EFFECTS OF THERMAL ANNEALING AND ION IRRADIATION ON THE MICROSTRUCTURES OF HT-9 FERRITIC STEEL (PRECIPITATES, CAVITIES, TEM, DISLOCATION LOOPS, CROSS-SECTION)

Ferritic steels have now become the leading candidates for the cladding and structural materials of fast breeder reactors and the first walls and blankets in conceptual fusion reactor designs. The purpose of this thesis research is to examine the effects of high temperatures and radiation levels on the microstructural stability of the HT-9 ferritic steel (12Cr-1MoVW).; The microstructures of as-received HT-9 consisted of tempered martensite laths, carbides and a small amount of ferrite. There were four types of carbides identified in the as-received alloy; namely, equiaxed M(,23)C(,6), needle-like M(,2)X, platelet MX, and elongated M(,23)C(,6). Most of these precipitates were Cr-enriched. The dislocation density in this alloy is very high (greater than 1 x 10('11) cm('-2)).; Thermal annealing studies showed that some major microstructural changes occurred after annealing above 600(DEGREES)C which indicates that it is not practical to use this material at 600(DEGREES)C or above. The stability of carbide phases during thermal annealing in increasing in the following order: M(,2)X, elongated M(,23)C(,6), MX, and equiaxed M(,23)C(,6).; No cavity formation was observed in HT-9 following ion irradiation without helium preimplantation at temperatures of 300 to 600(DEGREES)C (0.3 to 0.5 T(,m)) to a peak damage level of 200 dpa. However, with 100 appm He preimplantation, there were cavities formed in the specimens irradiated at 500 and 600(DEGREES)C to a damage level as low as 10 dpa. This result indicates that free gas atoms are essential in cavity formation and growth in ion-irradiates HT-9. Radiation-induced phases were observed, chi phase at 500(DEGREES)C and (alpha)' phase at 400(DEGREES)C, which may cause embrittlement of this alloy during irradiation. Interstitial dislocation loops are the major microstructure in HT-9 following ion irradiation at 300 and 400(DEGREES)C which may be the major cause of the irradiation hardening.; It is concluded from this study that void swelling may not be a major concern for the use of HT-9 in future reactor systems because of the superior swelling resistance of this alloy. In contrast, the irradiation embrittlement produced by the radiation-induced second phases and the high density of small dislocation loops might be a critical factor and needs further study.