Positron Annihilation in Multiprincipal Element Alloys for the Identification of Chemical Short-Range-Order Structures

Chemical short-range order (SRO) is an intrinsic feature of the atomic structures in multiprincipal element alloys (MPEAs). Effective control of SRO structures could considerably improve the strength and ductility of MPEAs. However, the experimental characterization of SRO remains highly challenging due to their extremely high complexity and small sizes. This Letter presents that, by integrating experiments and atomistic modeling, positron annihilation spectroscopy could directly identify SRO structures in bcc NbTiZr MPEA. It was revealed that both Zr-rich and Nb-rich regions are formed in the SRO structure. The effects of lattice relaxation and electron charges lead to an increased positron annihilation contribution from Zr and a corresponding decrease from Nb. This subtle change was directly captured by integrating theoretical and experimental positron annihilation Doppler broadening spectra. The evolution of SRO was further validated by synchrotron small angle x-ray scattering. Unlike conventional localized techniques, the proposed approach provides a nondestructive, statistically averaged and element-sensitive probe, establishing a unique route to identify local chemical fluctuations in complex materials.