Annealing Studies on Ion Tracks in Metallic Glasses

High energetic ions can lead to the formation of long columnar defects along the ion trajectories, so called “ion tracks”. In amorphous metals, overlapping tracks produce macroscopic anisotropic growth phenomena [1]. Characterization and imaging of single ion tracks in these materials is challenging and has been successful only in a few cases using chemical etching [2], scanning probe [3] and transmission electron microscopy [4], and small angle Xray scattering (SAXS) [5]. SAXS is a non destructive technique, highly sensitive to density changes at the nanometer length scale, and thus can yield important information about the damage morphology of ion tracks without modifying the studied samples. SAXS is also suitable to study ion track damage recovery by annealing experiments. Here, we investigate the ion damage recovery and the influence of the high energy irradiation on the recrystallization process with simultaneous SAXS and wide angle X-ray scattering (WAXS) measurements. The ion tracks were produced by irradiating amorphous Fe80B20 with 11.1 MeV/u Au ions at a fluence of 1×10 11 ions/cm at the UNILAC accelerator. The track structure was studied using SAXS performed at the Australian Synchrotron. Measurements as a function of angle between the X-ray beam and the track orientation show a high anisotropy of the scattering pattern. This is consistent with aligned structures of high aspect ratio and provides clear evidence for the existence of tracks. SAXS intensities of the ion tracks extracted from detector images can be well modelled assuming a cylindrically symmetrical density distribution with a constant density that is different from that of the matrix material. This model assumes continuous track damage and approximately constant energy loss throughout the sample. To study the kinetics of track annealing, we performed isochronal annealing experiments applying temperatures between 200°C and 450°C. The annealing of the irradiated samples leads to a gradual decrease of the track radii due to the relaxation of the ion track boundaries. The track radii obtained from the SAXS analysis of the irradiated Fe80B20 sample are plotted in figure 1, showing the recovery of the damage as a function of temperature. The annealing of this sample is characterized by an activation energy for the ion track damage recovery of 0.4±0.1 eV. At 360°C, after the onset of recrystallisation, we could not detect a SAXS signal from the ion tracks. The WAXS spectrum shown in figure 2 indicates that the presence of ion tracks enhances the recrystallization of the sample. Once the recrystallization process is complete, no differences were found in the WAXS spectra of the irradiated and unirradiated samples. A possible explanation for the observed differences during the recrystallization of both samples is the formation of nucleation centers during the ion irradiation process.