Precipitation Behaviour of Reduced Activation EUROFER ODS Alloy After Annealing During 3600 Hours at 700 deg C

Reduced activation ferritic-martensitic alloys (RAFM) are the most promising candidates for structural materials in the future thermonuclear reactors. They combine reduced activation with good radiation resistance. Nevertheless, their use is limited to temperatures below 500-550 deg C due to the loss in mechanical properties, especially creep. Reinforcing these alloys with a dispersion of oxides (ODS alloys) allow to increase the operation temperature to 700 deg C while maintaining good mechanical properties. The characterisation of these alloys after aging treatments is fundamental to determine their service conditions. In this work we present the results on the research carried out on two reduced activation ferritic-martensitic ODS alloys based on Eurofer 97. They were studied before and after annealing in air at 700 deg C during 3600 hours by means of TEM. Results indicate two types of precipitates M23C6 Cr-W carbides and yttrium oxide nanoparticles being the firsts of much larger size than the small Yttria particles. Before annealing, there are very few small precipitates in the martensitic structure. Chemical composition indicates that these are yttrium oxide with some Mn in their composition. After annealing, we have detected the presence of numerous precipitates and cavities. EDS analysis of their chemical composition reveals that Mn has diffused out of the precipitates. The larger Cr-W carbides are distributed in a similar way before and after the annealing experiments indicating that no precipitation is induced by the aging treatment. Nevertheless, we have detected by EDS analysis that some V is present originally in the precipitates while after annealing we saw no V in the carbides. (author)