Radiation damage during HRTEM studies in pure Al and Al alloys

During transmission electron microscopy (TEM) investigations of Al alloys, defects caused by the electron irradiation can occur. Since the image contrast of these irradiation defects is similar to that of early stages of precipitates, care is needed to avoid confusion. In the present paper the formation and the coarsening of radiation damage defects were studied by in situ TEM in both, pure Al and Al alloys. High-resolution (HR)TEM images show on an atomic level that the radiation defects are extrinsic Frank loops (some converted into unfaulted prismatic loops); they are distributed homogeneously within the TEM foil but inhomogeneously on the four {111} planes. Applying HRTEM imaging conditions, the minimum electron energy causing defects is found to be as low as 110 keV using a [110] beam direction. The results for pure Al are very similar to those of the Al alloys. Therefore, during HRTEM studies (using accelerating voltages > 100 W) the formation of radiation defects seems inevitable; they can be distinguished from the early stages of precipitates if they lie on different planes.