Effects of local elemental ordering on defect-grain boundary interactions in high-entropy alloys

Abstract

The way in which defects interact with grain boundaries (GBs) has profound influences on materials performance. In this work, we study defect-GBs interactions in a model CuNiCoFe high-entropy alloy (HEA) based on atomistic simulations. Five representative GBs are considered, namely Σ3 < 101 > {11−1} coherent twin GB, Σ5 < 103 > {010} twist GB, Σ11 < 1–10 > {113} symmetric tilt GB (STGB), Σ11 < 1–10 > {554} asymmetric tilt GB (ATGB), and Σ45 < 1–20 > {001} tilt GB. A particular focus is placed on the role of chemical disorder and local elemental segregation in influencing the defect-GBs interactions. Specifically, we compare the results obtained within an averaged atom model, the random HEA with randomly distributed elements, and the equilibrated HEA with Cu segregation after a combined Monte-Carlo/Molecular statics algorithm. For the pristine CuNiCoFe HEA without GBs, we find chemical occupancy fluctuations tend to lower the formation energies of defects, especially for interstitials because of the larger lattice distortion. For defect-GBs interactions, we find GBs strongly interact with interstitials over vacancies. We further reveal that elemental segregation can enhance the sink strength of GBs towards vacancies, but at the same time, reduce the sink strength toward interstitials. Therefore, the bias effects of GBs toward interstitials and vacancies are suppressed in HEAs due to local ordering, promoting efficient defect annihilation within the grain interiors. We highlight that the local ordering tendency and elemental segregation in HEAs play dominant roles in influencing the defect-GB interactions.