Multicomponent Si‐Rich σ Phase Induced Deterioration of Ductility and Strain Hardening in a (CoCrNi) 9 Si Medium‐Entropy Alloy

This work investigates the effect of multicomponent Si‐rich σ phase with a composition of Co 22.2 Cr 27.3 Ni 31.9 Si 18.6 on microstructural evolution, tensile properties, and fracture mechanisms of a (CoCrNi) 9 Si medium‐entropy alloy (MEA). The as‐cast MEA exhibits a dendritic microstructure containing abundant intergranular and intragranular Si‐rich σ phase (volume fraction of ~6%), yielding poor strength–ductility synergy, with an ultimate tensile strength (UTS) of ~596 MPa and a uniform elongation of ~12.7%. Solution heat treatment removes the dendritic morphology, dissolves the σ phase, and forms a homogeneous single‐phase face‐centered‐cubic (FCC) structure consisting of equiaxed grains. Such microstructural optimization triggers a brittle‐to‐ductile fracture transition, substantially increasing the uniform elongation to ~72.3% and the UTS to ~693 MPa. The excellent strain‐hardening capacity of the solution‐treated MEA mainly arises from multislip dislocation interactions and dense stacking faults, with only minor strengthening from local FCC to hexagonal close‐packed (HCP) phase transformation. The Si‐rich σ phase constitutes the major origin of ductility degradation in the as‐cast MEA, which produces interfacial stress concentration, facilitates microcrack initiation and propagation, and consequently triggers premature fracture. These findings provide valuable insights for optimizing the strength–ductility synergy of Si‐containing medium‐/high‐entropy alloys by avoiding the formation of Si‐rich segregated phases.