Vacancy effects on one-dimensional migration of interstitial clusters in iron under electron irradiation at low temperatures

Abstract

We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110–300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 10⁻⁴–10⁻², using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 10⁻³. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.

Keywords:

• Point defects
• irradiation effects
• dislocation loops
• defect interaction
• radiation-induced diffusion
• in situ observation
• high-voltage electron microscope
• molecular statics simulation

Acknowledgements

We are grateful to Messrs. K. Ohkubo, T. Tanioka, R. Oota and Y. Yamanouchi of the High Voltage Electron Microscopy Center at Hokkaido University for technical support in the electron irradiation experiments. A part of this work was performed at the Center for Computational Materials Science, Institute for Materials Research, Tohoku University [Proposal No. 15S0407 and 16S0402].

Funding

This work was supported in part by the ‘Advanced Characterization Nanotechnology Platform Program (MEXT)’ of the High Voltage Electron Microscope Laboratory at Hokkaido University, ‘Inter organization Atomic Energy Research Program’ of the Japan Atomic Energy Agency and Japan Society for the Promotion of Science (JSPS) KAKENHI [grant number 15K06663].