Evolution of the EUROFER97 microstructure during thermal treatment up to 122,000 h

Detailed knowledge of the microstructural evolution of reduce activation ferritic-martensitic steel EUROFER97 after exposure at high temperatures is essential for determining its applications potential. For this proposal, EUROFER97 was annealed in the temperature range between 450 °C and 650 °C for up to 122,000 h (≈14 years) and subsequently analyzed using transmission electron microscopy (TEM) including high resolution TEM and two-dimensional energy dispersive X-ray (EDX) mapping. The study demonstrates the effects of thermal treatment on the size and composition of the precipitates and allows conclusions about their stability. Application of the extraction replication technique was used to analyze composition and morphology of four particle types present in the untreated EUROFER97: M23C6, VN, TaC and TiN with sufficient statistics. The rapid coarsening of the M23C6 precipitates was observed at 650 °C, while the MX particles were found to be more stable upon thermal treatment. It has been proved that new Laves (WFe2) and modified Z-phases (Cr(V,Ta)N) precipitates are formed in the temperature range from 500 °C to 600 °C. The detailed analysis allows the drawing a time–temperature formation diagram for these two phases, which could be valid for alloys with composition similar to EUROFER97

Transmission electron microscopy study of platinum clusters on Al₂O₃/NiAl(110) under the influence of electron irradiation

Using transmission electron microscopy we have studied the influence of the electron beam in an electron microscope onto platinum clusters deposited on a thin single crystalline γ-Al2O3 film grown by oxidation of NiAl(110). At electron current densities below j≈1 A/cm2 no influence is observed. Movement and coalescence of clusters occur at electron beam current densities between j=2 and some 10 A/cm2. For current densities around j=50 A/cm2 decoration of steps takes place. Further increase to j=100  A/cm2 and above induces drilling of holes into the substrate by clusters. At such current densities also melting of the clusters may occur. Due to the heat capacity of the system the result does not only depend on the electron current density but also on the irradiation time

Small-angle neutron scattering (SANS) characterization of 13.5 Cr oxide dispersion strengthened ferritic steel for fusion applications

Small-angle neutron scattering (SANS) has been utilized for micro-structural investigation on laboratory heats of oxide dispersion strengthened (ODS) 13.5 Cr wt % ferritic steel, with 0.3 wt% Y$_{2}$O$_{3}$ and with variable Ti and W contents. The results show that increasing the Ti content from 0.2 to 0.4 wt% a distribution of nano-clusters develops, tentatively identified as Y$_{2}$Ti$_{2}$O$_{7}$, with average radii as small as 6.5 Å and volume fractions increasing from 0.021 to 0.032. The measured SANS cross-sections show also the growth of much larger defects, possibly Cr oxides. Furthermore, the ratio of magnetic to nuclear SANS components shows that the defect composition varies both with their size and with the Ti and the W content. These results are in qualitative agreement with transmission electron microscopy (TEM) observations, showing a striking influence of Ti addition on particle size refinement. However, while TEM is limited in statistics and minimum observable size of the Ti-rich nano-clusters, the defect distributions obtained by these SANS measurements provide complementary information useful for morphological characterization of the micro-structure in the investigated material

Micro-structural effects of irradiation temperature and helium content in neutron irradiated B-alloyed Eurofer97-1 steel

The micro-structural effects of different neutron irradiation temperatures and helium contents, for 16 dpa dose, have been investigated by means of small-angle neutron scattering (SANS) in B-alloyed ferritic/martensitic steel Eurofer97-1 (0.12 C, 9 Cr, 0.2 V, 1.08 W wt%, B concentrations up to 1000 ppm); due to B transmutations, fusion relevant He/dpa values are expected to be produced under neutron irradiation. SANS measurements have been carried out on a sample irradiated at 350 °C, with estimated helium content of 5600 appm, and compared to previous SANS results, obtained on two other irradiated samples of this same B-alloyed steel. These new measurements confirm that for such high helium contents the SANS cross-section increases in order of magnitude and the magnetic SANS component is strongly reduced, compared to lower helium content (400 appm). Such effects are attributed to increase in helium bubbles density and to the presence of micro-cavities, produced after dissolution of large B-carbides. The SANS data analysis procedure has been improved, also thanks to the additional information provided by the new measurements, and more accurate helium bubble size distributions have been obtained for all the investigated samples. For 5600 appm helium content, bubble volume fractions are found of 0.025 for the sample irradiated at 350 °C and of 0.041 for the previously investigated sample irradiated at 400 °C, significantly increasing with the irradiation temperature. These values are approximately one order of magnitude larger than the value of 0.003 previously found for the sample with 400 appm helium. The size distributions are compared with electron microscopy observations of these same samples. It appears that the occurrence of complex micro-structural changes in irradiated Eurofer97-1 steel should be taken in due account when considering its application under high He/dpa ratio values. Keywords: Helium effects, Neutron irradiation, Small angle neutron scattering, Electron microscop