Effects of Heavy-Ion Irradiation on HT-9 Ferritic Steel Examined in Cross Section

Specimens of HT-9 have been irradiated with 14 MeV Ni+³ ions in the temperature range from 300 to 600°C and up to the peak damage level of 200 dpa. The specimens were prepared for transmission electron microscopy (TEM) studies using the cross-section technique that allows the whole damage region to be examined in a single foil. The most significant result of this study is that without prior implantation of helium gas atoms virtually no cavities were found in any of the irradiated specimens. Other microstructural features resulting from the irradiation were also examined in detail, including dislocation loops and precipitates. After irradiation at 300°C, the microstructure consisted of a very high number density of small dislocation loops. The average diameter of these loops is 8 nm and is relatively independent of the dose level. The number density of the loops saturates at higher dose levels. The microstructure after irradiation at 400°C is very similar to the 300°C case except that the average diameter of the dislocation loops is about 24 nm. However, a new phase forms at the end of the damage range during the irradiation at 400°C. The particle diameter of this new phase is about 5 nm, and it has been provisionally identified as a' phase, which is a chromium enriched bcc phase. It forms largely in association with the dislocation loops. After irradiation at 500°C, the microstructure consisted of a few large dislocation loops, dislocation segments, and a group of new precipitates. The dislocation loops have the same characteristics as in the 300 and 400°C cases. The size of the loops is independent of dose level and about 100 nm in diameter, and the number density of the loops saturates at higher dose levels. The new phase has been identified as the Chi phase, which is an intermetallic phase with a bcc structure. The precipitates of Chi phase have the tendency to grow to larger diameter and the number density decreases with increasing dose levels. There is virtually no difference between the microstructure in irradiated and control regions after irradiation at 600°C. This result indicates that thermal effects dominate the microstructural evolution at this temperature and above in ion irradiated HT-9. © 1987 ASTM International. All rights reserved.