He-enhanced heterogeneity of radiation-induced segregation in FeNiCoCr high-entropy alloy

Radiation-induced segregation (RIS) is a typical non-equilibrium process that can dramatically alter the behavior of defect sinks and material properties under irradiation. However, RIS mechanisms have been rarely studied around small He bubbles owing to the technical challenges involved in direct measurements of local chemistry. Here, using state-of-the-art atom probe tomography, we report the RIS behavior near He bubbles in the FeNiCoCr high-entropy alloy that indicates Co segregates most strongly, followed by weaker Ni segregation, whereas Fe and Cr are depleted almost to the same degree. Exceptionally, the magnitude of Co segregation around He bubbles is higher than previously measured values at voids and dislocation loops. Electron energy-loss spectroscopy was used to measure the He density and pressure inside individual bubbles. We demonstrate that He bubbles are over-pressurized at the irradiation temperature that could result in the vacancy bias and the subsequent vacancy-dominated RIS mechanism. First-principles calculations further reveal that there are repulsive interactions between He and Co atoms that may reduce the frequency of Co-vacancy exchange. As a result, He atoms likely retard Co diffusion via the vacancy mechanism and enhance the heterogeneity of RIS in Co-containing multicomponent alloys. These insights could provide the basis for understanding He effects in nuclear materials and open an avenue for tailoring the local chemical order of medium-and high-entropy alloys. (c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

This study investigates the radiation-induced segregation behavior near small helium bubbles using atom probe tomography. It is found that cobalt (Co) exhibits the strongest segregation, followed by weaker segregation of nickel (Ni), while iron (Fe) and chromium (Cr) are depleted in a similar degree. Over-pressurization of helium bubbles at the irradiation temperature is demonstrated, which could result in a vacancy-dominated segregation mechanism.