MICROSTRUCTURAL RESPONSE OF TITANIUM-MODIFIED AUSTENITIC STAINLESS-STEELS TO NEUTRON EXPOSURE OF 70-DPA IN FFTF MOTA/

JPCA, a titanium-modified austenitic stainless steel, in solution-anne aled or cold-worked condition and a compositionally modified JPCA in s olution-annealed condition were examined by transmission electron micr oscopy following irradiation in FFTF/MOTA to an exposure level of up t o about 70 dpa at 390 to 600 degrees C. At lower temperatures, ail the materials developed qualitatively similar cavity-, dislocation- and p recipitate-microstructures. The lower-temperature swelling peak, which appeared at near 410 degrees C, was more efficiently suppressed by ph osphorus addition than cold-working. Irradiation at or above 520 degre es C produced substantially large swelling in solution-annealed JPCA. The cavities contributed to this higher-temperature swelling developed in association with M(6)C-type precipitates. Neither cavities other t han very small helium bubbles nor massive particles of M(6)C-type prec ipitates were observed in cold-worked and phosphorus-modified material s, in which MC-type precipitates developed at very high concentration. The effect of pre-irradiation microstructure and compositional modifi cation on the behavior of these precipitates is discussed.