Effect of milling time on microstructure and properties of Ni-Free Co-28Cr-6Mo-0.425Si3N4 alloy developed by powder metallurgy

This study focuses on fabricating a nickel-free cobalt-based alloy with a composition according to ASTM F1537. The process combined mechanical alloying (MA) of elemental powders with silicon nitride (Si 3 N 4 ) addition—serving as a controlled nitrogen source—followed by consolidation via cold pressing and sintering. Milling was conducted using a planetary ball mill. XRD analysis confirmed the successful formation of a single-phase (ε-HCP) solid solution upon completion of 30 h of milling. With prolonged milling, a significant reduction in crystallite size was observed, concurrent with an increase in lattice strain, lattice parameter, and dislocation density. Between 24 and 48 h of milling, dynamic recrystallization was visible; thus, at various milling periods, a minor increase in crystallite size and a slight decrease in lattice strain and dislocation density were noted. The results indicated that Si 3 N 4 is a suitable source for the controlled supply of the required amount of N in the chemical composition through the mechanical milling method. The milled powders, corresponding to different milling durations, were consolidated under a uniaxial cold pressure of 150 MPa and subsequently sintered at 1200°C. With increasing milling time—apart from a brief decrease at 30 h—the density of the samples increased, reaching 91.44 % of theoretical density at 48 h. This milling time was thus identified as optimal. A comparison between the crystallite size of the 48 h milled powder (5.8 nm) and the resulting bulk sample (29.5 nm) revealed considerable growth, yet the microstructure remained nanocrystalline, attributed to nitrogen addition preventing rapid grain growth. • A Ni-free Co-Cr-Mo alloy was developed via powder metallurgy as a safer option for orthopedic implants. • Si 3 N 4 was introduced as a solid-state N source during mechanical alloying to achieve precise N incorporation. • A stable nanocrystalline structure (