/strength properties. Results reveal that the hot mounted samples yielded greater accuracy and higher hardness values than compared to those of the cold mounted samples. Additionally, although the low-load sensitivity of AlCoCrFeNix provided consistent nano-scale hardness values across selected loads, their hardness values were found to be depth-dependent. Overall, the study concludes with a methodology for the nano-scale hardness/strength measurement of HEA particles that must account for particle size, sample preparation technique, and nanoindentation test parameters.