TEM Characterization of Crept and Irradiated Nano-structured Ferritic Alloys

Abstract

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 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 [3]. 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 [2]. 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% [5] 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].The NC in MA957 also appear to be unchanged after creep testing in air for a remarkable 38,555 h at 800°C and 100 MPa (Fig. 1). Cavities of >1 m diameter present in the crept specimen may have formed from the release of gases entrained during the original material processing (a potential problem for all MA materials) and may have initiated specimen rupture after a uniform strain of <0.4%. In TEM specimens prepared by electropolishing these cavities caused premature perforation with little or no thin regions, so it was necessary to use FIB lift-out specimens. A Japanese 9Cr oxide-dispersion-strengthened (ODS) steel .%Cr-1.95%W-0.23%Ti-0.27%Y-0.14%O) was irradiated at 525°C with 2.6 MeV protons at ~5 x 10 -6 dpa/s to 1 dpa at the University of Wisconsin [6]. EFTEM results showed that the mean diameter (~3.2 nm) and concentration (~1 x 10 23 m -3) of NC were approximately the same in irradiated and unirradiated material. However, the NC were heterogeneously distributed with factors of >2.5 (unirradiated) and >5 (irradiated) variation in the mean concentration over 6 mapped regions in each sample and with a ~2-fold variation in mean dia...

Effects of single- and simultaneous triple-ion-beam irradiation on an oxide dispersion-strengthened Fe12Cr steel

“…These methods minimize diffraction contrast and in combination provide the information needed to investigate the chemical evolution of the particles after irradiation. Only very few reports have been found that apply these advanced techniques to accomplish such analyses [13,28,44]. Some authors use Fe EFTEM maps to visualize the nanoparticles and APT to study their chemical evolution because attaining reliable Y EFTEM maps is a challenging task due to the locations of its three electron energy loss edges [13,44].…”

A review of advantages of high-efficiency X-ray spectrum imaging for analysis of nanostructured ferritic alloys

“…These NFA materials, such as the 14YWT alloy studied here, have a complex non-equilibrium microstructure produced by mechanical alloying and extrusion. Previous work [9][10][11][12][13] has indicated the microstructure consists of a ferritic matrix with a grain size of $100-500 nm, with strong enrichment of Cr and W solutes at the grain boundaries over a width of $1.5 nm [14]. Titanium oxycarbonitride (Ti(O,C,N)) precipitates of $20-75 nm size are also found in the microstructure.…”