Ion Irradiation Damage of FeCoNiCr-based High-entropy Alloys at Elevated Temperatures

鐵鈷鎳鉻基高熵合金在高溫下的離子輻照損傷研究

Student thesis: Doctoral Thesis

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Award date6 Jan 2020

Abstract

The structural materials applied in the Generation IV nuclear reactors or fusion reactor are usually expected to serve at higher temperature and suffer more extensive radiation damage. Additionally, more He production from the transmutation reaction is another great issue, which will induce the He bubble formation and severely degrade the materials’ performance. Hence developing the materials which can stand such harsh environment has been a great challenge for the application of advanced nuclear energy. High-entropy alloys (HEAs), as emerging metallic materials composing with four or more principal elements in equal-molar or near equal-molar constituents, open a new way for alloy design. Recently, some of fcc-based HEAs have been demonstrated with the promising radiation damage tolerance, such as the void swelling resistance, suppressed defects evolution and great phase stability under irradiation damage. In such background, the thesis is concerned with understanding the radiation effects of FeCoNiCr-based HEA at elevated temperatures. Ion irradiations were performed using the accelerator, and the post irradiation characterization was conducted by the transmission electron microscopy (TEM). Based on the experimental strategy, the thesis could be divided into the following three parts.

The first part was designed to explore the helium bubble formation in the single phase fcc-based FeCoNiCr HEA in the temperature regime of 0.31 ~ 0.4 Tm. The single phase face centered cubic (fcc) alloy was irradiated by 275 keV He+ at 523 K, 573 K and 673 K, respectively. The results showed that the number density of helium bubbles has a temperature dependence while the bubble size shows little coarsening effect with increasing temperature. The bubble size in FeCoNiCr is smaller than that in pure Ni and steels, which are irradiated at the similar conditions. Based on the Trinkaus’s model for helium bubble evolution, the helium diffusion is identified as the self-interstitial/He replacement mechanism, and the activation energy of helium diffusion was obtained as ~1.075 eV. Furthermore, helium retention during irradiation was calculated when considering the diffusion effect and a high-pressure Equation of State (EOS) for helium bubble. We found that the fraction of helium diffused into bubbles is only about 14.3%, 31.4% and 51.4% at 523 K, 573 K and 673 K in the FeCoNiCr HEA.

In the second part, the helium bubble evolution in FeCoNiCr HEA was further examined in the half-melting temperature regime of 0.46 ~ 0.57 Tm. The HEA was irradiated in parallel with a model fcc metallic system of pure Ni by 2 MeV He ions at 773 K, 873 K and 973 K. The results show that in any designated temperature, He bubbles have a smaller size, higher number density and denser distribution in the HEA sample, comparing to that of pure Ni. The volume fraction of He bubbles is also less in the HEA, indicating a suppressed bubble evolution. For the underlying mechanism, we suggest that the featured energy barriers for point defects migration in the HEA will promote the recombination of defects and somewhat reduce the vacancy concentration during irradiation. So that the He diffusion was restrained where the vacancy mechanism was operated in the half-melting temperature regime, the effect would finally suppress the He bubble evolution in the HEA. Furthermore, the evaluation on the He retention in bubbles indicated that all the implanted He atoms could be trapped by the vacancies during irradiation at or above 0.57 Tm, where the interactions of He and vacancy become significant.

Thirdly, two types of HEAs that FeCoNiCr and γ' strengthened FeCoNiCrTi0.2 were irradiated by 6.4 MeV Fe3+ to a peak damage level of ~ 13 dpa at 673 K, 773 K, and 873 K. In addition, one set of high fluence experiment was conducted by 3 MeV Ni12+ at 873 K. The results showed that the ordering L12 structure of γ′ precipitate was susceptible to the radiation damage at a lower temperature, but their basic compositional features could sustain up to ~ 13 dpa at the above temperatures. When the radiation damage is up to ~ 70 dpa, all the γ′ precipitates will be dissolved. For the evolution of radiation defects, the faulted dislocation loops, perfect loops and dislocations exhibited a normal evolution pathway in FeCoNiCr HEA along temperature effects, while the conventional void structures were observed at 873 K and their volume fraction increased from 0.45% of ~ 5 dpa to 3.09 % of ~70 dpa. In the irradiated FeCoNiCrTi0.2 HEA, the planar defects including faulted dislocation loops and stacking faults dominated the microstructural evolution. At the peak swelling temperature of 873 K, only some 2-D vacancy discs with an extremely low volume fraction of < 0.3% were detected at the damage level of ~70 dpa. The underlying reason for superior void swelling resistance of FeCoNiCrTi0.2 HEA is attributed to the lower SFE induced by Ti addition, that could stabilize the faulted planar defects and alter the development of biased sinks of dislocations. Such effect will finally affect the vacancy supersaturation for void formation.

Overall, this thesis not only systematically reported the behaviors of radiation defects in the FeCoNiCr-based HEA which were irradiated at a wide temperature regime, but also elucidated the behind mechanism of defects evolution. I believe that our findings could provide some insights for the future developing of nuclear structural materials.

    Research areas

  • High-entropy alloy, Radiation damage, Helium bubble, Dislocation loop, Precipitates