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Abstract: The goal of this study is to characterize microstructure of a friction stir welded oxide dispersion strengthened alloy. The welded material is constituted by two sheets of an yttria-dispersion-strengthened PM 2000 ferritic steel. Different areas of the friction stir welded product were analyzed using field emission gun secondary electron microscopy (FEG-SEM) and electron microprobe whereas nanoindentation was used to evaluate mechanical properties. The observed microstructural evolution, including distribution of the yttria dispersoids, after friction stir welding process is discussed and a correlation between the microstructure and the results of nanoindentation tests is established. [Copyright &y& Elsevier]

Abstract: Reactive-inspired ball-milling is proposed as a new production route for oxide dispersion strengthened (ODS) steels. So a Fe–14Cr–2W–1Ti–0.8Y–0.2O(wt.%) ODS steel is elaborated by ball-milling of FeCrWTi and YFe3 plus Fe2O3 powders instead of Y2O3 and then by annealing at 800°C for 5min. Characterizations by Electron Probe MicroAnalysis and Atom Probe Tomography (APT) are performed after milling and after annealing. For the very first time, nanoclusters are observed after ball-milling by APT. Those nanoclusters are enriched in titanium, yttrium and oxygen and their mean radius is 0.8nm. With annealing, the mean radius rises up to 1.4nm and the number density as well as the enrichment factor in O, Ti and Y increase. So a new formation mechanism of nanoclusters is observed in those conditions of synthesis: ball-milling initiates the nanoclusters nucleation and during annealing, nucleation continues, accompanied by a slight growth of nanoclusters. Thus reactive-inspired ball-milling appears as a promising route for synthesizing ODS steels with a fine and dense dispersion of oxides. [Copyright &y& Elsevier]

Abstract: Threshold stress of ODS (Oxide Dispersion Strengthened) alloy is well known as a value which can quantitatively shows the effect of oxide dispersion strengthening. However, the threshold stress cannot be clearly observed in the result of creep tests for the commercial ODS alloy MA957. In this work, the threshold stress of MA957 was carefully evaluated using TEM (Transmission Electron Microscope) with EDS (Energy Dispersive X-ray Spectrometer). From the result, it was revealed that MA957 has an inhomogeneous dispersion of oxide particles. The inhomogeneous dispersion caused different threshold stresses between observed areas. This is probably a reason why the threshold stress cannot be clearly seen in the result of creep tests. This inhomogeneous dispersion appeared to be partly caused by aluminum contamination, because aluminum made much larger complex oxide particles than the fine oxide particles with no aluminum. Consequently, aluminum should be strictly controlled or eliminated for the homogeneous dispersion of oxide particles of MA957. [Copyright &y& Elsevier]