Formation of chemical short-range orders of two kinds and the co-existence with medium-range orders in an equiatomic VFeCoNi alloy

Abstract

The chemical short-range orders (CSROs) have aroused the growing concern in the emerging multi-principal-element alloys. The CSROs as the built-in nanoscale heterogeneities cause the difference in the microstructures from conventional alloys and are expected to have an effect on mechanical properties. The formation of CSROs relates to elemental species and recent verifications of CSROs are mainly in ternary alloys. Here, the CSROs are investigated in an as-annealed, quaternary, and equiatomic VFeCoNi alloy of face-centered cubic-structured single phase before and after tensile deformation. Both the lattice structure and elemental species occupation are investigated in detail by means of electron diffraction, dark-field imaging, and atomic-resolution chemical mapping in a high-resolution transmission electron microscope. The CSROs are characterized crystallographically by two M (M: specific element)-enriched planes to sandwich one M-depleted plane. Particularly, M can be either V or Fe, instead of single elemental species reported previously. This indicates that two kinds of CSROs have been simultaneously formed in terms of the elemental species occupation, while both having the same lattice structure. Further, these two kinds of CSROs may grow up into the medium-range orders in sizes larger than 1 nm. Meanwhile, both short- and medium-range orders are mechanically stable during tensile deformation to maintain an almost unchangeable average sizes and areal fractions as well. The Moran's index analyses predict the presence of more V-enriched CSROs and CMROs than Fe-enriched ones. The present results provide the insights into understanding the production of CSROs and their growth as a result of the complex and diverse chemical species in the multi-principal-element alloys.