Elucidating the Origination of Annealing‐Induced Hardening in an Equiatomic Medium‐Entropy Alloy

FeCoNi‐based medium‐entropy alloys (MEAs) have attracted lots of attention owing to their potential for achieving excellent mechanical and soft magnetic properties. Herein, an anomalous low‐temperature annealing‐induced hardening behavior in a prototyping equiatomic FeCoNi MEA subjected to severe cold‐rolling deformation is reported. The hardening upon annealing at 300–500 °C is confirmed by both microhardness measurements and tensile tests. Microstructural characterization via transmission Kikuchi diffraction suggests that hardening is correlated with the annealing‐modified nanosubgrained structure. More specifically, the distribution heterogeneities of shear bands and nanosubgrains are reduced upon annealing, which can relieve the strain localization during plastic yielding. Nanoindentation maps further verify that the rearrangement of nanosubgrains upon annealing relieves the severe heterogeneity of nanohardness distribution, ensuring higher macroscopic strength and hardness. Herein, the novel annealing‐induced hardening phenomenon in FeCoNi MEA is rationalized, which can serve as a guideline for optimizing the thermomechanical treatment strategies of this alloy family to obtain enhanced mechanical properties.