Lattice distortion and atomic ordering of the sigma precipitates in CoCrFeNiMo high-entropy alloy

Abstract

This study aims to understand the critical lattice distortion effect of σ-phase precipitation in CoCrFeNiMo high-entropy alloys (HEAs) using various quantitative methods. The high-angle annular dark-field (HAADF) STEM image illustrates the actual atomic arrangement of materials. From the σ-phase atom image, we can distinctly observe the atomic distortion and realize that the lattice constant coefficient of variation is approximately 2.9%, which is similar to the difference in the lattice constant calculated using XRD (ε∼2.74%). The values 2.9% and ∼2.74% represent the values of lattice distortion from the microscopic and macroscopic perspectives, respectively; however, they are significantly different from the lattice distortion value (∼9.9%) obtained from atomic size difference. Therefore, it is speculated that factors such as atomic bonding and electronic structure can significantly affect the lattice distortion effects. In addition, we use EELS elemental mappings at the atomic scale for understanding compositional distributions in the σ phase. We found that 8j sites have Mo, Fe, Co, and Cr signals; however, they do not contain Ni signal. Based on these findings, it can be explained that the ordering degree of the σ phase is neither entirely ordered nor disordered. This study not only provides the most direct evidence of the critical lattice distortion effect of high-entropy alloys but also proves that the σ phase is not entirely disordered, unlike the conventional HEAs.