Size-dependent mechanical behaviors and mechanisms in CoCrFeNi microfibers

High-entropy alloys (HEAs) exhibit a wide diversity of crystalline defects for property control. Fabricating HEAs in microfiber forms further enhances property controllability due to intrinsic and extrinsic size effects. In this study, CoCrFeNi high entropy alloy microfibers with 30–100 μm diameters (D) and grain sizes (d) of 2.1–60.6 μm, were obtained through drawing, electric current annealing, and electropolishing, and subjected to uniaxial tensile testing. As D/d > 3, the yield strength obeys the Hall-Petch relation concerning d and a smaller-is-weaker effect or is insensitive to D. When D/d < 3, the yield strength deviates positively from the Hall-Petch relationship with respect to d and a smaller-is-stronger effect to D. The D/d > 3 behavior is due to grain boundary strengthening and surface-grain softening, while the D/d < 3 behavior is driven by reduced dislocation accumulation and size effects influenced by the limited number of grains spanning the diameter. These findings illustrate that in small-diameter microfibers, strengthening and weakening mechanisms intertwine to yield complex size effects, thus offering the potential to tailor the mechanical properties of micro-sized polycrystalline components through grain-size control and external-size adjustment.