Quantitative identification of bimodal L12 precipitate populations and their deformation-mediated strengthening mechanisms in a NiCoCr-based multi-principal element alloy

L1 2 precipitation-strengthened multi-principal element alloys (MPEAs) have attracted increasing attention due to their excellent combination of strength and ductility. Recently, bimodal L1 2 precipitation structures containing precipitates with different characteristic length scales have emerged as a promising microstructural strategy, owing to the distinct deformation responses of different-sized precipitates. However, quantitatively distinguishing different L1 2 precipitate populations and clarifying their individual contributions to deformation and strengthening remain challenging. Here, a Ni 47.5 Co 25.5 Cr 12 Al 5 Ti 4.5 Mo 3 W 2 Nb 0.5 (at.%) MPEA comprising a bimodal L1 2 precipitation structure, nanosized and submicron precipitates, was investigated. By combining neutron diffraction (ND) and small-angle neutron scattering (SANS), the total L1 2 phase fraction and nanosized L1 2 precipitate characteristics were quantitatively determined, enabling the quantitative determination of submicron L1 2 precipitates within the bimodal precipitation structure. The quantitative analysis reveals that nanosized L1 2 precipitates exhibit an average size of ∼24.2 nm and a volume fraction of ∼31.5 vol%, while submicron L1 2 precipitates show an average size of ∼363 nm and a volume fraction of ∼13.68 vol%. The studied alloy exhibits a favorable combination of strength and ductility with a yield strength of ∼977 MPa, an ultimate tensile strength of ∼1290 MPa and an elongation of ∼13.6%. Furthermore, TEM observations of the deformed microstructure reveal that nanosized L1 2 precipitates undergo dislocation shearing, whereas submicron L1 2 precipitates promote dislocation accumulation through bypassing and pinning interactions. The distinct deformation behaviors of different L1 2 precipitate populations contribute to the high strength of the alloy. This work provides a quantitative framework for understanding bimodal L1 2 precipitation structures and their roles in deformation and strengthening of L1 2 -strengthened MPEAs.