| Summary: | As leading candidates of sheet steels for advanced nuclear reactors, three types of Ni⁻Mo⁻Cr high-strength low alloy (HSLA) steels named as CNST1, CNST2 and CNSS3 were irradiated by 400 keV Fe<sup>+</sup> with peak fluence to 1.4 × 10<sup>14</sup>, 3.5 × 10<sup>14</sup> and 7.0 × 10<sup>14</sup> ions/cm<sup>2</sup>, respectively. The distribution and morphology of the defects induced by the sample preparation method and Fe<sup>+</sup> irradiation dose were investigated by transmission electron microscopy (TEM) and positron-annihilation spectroscopy (PAS). TEM samples were prepared with two methods, i.e., a focused ion beam (FIB) technique and the electroplating and twin-jet electropolishing (ETE) method. Point defects and dislocation loops were observed in CNST1, CNST2 and CNSS3 samples prepared via FIB. On the other hand, samples prepared via the ETE method revealed that a smaller number of defects was observed in CNST1, CNST2 and almost no defects were observed in CNST3. It is indicated that artifact defects could be introduced by FIB preparation. The PAS <i>S</i>-<i>W</i> plots showed that the existence of two types of defects after ion implantation included small-scale defects such as vacancies, vacancy clusters, dislocation loops and large-sized defects. The <i>S</i> parameter of irradiated steels showed a clear saturation in PAS response with increasing Fe<sup>+</sup> dose. At the same irradiation dose, higher values of the S-parameter were achieved in CNST1 and CNST2 samples when compared to that in CNSS3 samples. The mechanism and evolution behavior of irradiation-induced defects were analyzed and discussed.
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