Real and Reciprocal Space Imaging of Radiation-Induced Defects in BCC Fe
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REAL AND RECIPROCAL SPACE IMAGING OF RADIATION-INDUCED DEFECTS IN BCC Fe R.E. Stoller, G.E. Ice, R.I. Barabash Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge TN 37831-6118 Abstract It is important to determine the number and configuration of point defects that survive atomic displacement cascades in irradiated materials since only those defects that escape recombination are able to contribute to radiation induced property changes such as void swelling, hardening, embrittlement, and irradiation creep. Simulations of displacement cascades using molecular dynamics are being used to provide a description of the most probable surviving defects. In the case of interstitial-type defects in iron, these are predicted to be and type dumbbells and crowdions, along with small clusters of these same defects. Diffuse x-ray and neutron scattering provides a direct method for obtaining detailed information on the displacement fields both near to and far away from the defects in addition to information on the particular position of the defect. To analyze such radiation damage with diffuse scattering we have modeled reciprocal space diffraction from crystals with variously oriented dumbbells and small clusters. Diffuse scattering measurements between and close to Bragg reflections are sensitive to the orientation of the dumbbells, and the size and type of the small clusters at sizes that are too small to analyze by electron microscopy. Displacement of the near neighbors induces diffuse scattering in regions between the Bragg peaks, whereas the long range part of the displacement field results in Huang scattering close to the Bragg reflections. Simulation of diffuse scattering by different interstitial defects around (h00), (hh0), (hhh), (3h,h, 0) reflections demonstrates that each defect type leads to distinct isointensity contours. These scattering signatures can be used to determine the type and configuration of the surviving interstitials. Introduction In recent years, molecular dynamics (MD) simulations of high energy displacement cascades have provided a detailed picture of primary damage formation in irradiated materials [1-7]. An unanticipated observation in these studies was extensive formation of small interstitial clusters and the very high mobility exhibited by these clusters. In the case of iron, two primary stable interstitial configurations are found, the and dumbbells [3,4]. The difference in the formation energy between these configurations is only about 0.1 eV. The dumbbell migrates with a low activation energy of ~0.1 eV, while the dumbbell is essentially immobile near room temperature. The dumbbell migrates by first rotating into a . The interstitial clusters in iron are primarily clusters of dumbbells, and they migrate with a low activation energy similar to that of the single dumbbell, ~0.1 eV. These interstitial properties are dependent on the interatomic potential employed, but similar results have been obtained with several different potentials. However, the iron potentials cu
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