Mobility of point defects in CoCrFeNi-base high entropy alloys

Significance 

Unlike conventional alloys, where small amounts of one or two elements are added to the main element, high entropy alloy (HEA) have its components configured in almost equal ratios. Generally, HEAs are characterized by ductile solid structure solutions comprising of either body-centered cubic (BCC) or face-centered cubic (FCC) phases or a mixture of the two due to their random atom arrangements. This suggests that most HEAs do not form brittle intermetallic compounds. HEAs have attracted significant research attention due to their remarkable mechanical and physical properties than conventional alloys. For example, they exhibit high radiation resistance and high strength at elevated temperatures and are thus promising candidates for high-temperature fusion and fission structural applications. To this end, HEAs are also a potential candidate for nuclear energy applications. However, this requires a thorough understanding of their radiation resistance properties at elevated temperatures that currently remains a largely underexplored. Nuclear energy has been singled out as one of the most reliable and cost-effective energy sources with a very low carbon footprint.

It is speculated that the different radiation stabilities associated with HEAs are due to the effects of its high mixing entropy that influences the recombination of point defects in irradiated materials by modifying different mechanisms, including the solute diffusivity and vacancy-interstitial recombination distances. Moreover, the available austenitic stainless steels that are commonly used in light water reactors exhibit insufficient radiation damage resistance at elevated temperature applications and are not effective for next-generation nuclear energy systems. Solving this puzzle requires a thorough understanding of the mobility of point defects in HEAs at elevated temperatures.

On this account, Professor Naoyuki Hashimoto and graduate student Yuta Ono from Hokkaido University studied the mobility of point defects and microstructural changes in HEAs irradiated at elevated temperatures. Specifically, three distinct single-phase FCC-type materials were used, namely, 316 austenitic stainless steel (316SS), CoCrFeNiAl0.3-HEA and CoCrFeNiMn-HEA. The mobility of the vacancy and interstitial in both the 316SS and two CoCrFeNi-base HEAs were experimentally estimated through in-situ observation of the microstructural evolution under high voltage electron irradiation. Additionally, positron annihilation measurements were carried out to investigate the lattice defects vicinity in HEAs. The vacancy and interstitial migration energy were compared in the three alloys. The work is currently published in the journal, Intermetallics.

Results showed that the vacancy migration energy was lower in CoCrFeNiMn-HEA and higher in CoCrFeNiAl0.3-HEA compared with that of 316SS. The increase in vacancy migration energy observed in CoCrFeNiAl0.3-HEA could be attributed to the strong interaction between aluminum and vacancy. In contrast, the interstitial energy migration in the three allows appeared to be almost the same. The structure and positron annihilation analysis showed that the vacancies existed closer to Ni and Co elements in CoCrFeNi-base HEAs, and they could be captured by Mn in CoCrFeNiMn and Al in CoCrFeNiAl0.3. Furthermore, ion- and electron irradiation at 300 °C and 400 °C produced precipitated intermetallic compounds like Ni3Al in CoCrFeNiAl0.3 and NiFe and CoFe in CoCrFeNiMn.

In summary, the point defect mobility and microstructural evolution in CoCrFeNi-base HEAs and 316SS were investigated through an in-situ electron-irradiation experiment. From the results, the vacancy mobility in HEAs was affected by Mn, Al, Ni and Co atoms that played a key role in delaying the microstructural evolution and precipitation enhancement under irradiation. The study findings provided a thorough understanding of the microstructural evolution and point defect mobility in CoCrFeNi-base HEAs. In a statement to Advances in Engineering, Professor Naoyuki Hashimoto pointed out that the findings would improve the radiation resistance of high entropy alloys at elevated temperatures, making them potential candidates for nuclear energy systems.

Mobility of point defects in CoCrFeNi-base high entropy alloys - Advances in Engineering

About the author

Naoyuki HASHIMOTO, a professor in Graduate School of Engineering of Hokkaido University since 2006. He earned Dr.(Eng) in Metallurgy from Hokkaido University in 1996, and then studied the nuclear and fusion materials science at Oak Ridge National Laboratory and the University of Tennessee from 1997 to 2006.

His research has been on experimental and computational studies of the microstructural evolution of structure materials using scanning and transmission electron microscopy. Topics he has researched include the in-situ observation of microstructure and mechanical property changes in irradiated materials, such as austenitic and ferritic/martensitic steels and high entropy alloys for nuclear application.

Website: https://loam.eng.hokudai.ac.jp/en/

Reference

Hashimoto, N., & Ono, Y. (2021). Mobility of point defects in CoCrFeNi-base high entropy alloysIntermetallics, 133, 107182.

Go To Intermetallics

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