SWELLING OF ADVANCED AUSTENITIC STAINLESS-STEELS DEVELOPED FOR THE ENVIRONMENT OF HEAVY NEUTRON EXPOSURE

Modified Type-316 and higher-nickel advanced austenitic steels were ir radiated in the FFTF and JOYO fast reactors to a neutron dose as high as 210 dpa. At temperatures greater than 500 degrees C, P-, Si-, and T i-bearing cold-worked austenitic steels showed significant swelling su ppression even after 150 dpa. The dominant mechanism is associated wit h formation of stable phosphide precipitated by addition of Ti and/or increased Si. In the lower-temperature regime, where the phosphide pre cipitate is not present, silicon acts to suppress swelling at contents of about 0.8 wt%. Only at this higher silicon level, increased amount s of nickel reduced the swelling. At 0.5 wt% silicon, the swelling was insensitive to the alloying elements, such as Ni, Ti and P. The effec t of Si and Ni could be explained by their influence on void nucleatio n through effects on effective vacancy diffusion coefficient. Further improvement in swelling was expected by increasing the nickel and poss ibly the phosphorus contents of the advanced austenitics.