TEM Characterization of Crept and Irradiated Nano-structured Ferritic Alloys
Microscopy and Microanalysis
The past ten years or so have seen the development of an exciting new class of mechanically alloyed (MA) nano-structured ferritic alloys (NFA) with outstanding mechanical properties that are mostly due to the presence of high concentrations (>10 23 m -3 ) of Ti-, Y-, and O-enriched nano-clusters (NC). Because NC may promote point defect recombination and trap transmutation-produced He in small clusters, NFA have the potential to be highly resistant to radiation damage in fission and fusion
... ion and fusion environments [1, 2] , and thus are being characterized following neutron and ion irradiation. Energy-filtered transmission electron microscopy (EFTEM) performed at 300 kV on a LaB 6 Philips CM30 equipped with a Gatan imaging filter (GIF) has been especially beneficial for imaging NC. In particular, Fe-M jump-ratio images produced from component images recorded with 10-eV slits at energy losses of 46 and 62 eV reliably reveal NC in dark contrast. Such images are insensitive to surface oxide films or modest surface contamination and for sufficiently thin regions (<50 nm) 2-nm diameter NC are visible  . Additional EFTEM elemental mapping (e.g. O, Ti-L 23 , Cr-L 23 ) has also been usefully applied to NFA, and focused-ion-beam (FIB) lift-out specimens have been used to good advantage  . Fabrication of an Fe-14.2wt.%Cr-1.95%W-0.22%Ti-0.25%Y 2 O 3 NFA, designated 14YWT, has been described elsewhere, as have the contributions of TEM to help optimize material processing parameters [3, 4] . It was also previously shown that NC in 14YWT are not detectably changed by tensile testing at 25 and 700°C with total strains of up to 39%  and that in MA957 (an INCO-patented Fe-14wt%Cr-1%Ti-0.3%Mo-0.27%Y 2 O 3 NFA) neutron irradiated at 500°C to 9 displacements per atom (dpa) and with ~380 appm He, the diameter (~3 nm) and concentration (~4 x 10 23 m -3 ) of the NC differ little from those of unirradiated MA957 [1,2].