Temperature dependent strengthening contributions in austenitic and ferritic ODS steels

We aim on the model-based description of the strength of ferritic and austenitic oxide dispersion strengthened (ODS) steels in the temperature range from room temperature (RT) up to 800 degrees C. Therefore, we present two approaches for the synthesis of austenitic alloys by mechanical alloying Y2O3, namely with (i) elemental powders at RT and (ii) with a gas-atomized master-alloy. Consolidation of both powders by field assisted sintering technique leads to a more homogenous distribution of grain size and particles in specimens from elemental powders. In the entire temperature range, the compressive strength of the austenitic ODS steels is shown to be lower compared to the one of ferritic counterparts. Above approximately 500 degrees C, a strong decrease in strength is observed for all ODS variants due to the onset of creep-based deformation. Multi-scale materials characterization is performed to quantitatively assess microstructural materials parameters crucial for the modeling of the temperature dependent yield strength. These data are utilized to quantitatively describe the strength contribution by Hall-Petch and Orowan strengthening as well as dislocation strengthening at RT. Lower amounts of grain boundary and dislocation strengthening are found to be crucial for the lower strength of austenitic ODS steels. Meaningful calculation of materials strength is only achieved, when both interactions of strengthening contributions and experimental uncertainties are considered. Models describing diffusion-based creep (by Coble) and dislocation-based creep (by Blum and Zeng), which are shown to provide a more appropriate description of high temperature strength, are critically assessed for temperatures at and above the strength drop. It is shown that the deformation at high temperatures is possibly dominated by the formation and annihilation of dislocations at grain boundaries.