A review of the primary damage induced by irradiation in ultra high purity (UHP) Fe(Cr) alloys investigated by transmission electron microscopy (TEM) is given, with a critical analysis of results with respect to the presence of the free surfaces inherent to the TEM thin foil. Indeed, free surfaces induce so-called elastic “image forces” that can bias the resulting irradiation induced damage. This was soon remarked, first by Masters in 1963 [1], in the study of the type of dislocation nanometric loops induced in pure Fe thin foils at 500-600°C. Since then many studies have confirmed that in such a thin foil the loop population is dominated by a0<100> loops, counterbalancing the ½ a0<111> ones observed in bulk irradiated Fe, even at lower temperatures. Here we review results of UHP Fe(Cr) irradiations at room temperature in situ in a TEM coupled to two ion accelerators, providing simultaneously 500 keV Fe+ and 10 keV He+ ions [2,3]. Single Fe ions and dual Fe and He ions beam experiments were performed up to a dose of 1 dpa and to a He content of up to 1000 appm. Defects appear in TEM bright field imaging in the form of nanometric black dots with sizes between 1 and 5 nm; they stem from nanometric dislocation loops. From these studies it is concluded that the primary loop population is dominated by ½ a0<111> loops, which in thin foils escape to free surfaces, contrary to a0<100>’s. This is now for the first time quantitatively explained by a proper analysis of the image forces using recent elasticity of the continuum development [4]. Figure 1 shows that the bulging of the free surface induced by a nanometric loop just beneath it is remarkable. These recent calculations of the image forces indicate that free surfaces have a strong effect on the loop population within the Fe foil, with a much larger and deeper impact within the foil on the ½ a0<111> relative to the a0<100> loops because of the strong anisotropy of Fe. This is shown in Fig. 2, comparing the stress field induced by both types of loop. Results will be presented here, with the first experimental validation of image forces using TEM on dislocations in Fe.
[1] B.C. Masters, “Dislocation loops in irradiated iron”, Nature, 200, 254 (1963).
[2] A. Prokhodtseva, B. Décamps, A. Ramar, R. Schaeublin, Acta Materialia, 61, 6958 (2013)
[3] A. Prokhodtseva, B. Décamps, A. Ramar, R. Schaeublin, Journal of Nuclear materials, 442, S786 (2013)
[4] W. Wu, J. Chen, R. Schaeublin, Journal of Applied Physics, 112, 093522 (2012)
Financed by the EC for EFDA, the Swiss National Science Foundation and PSI. Experiments were done at JANNuS, Orsay, France, and supported by the French Network EMIR and by EFDA. The authors acknowledge the outstanding work of the JANNuS team.