Microstructure and properties of high-entropy-superalloy microlattices fabricated by direct ink writing

Ni-Co-Fe-based high-entropy superalloys (HESA) are fabricated into microlattices via a three-step process: (i) layer-by-layer extrusion of inks containing elemental powders (Ni, Co, Fe, Cr, Ti) and TiAl3 powders; (ii) sintering to densify and homogenize the struts; (iii) aging to achieve a γ/γ' microstructure. The struts of the microlattices show a nearly pore-free and fully-homogenized microstructure. Increasing the Ti concentration from 4 at% (Al9Co26Cr7Fe16Ni38Ti4) to 9 at% (Al8Co25Cr7Fe15Ni36Ti9) leads to a significant increase in the volume fraction of strengthening γ' precipitates, from 51 to 78%. Furthermore, in the Ti-rich composition, the γ' precipitates exhibit a sharp-edged cubic morphology with larger sizes and higher lattice misfit (0.63%) with respect to the γ matrix. As a result, Ti-rich HESA microlattices show higher strength at ambient temperature than Ti-poor ones, while retaining high compressive ductility (> 60%). They also demonstrate superior specific strengths when compared to bulk Inconel 617 and other representative bulk HESA, up to 1000 °C. The combination of low-density (2.29–3.15 g/cm3), high strength at elevated temperatures, and high processability positions HESA microlattices from direct ink writing (DIW) as promising candidates for structural components in extreme operating conditions. The versatility of the process is demonstrated by printing and sintering three miniature HESA objects with complex shapes (hollow turbine blade, gyroid heat exchanger, and compressor wheel).