Direct Observation of Chemical Short-Range Order in CoCrNi Alloy Using Neutron Diffraction

Abstract: This study provides experimental evidence of chemical short-range order (CSRO) in the equiatomic CoCrNi alloy, identified through neutron diffraction. The phenomenon manifests as a distinct diffuse peak at Q = 1.85 A-1, the intensity increases under thermodynamically favorable conditions for CSRO development such as prolonged aging (100 h and 240 h) at 748 K or shorter aging (24 h) at slightly higher temperature (798 K). The degree of ordering was measured by integrating the diffuse scattering intensity, revealing that the gas-atomized sample, i.e. the sample with the least amount of CSRO, still displays approximately 70% of the CSRO level observed in the sample subsequently aged for 240 h at 748 K, i.e. the sample with the highest amount of CSRO produced in this study. Predictive atomistic simulations reproduced both the presence and position of the diffuse peak, while two-dimensional fast Fourier transform (FT-2D) analyses indicated that reflections at (1 1/2 0) within the <001> zone axis originate from some structural projections associated with like D022, Pt2Mo and D1a motifs. Complementary small-angle neutron scattering (SANS) measurements identified Ni-rich, disk-shaped domains with radii of approximately 11 A and thicknesses of about 1 A, consistent with nanoscale CSRO characteristic length scale. These findings demonstrate that CSRO is an intrinsic and energetically favorable feature of the CoCrNi system, remaining stable even under rapid solidification and further enhanced by low-temperature aging. Combined use of neutron diffraction and atomistic modeling provides a framework for probing local ordering phenomena in multi-principal element alloys (MPEAs).