Oxide-dispersion strengthened ferritic steel was produced by high-energy attrition, leading to a complex nanostructure with deformed ferritic grains. After mechanical alloying, the powder was then consolidated by spark plasma sintering (SPS) using various thermo-mechanical treatments. Hot isostatic pressing (HIP) was also performed on the same powder for comparison. Above 1123 K (850 !C), SPS consolidation-induced heterogeneous microstructure composed of ultra-finegrained regions surrounded by larger grains. Spatial distribution of the stored energy was measured in the bimodal microstructure using the Kernel average misorientation. In contrary to large recrystallized grains, ultra-fine grains are still substructured with low-angle grain boundaries. The precipitation kinetics of the nano-oxides during consolidation was determined by small-angle neutron scattering. Precipitation mainly occurred during the heating stage, leading to a high density of nanoclusters that are of prime importance for the mechanical properties. Other coarser titanium-enriched oxides were also detected. The multiscale charac-terization allowed us to understand and model the evolution of the complex microstructure. An analytical evaluation of the contributing mechanisms explains the appearance of the complex grain structure and its thermal stability during further heat treatments. Inhomogeneous dis-tribution of plastic deformation in the powder is the major cause of heterogeneous recrystal-lization and further grain growth during hot consolidation. Then, the thermal stability of coherent nano-oxides is responsible for effective grain boundary pinning in recovered regions where the driving pressure for recrystallization is lowered. This scenario is confirmed in both SPSed and HIPed materials.

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Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature. This paper is part of a special issue on Euromat 2013: Emerging technologies in powder metallurgy Highlights N An oxide dispersion strengthened ferritic steel was consolidated by spark plasma sintering. N Scale-up from 15 g to 1/2 kg compacts was achieved with reproducible microstructure and resulting properties. N The chemical homogeneity of the compacts was characterised by electron microprobe analysis and microhardness. N Carbon coming from the graphite foils at the interfaces between the graphite die, the punches and the powder diffused within few hundreds microns into the ferritic matrix. N Precipitation of large titanium oxides ranging from 50 to 200 nm was detected in the compacts.N These coarse particles were not detrimental to the mechanical properties, since both tensile strength and ductility were achieved.