Radiation damage in helium ion irradiated nanocrystalline Fe

K.Y. Yu, Y. Liu, C. Sun, H. Wang, L. Shao, E.G. Fu, X. Zhang, Journal of Nuclear Materials, 425 (2012) 140-146.

Abstract

Fe films with an average columnar grain size varying from 49 to 96 nm are deposited by magnetron sputtering technique. Sputtered films have predominant body centered cubic structure together with a small fraction of face centered cubic phase. Helium bubbles are observed in films irradiated by 100 keV helium ions to a fluence of 6 × 10²⁰ ions/m². Smaller grains lead to lower density and smaller diameter of He bubbles. Radiation hardening is a combined consequence of He bubble induced hardening and radiation induced compressive stress.

Brief summary

Fig. 1.(a) Lower magnification XTEM image of irradiated 49nm Fe film taken at an under focus of -400nm shows the generation of He bubblesboth inside grains and along grain boundaries. (b) Higher magnification XTEM micrograph shows the detail of a He bubble array at a grain boundary. (c) XTEM image of irradiated bulk Fe.

Fig. 2. Comparison of radiation depth dependent He bubble density (scattered data point) distributions in irradiated 49 nm, 96 nm Fe films and bulk Fe. Bubble density in 49nm Fe is slightly lower than that in 96 nm Fe film while bulk Fe shows significantly higher bubble density than films. Also shown is SRIM simulation (solid line) of depth profile of He concentration. The peak bubble density and He concentration occur at a similar depth.

Conclusions

The average grain sizes in sputtered Fe films have been varied from 49 to 96 nm. In irradiated films, both bubble size and density decrease with decreasing grain size. Grain and phase boundaries are preferential sites for accumulation of He bubbles. Radiation hardening is the result of pressurized He bubbles as well as radiation induced compressive stress.