Formation of dislocation loops in iron by self-ion irradiations at 40K

This letter reports on the first observation of dislocation loops formed in iron by bombardment with self-ions. The irradiations were performed with 50 and 100 keV self-ions to maximum ion doses of 8 x 1013 ions cm-2 in-situ in the High-Voltage Electron Microscope (HVEM) - Tandem accelerator facility at Argonne National Labs. During the irradiation and subsequent examination, the specimen temperature was maintained at 40 K (below stage I recovery in iron) to minimize the thermal rearrangement-of the defects. The microscope was operated at 200 kV to prevent the creation of additional Frenkel pairs; the threshold voltage for Frenkel pair production in iron is about 30 kV. These observations revealed that at low temperatures (40K) dislocation loops can be formed from displacement cascades generated by self-ion irradiation, but only at ion doses exceeding those of previous studies. It is thought that these dislocation loops are formed in iron, not from the direct collapse of individual displacement cascades as has been found in several other metals after both room temperature and low temperature, < 40 K, irradiations, but from the spatial overlap of cascades, as first suggested by. The relationship between the defect density and the ion dose is not understood. A model of single overlap of cascades would require the loop density to vary as the square of the ion dose. However, the measured defect density was found to be proportional to ion dose to the 3/2 power. Further experiments are planned to investigate this relationship