Development of low activation oxide-dispersion strengthened ferritic steels for fusion reactors

Oxide dispersion strengthened (ODS) ferritic steels are an attractive choice as a structural material for the first wall of a fusion reactor. Ferritic steels are suitable because of their high resistance to swelling under neutron irradiation, while the austenite phase is undesirable because of excessive swelling. The dispersion of fine oxides provides excellent microstructural stability and improves creep strength of the ferritic steels under an irradiation environment inside the fusion reactor. In the present investigation, the objective was to produce a low Cr ODS ferritic steel using only low activation alloying elements which do not produce long-lived radioactive nuclide under neutron irradiation. Five ODS ferritic steels, Fe-(5, 9, 11, 13 and 13.5)Cr-2W-0.5Ti-0.25 {dollar}rm Ysb2Osb3{dollar} were produced using the mechanical alloying (MA) process. The addition of 2% W provides solid solution strengthening and increases the fracture toughness of the ferritic steel. The Ti in the ODS steel works as a scavenger of O{dollar}sb2{dollar} and N{dollar}sb2.{dollar} The milled powders were characterized by x-ray diffraction and by differential thermal analysis techniques. Fe-13.5Cr-2W-0.5Ti-0.25Y{dollar}rmsb2Osb3{dollar} showed no austenite formation at any temperature and hence this composition was selected to be studied in detail. Powders were produced in three batches using both the SPEX mill and the Szegavari attritor. The first batch (sample # 1) was produced by SPEX milling only. The second batch (sample # 2) was produced by combined SPEX milling and Szegavari attrition. The first batch showed less nitrogen than the second batch. The third batch showed the presence of undissolved W and was discarded.; Transmission electron microscopy of the as-swaged materials showed a predominantly elongated grain structure in the longitudinal section, similar to commercial mechanically alloyed and extruded products, and the presence of a high dislocation density. The hardness of the sample # 1 was 65 R{dollar}rmsb {lcub}c{rcub}{dollar} which was slightly lower (70 R{dollar}rmsb {lcub}c{rcub}){dollar} than that observed in sample # 2 and a 1 h annealing showed only a slight decrease in the hardness which indicates that the microstructure is highly thermal stable. The dislocation substructures were still observed.; A Larson-Miller plot indicates higher creep resistance for the lower nitrogen material than that observed in the higher nitrogen material. In the present alloy (both sample # 1 and sample # 2) the amount of interstitial O{dollar}sb2{dollar} and N{dollar}sb2{dollar} is much higher than other similar ODS alloys. The ODS alloy produced during the present investigation appears to be superior to MA 956 and MA 957, similar commercially available ODS alloys. The creep resistance of the present ODS ferritic steel sample # 1 is apparently the best studied to date. A lowering of the interstitial content would give an even better alloy for first wall applications in a fusion reactor. The fabrication route via hot swaging used in the present investigation was novel and attractive because it gave good results and is also simple and inexpensive. (Abstract shortened by UMI.)...