Investigations of ion irradiation induced surface damage features in zirconium alloys by atomic force microscopy

“Continuous Kr ion irradiation at 400 °C with damage level up to 40 dpa was performed in Zircaloy-4 to investigate the surface damage features of α-Zr matrix. Atomic force microscopy confirms that the irradiation induced surface damage is primarily in the form of damaged pits. The increasing Ra value indicates the gradual surface roughening process with continuous irradiation. The irradiation-induced surface roughening mechanism is suggested as three stages: stage I, the formation and growth along depth direction of damaged pits at low doses; stage II, the extension of damaged region within the surface; stage III, layer by layer removal of surface atoms once the undamaged matrix is depleted.”

The materials investigated in this study were Zircaloy-4 sheets with chemical composition Sn of 1.3, Fe of 0.2, Cr of 0.1, O of 0.1 and Zr of 98.3 wt%. Prior to ion irradiation, mechanical polishing to a mirror finish and subsequent vibratory polishing to remove the stress layer were conducted on the pre-irradiation surface of Zircaloy-4. The irradiation temperature was designed to be 400 °C. The pre-polished surfaces were irradiated from the normal direction by 4.6 MeV Kr ions to increasing doses: 1 dpa, 5 dpa, 10 dpa, 20 dpa, 40 dpa. The damage rate in our study is about 4.98 × 10−4 dpa/s.

A BRUKER Dimension ICON atomic force microscope (AFM) was used to visualize the surface morphology and measure the height data of interested features. AFM analysis was conducted in peak force tapping mode using a silicon tip on nitride lever (Bruker ScanAsyst-Air). The acquisitions were made on areas of 50 μm × 50 μm for each map with a spring constant of about 0.4 N/m and a curvature radius of 2 nm. The scanning electron microscope (SEM) used in the study is ZEISS GeminiSEM 300 equipped with an Oxford Instruments X-MaxN Energy-Dispersive Spectroscopy System (EDS).

Deep insights into the surface damage features of Zircaloy-4 with continuous irradiation are investigated by AFM, as shown in Fig. 1. The unirradiated surface is essentially featureless, though some mounts or pits with the height range below ±10 nm are still present, as shown in Fig. 1(a). On the contrary, obvious surface damage features could be observed in the continuous irradiated surface. At an irradiation dose of 1 dpa, surface damage in the form of pits was present in the original smooth matrix. The most significant change by irradiation up to 5 dpa was the increase in the amount of damaged pits with large-size. At irradiation doses exceeding 10 dpa, there was no undamaged matrix left and the entire surface showed the typical damaged features. Especially, the damaged morphology at 40 dpa was actually different. The undamaged matrix was completely depleted. In addition, obvious grain structure could be observed by irradiation up to 40 dpa. It is believed that the grain morphology at high irradiation dose was caused by the crystallographic anisotropy of thermal induced atomic diffusion [10].

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Fig. 1AFM surface micrographs with increasing irradiation doses at (a) Unirradiated, (b) 1 dpa, (c) 5 dpa, (d) 10 dpa, (e) 20 dpa, (f) 40 dpa.

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