Strategic control of precipitate architecture and ultrafine grain-boundary engineering for enhanced mechanical performance in L1₂-strengthened FCC-type multi-principal element alloys

Multi-principal element alloys (MPEAs) offer exceptional mechanical properties for structural applications, yet their microstructural complexity poses challenges in optimizing performance. This study investigates the impact of initial microstructures—homogenized equiaxed grains (EG) with dot-like L1₂ nanoprecipitates versus as-cast columnar grains (CG) with rod-like L1₂ nanoprecipitates—on the mechanical behavior of Ni₄₀Co₃₅Cr₁₅Al₅Ti₅ MPEAs under identical thermomechanical processing. The processed EG samples develop a bimodal grain structure, comprising ultrafine recrystallized and coarse unrecrystallized grains. Detailed analysis reveals that coherent L1₂ nanoprecipitates predominantly form within unrecrystallized regions, while recrystallized grains contain both continuous and discontinuous L1₂ nanoprecipitates, alongside submicron semi-coherent L1₂ particles at grain boundaries (GBs). Particularly, Lamellar L1₂ precipitates in the recrystallized-unrecrystallized transition zone initiate microcracks, compromising strength-ductility synergy. Conversely, the processed CG samples exhibit a uniform ultrafine-grained matrix with comparable L1₂ precipitation but spatially modulated distributions, enhancing plastic deformation through stacking faults, Lomer-Cottrell locks, and distorted 9R structures near annealing twins. Submicron L1₂ particles at GBs impede crack propagation, resulting in superior mechanical properties: an ultimate tensile strength of ∼1833 MPa and a total elongation of ∼14.8 %. This study reveals the strategic control of initial microstructures and thermomechanical processing to optimize grain refinement and L1₂ phase precipitation, advancing the development of high-performance structural materials.