Microstructural and mechanical characterisation of an ODS steel manufactured by low-energy mechanical alloying

The aim of this thesis is a mechanical and a microstructural characterization of ODS steels produced by a novel route, namely the low-energy mechanical alloying (LEMA), compared to the most common process of high-energy mechanical alloying (HEMA). Indeed, ODS steels are commonly prepared by high-energy mechanical alloying 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. In general ODS microstructure is quite complex and several mechanisms contribute to the mechanical strengthening with different effects depending on the temperature. 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 role played by the following strengthening mechanisms at increasing temperature have been studied by considering the specific microstructural features: (i) solid solution; (ii) Bailey-Hirsch; (iii) Hall-Petch; (iv) Orowan; (v) Coble creep and (vi) Arzt-Rősler-Wilkinson. Furthermore, a comparison of the mechanical properties and the involved strengthening mechanisms between LEMA-ODS steel and the correspondent unreinforced steel (namely steel matrix) has carried out. The analyses evidence advantages and drawbacks of different preparation routes and suggest some criteria for further improving the mechanical properties of these materials.