Analysis of hardening limits of oxide dispersion strengthened steel

Oxide dispersion strengthened (ODS) ferritic/martensitic (F/M) steels are promising materials for high temperature applications. The hardening limits from room temperature to 1000 °C of one of such steel, ODS EUROFER97, together with the impact of the material production steps, are investigated at a microstructural level by coupling hardness, tensile tests and transmission microscopy, including in situ heating experiments. The oxides, ytttria and complex yttrium titanium oxides, reinforce the material by forming more or less stable obstacles to dislocations, and by promoting grain refinement by pinning grain boundaries. It appears that part of the yttrium titanium oxides particles dissolves from about 600 °C while pure yttria particles are stable at least to 1000 °C in the steel. The concurrent roles of the oxides and the dislocation structure in the hardening are rationalized using the dispersion barrier hardening model. It appears that hardening due to dislocations can overcome the one due to oxides but is more sensitive to temperature than the one due to oxides, and that the main limiting factor is the thermal stability of the oxides.