Engineering harmonic structure in CrFeNi medium-entropy alloy via milling parameter optimization for enhanced mechanical performance

The CrFeNi medium-entropy alloy (MEA) is a promising candidate for advanced engineering applications. However, its insufficient strength limits its use in structural components. To improve the strength-ductility synergy, a harmonic structure (HS) was engineered in this alloy, featuring coarse-grained “core” regions surrounded by a network of fine-grained “shell” zones, through controlled mechanical milling followed by sintering. By varying two milling parameters — filling ratio (FR) and milling time — the HS architecture and mechanical properties of alloy were precisely tuned. The results show that stepwise reductions in FR from 0.350 to 0.117 and then to 0.058 progressively modify the deformation behavior of milled powders, from incomplete peripheral deformation to substantial deformation including distortion, and finally to moderate deformation. These changes correlate with shifts in the ball motion pattern from centrifugation/rolling through cataracting to cascading. The microstructural characteristics, specifically the “shell” fraction ( f shell ), and the alloy strength exhibit a rise-and-fall trend with decreasing FR. The optimal FR value of 0.058 promotes a favorable HS configuration with moderate f shell and a continuous “shell”, thereby enabling the alloy to achieve a superior strength-ductility balance. Prolonged milling from 50 h to 200 h thickens surface deformation layers and promotes agglomeration in the milled powders, significantly increasing f shell and undesirable lamellar structures in HS. This evolution enhances alloy strength mainly through hetero-deformation induced (HDI) strengthening, but gradually reduces ductility, as the increased HDI hardening resulting from elevated f shell fails to offset the diminished forest hardening due to grain refinement. This study identifies the optimal milling parameters for producing a strong and ductile CrFeNi alloy. • Harmonic structured (HS) CrFeNi alloy was prepared by milling and sintering. • Milling parameters, filling ratio (FR) and milling time, were tuned and optimized. • FR affects the ball motion pattern and the deformation behavior of milled powders. • Prolong milling increases the “shell” fraction but causes “core” distortion in HS. • The efficacy of hetero-deformation induced strengthening dominates alloy strength.