Oxide dispersion strengthened (ODS) steels are promising materials for applications at temperatures up to 800 °C in harsh environments as they occur in conventional or fission power plants. Compared to common stainless steels, they show an outstanding resistance against swelling under radiation and a superior creep and oxidation resistance [Schneibel et al., Scr. Mater., 2009]. The key to achieve these extraordinary properties are homogenously distributed, nanometer-scaled oxide dispersoids with an average size of < 4 nm.
We successfully manufactured ferritic (Fe-14Cr-0.4Ti-0.25Y2O3 in wt.%) and austenitic (Fe-25Cr-20Ni-0.4Ti-0.25Y2O3) ODS steels by mechanical alloying and subsequent consolidation by field assisted sintering technique. Multiscale characterization of the microstructure including X-ray diffraction, electron backscatter diffraction and atom probe tomography was performed on the consolidated material to obtain relevant microstructural information about dislocation density, grain size and oxide particle size. Subsequently, the temperature-dependent yield strength was analyzed by means of compression tests up to 800 °C. Contrary to what was anticipated regarding the packing density of the respective crystal structures, it is shown that the yield strength of the ferritic ODS alloy is always significantly higher than that of the austenitic ODS alloy. Besides, both materials show substantial decrease in strength at temperatures above 450 to 500 °C.
The results of the microstructural analyses were combined with traditional strengthening models, namely Orowan, Hall-Petch and dislocation strengthening to predict the room temperature strength. This analysis shows that in both alloys Orowan and Hall-Petch type strengthening are the dominating contributions. Furthermore, a significant influence of dislocation strengthening on the yield strength was identified for the ferritic ODS alloy, only. Applying linear superposition of the aforementioned strengthening contributions seems to overestimate the experimentally determined yield strength. Nevertheless, we may conclude that the model-compatible calculation of the yield stress describes the experimental data satisfactorily if uncertainties resulting from the determination of microstructural parameters are taken into account. Finally, it is shown that at high temperatures, the decreasing strength is best described by the Blum and Zeng model, based on dislocation creep [Blum and Zeng, Acta Mater., 2009].