Non-monotonic regulation of the intrinsic mechanical properties of electrodeposited FeNiCo alloys by Co content

Electrodeposited FeNiCo films (0–51.61 at.% Co) were studied by in-situ micropillar compression, first-principles SQS calculations and nanoindentation finite-element modelling. Yield strength and strain-hardening capacity vary non-monotonically with Co content but peak at different compositions: yield strength at (FeNi) 84 Co 16 , strain hardening and high-strain flow stress at (FeNi) 66 Co 34 , so the stress–strain curves cross. A Hall–Petch decomposition using TEM and X-ray grain sizes shows that grain refinement accounts for most of the initial rise in yield strength but only about 38% of its subsequent fall; the grain-size-corrected strength still peaks at (FeNi) 84 Co 16 and follows the calculated shear modulus. The calculated bulk modulus rises monotonically, whereas the shear and Young’s moduli peak at (FeNi) 84 Co 16 ( G = 100.95 GPa, +16% over the Co-free alloy). Density-of-states and Bader analyses associate this maximum with balanced charge partitioning among Fe, Co and Ni; a charge-dispersion index is minimal at (FeNi) 84 Co 16 and varies together with the shear modulus across the seven models ( r = −0.91, suggestive rather than conclusive). A finite-element model whose only composition-dependent inputs are the micropillar yield strength and hardening exponent reproduces the measured rise-then-fall of hardness ( r = 0.99). These results give a plausible electronic-structure basis for FeNiCo compositional design.