Subtle microstructural changes during prolonged annealing of ODS-Eurofer steel

Reduced-activation ferritic-martensitic oxide-dispersion-strengthened (RAFM-ODS) Eurofer steel is a potential candidate material for structural applications in fusion reactors. Microstructural stability during long-term exposure at high temperatures is a key issue. Depending on the amount of prior cold-rolling strain and service temperature, important solid-state restoration reactions occur such as recovery, recrystallization and particle coarsening. ODS-Eurofer steel was cold rolled up to 80% reduction in thickness and annealed at 800 °C for durations up to 4320 h. Changes in microstructure were tracked by X-ray diffraction measurements using synchrotron radiation in post-mortem specimens to estimate dislocation character and density. The volume fraction of recrystallized grains was estimated using grain orientation spread (GOS) maps from electron backscatter diffraction (EBSD). Most of the softening occur in the first hour of annealing and it seems to be closely related to discontinuous recrystallization where a few special grain boundaries overcome Zener-Smith pinning effects caused by fine and stable Y2O3-based particles. M23C6 carbides undergo coarsening upon annealing and, as a result, extended recovery is the predominant softening mechanism as annealing proceeds, although only about 15% softening is noticed after annealing for 4320 h. Using thermodynamic and kinetic calculations, the results were extrapolated to the predicted service temperature of 650 °C. The results suggest that the remarkable microstructural stability of ODS-Eurofer would withstand almost 180 years at high service temperatures without major loss of the mechanical properties of the materials

An atomic force microscopy study of Eurofer-97 steel

In recent years the microstructure, mechanical and magnetic properties of Eurofer-97 steel are studied intensively due to its application in nuclear fusion power plants. Its microstructure is usually accessed by means of electron microscopy. Here we present an alternative approach utilizing Atomic Force Microscopy (AFM) to study as-received Eurofer-97 steel. We recorded both the Height Signal (HS) and Phase Signal (PS) that provided information on the morphologic and inelastic topography, respectively. With the HS we detected spherical particles (SPs) of size 50-2000 nm. Interestingly, micrometer SPs (0.1-2.0 μm) are randomly distributed, while nanometer SPs (50-100 nm) are sometimes arranged in correlation to grain boundaries. The PS clearly revealed that the micrometer SPs exhibit inelastic properties. Though we cannot identify the elemental composition of the SPs with AFM, based on relevant electron microscopy data we ascribe the nanometer ones to the TaC, TiN and VN and the coarse micrometer ones to M23C6 (M=Cr, Fe). The latter class of SPs can probably be active sites that influence the mechanical properties of Eurofer-97 steel upon annealing as observed in relevant electron microscopy based studies

Protrusions formed during primary recrystallization of cross-rolled coarse-grained niobium

The growth of recrystallization nuclei towards the deformed matrix generally does not proceed isotropically. Local disturbances in the migration along grain boundaries lead to the formation of protrusions. This important aspect in recrystallization is rarely explored in the literature, especially in the sense of understanding which microstructural mechanisms explain its formation. The aim of this work is to study the formation of protrusions during the recrystallization of a niobium oligocrystal. A coarse-grained niobium plate was cut out from the longitudinal section of an electron-beam cast ingot. Five coarse grains (A, B, C, D and E) were randomly selected for this study to evaluate likely orientation effects on protrusion formation. The plate was cold rolled with several passes up to 50% thickness reduction. The sheet was then cut into two parts. The former was straight rolled until 70% thickness reduction while the latter was rotated 90° around the normal direction with respect to the initial rolling direction. Important orientation and rolling mode effects were noticed regarding their recrystallization behavior. Protrusions were more abundant in the cross-rolled specimens. It was not possible to identify and associate significant differences regarding either CSL-type special boundaries or highly-misoriented boundaries ahead of the boundaries showing protrusions.Fil: Brekailo, T.. Universidade de Sao Paulo; BrasilFil: Almeida, D.R.. Universidade de Sao Paulo; BrasilFil: Avalos, Martina Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Bolmaro, Raul Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Sandim, H.R.Z.. Universidade de Sao Paulo; Brasi

An atomic force microscopy study of Eurofer-97 steel

In recent years the microstructure, mechanical and magnetic properties of Eurofer-97 steel are studied intensively due to its application in nuclear fusion power plants. Its microstructure is usually accessed by means of electron microscopy. Here we present an alternative approach utilizing Atomic Force Microscopy (AFM) to study as-received Eurofer-97 steel. We recorded both the Height Signal (HS) and Phase Signal (PS) that provided information on the morphologic and inelastic topography, respectively. With the HS we detected spherical particles (SPs) of size 50-2000 nm. Interestingly, micrometer SPs (0.1-2.0 μm) are randomly distributed, while nanometer SPs (50-100 nm) are sometimes arranged in correlation to grain boundaries. The PS clearly revealed that the micrometer SPs exhibit inelastic properties. Though we cannot identify the elemental composition of the SPs with AFM, based on relevant electron microscopy data we ascribe the nanometer ones to the TaC, TiN and VN and the coarse micrometer ones to M23C6 (M=Cr, Fe). The latter class of SPs can probably be active sites that influence the mechanical properties of Eurofer-97 steel upon annealing as observed in relevant electron microscopy based studies