Analysis of Strengthening Mechanisms in Nano-ODS Steel Depending on Preparation Route

Oxide dispersion strengthened (ODS) steels are promising materials for high temperature applications, in particular in fission and fusion nuclear reactors. In comparison to common reduced activation ferritic/martensitic steels they exhibit better resistance to neutron irradiation and creep owing to an uniform dispersion of nano-oxides particles (~5 nm) and a very fine grain structure (~500 nm). ODS steels are commonly prepared by high-energy mechanical alloying (HEMA) of a mixture of steel powder and Y2O3 particles followed by a consolidation stage consisting of hot extrusion (HE) or hot isostatic pressing (HIP). The samples are then submitted to annealing around 1100°C for 1-2 hours. Recently, the present authors proposed a novel method based on low-energy mechanical alloying (LEMA). In general ODS microstructure is quite complex and several mechanisms contribute to the mechanical strengthening with different effects depending on the temperature. The present work analyses the role played by each single mechanism at increasing temperature by considering the specific microstructural features. ODS steels prepared through different routes and process parameters display different grain size distribution and homogeneity of particles dispersion, factors which strongly affect the mechanical properties. Yield stress values measured in tensile tests performed at increasing temperature up to 700°C, either taken from literature or achieved by authors, have been examined and the following strengthening mechanisms have been considered to fit the experimental data: (i) solid solution; (ii) Bailey-Hirsch; (iii) Hall-Petch; (iv) Orowan; (v) Coble creep and (vi) Arzt-Rősler-Wilkinson. The analyses evidence advantages and drawbacks of different preparation routes and suggest some criteria for further improving the mechanical properties of these materials.