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 properties. This work reports an anomalous low-temperature annealing-induced hardening behavior in a prototyping equiatomic FeCoNi MEA subjected to severe cold-rolling deformation. The hardening upon annealing at 300-500 °C was confirmed by both microhardness measurements and tensile tests. Microstructural characterization via transmission Kikuchi diffraction suggests that the hardening is correlated with the annealing modified nano-subgrained structure, which reduces the distribution heterogeneity of pre-existing bundled shear bands and strain localization during the plastic deformation. Nanoindentation maps further verify that the rearrangement of nano-subgrains upon annealing relieves the severe heterogeneity of nano-hardness distribution, ensuring higher macroscopic strength and hardness. The work rationalizes the novel annealing-induced hardening phenomenon in the FeCoNi MEA, which could serve as a guideline for optimizing the thermomechanical treatment strategies of this alloy family to obtain enhanced mechanical properties.