Electron microscopy and atom probe tomography of nanoindentation deformation in oxide dispersion strengthened steels

Abstract

Oxide Dispersion Strengthened (ODS) steels are candidates for fuel cladding materials in sodium-cooled fast reactors and for structural materials in nuclear fusion power reactors. The effect yttrium-titanium-oxygen (Y-Ti-O) nano-oxide precipitates within ODS steels have on the micromechanical deformation mechanisms has been investigated. The aim is to assess the extent of any direct link between the Y-Ti-O dispersion and nanoindentation hardness, using electron backscatter diffraction, transmission electron microscopy (TEM), and atom probe tomography (APT) studies of a Fe-14Cr-3W-0.2Ti-0.25Y2O3 (wt%) ODS steel at room temperature. Y-Ti-O nanoclusters had a non-uniform distribution and average number density that ranged from 5.8±0.1×10 23 to 1.3±0.1×10 24 m -3 and Guinier radii ranging from 1.8±0.2 nm to 2.1±0.2 nm. Surprisingly, the local Y-Ti-O distribution did not correlate strongly with the nanohardness, indicating that the dominant hardening mechanisms was at best only weakly related to the Y-Ti-O distribution. Instead, scanning TEM and APT confirmed that the dominant hardening mechanism was due to the grain boundary refinement, and was further enhanced by tungsten enrichment to ~3.5 at% at grain boundaries.

Nanocluster evolution and mechanical properties of ion irradiated T91 ferritic-martensitic steel

“…These clusters act as dislocation motion barriers; however, direct observation of this pinning would involve utilising transmission electron microscopy (TEM) to image the plastic zone beneath the nanoindent. This is beyond the scope of this study, however, Davis et al [69] performed a similar study in an oxide dispersion strengthened steel, imaging the dislocations becoming pinned by nanoparticles beneath nanoindents. The size of the nanoparticles was 2 -5 nm, which is comparable to the size of the MNSP observed within this study.…”