Development of new ferritic 14 cr ods steels with four oxides formers (y, ti, zr, al) for nuclear applications

• Autores: Eric Macia
• Directores de la Tesis: Mónica Campos Gómez (dir. tes.)
• Lectura: En la Universidad Carlos III de Madrid ( España ) en 2019
• Idioma: español
• Tribunal Calificador de la Tesis: Carlos Capdevila Montes (presid.), María Teresa Pérez-Prado (secret.), Eberhard Altstadt (voc.)
• Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
• Materias:
  • Física
    • Física atómica y nuclear
      • Reactores nucleares
  • Ciencias tecnológicas
    • Tecnología metalúrgica
      • Pulvimetalurgia
• Enlaces
  • Tesis en acceso abierto en: e-Archivo
• Resumen

  • Throughout the last decades, the population is globally growing, demanding more energy to persevere the required lifestyle. Investigations are focused on several sources of energy (renewable, thermal…), nevertheless, nuclear power plants are postulated as an important alternative due to their huge electrical productivity. Contemporary ideas lead the energetic industry to low emission highly efficient design systems. Therefore, raising the service operated temperature leads to higher energetic productions. However, safety and durability are the main conditioning and limitation factors on the development of nuclear materials. On this complex scenery, the new GEN IV reactors project has emerged.

    As it is well known, regarding nuclear reactors, phenomena such as irradiation, corrosion or high temperature environments are faced. Consequently, designing the structural material has to be prioritized to ensure safe and productive power plants.

    Along with, oxide dispersion strengthened (ODS) ferritic steels are one of the main candidates for nuclear cladding and tubes. During the last years, researchers were especially focused on the evolution of the alloy design. Therefore, the high potential resistance under extreme temperature and neutron exposure environments is provided by BCC crystal structure together with an explicit material composition. In this work, Cr and W allow to improve the final service temperature, thanks to their solid solution strengthening. Besides, Cr and Al are selected to improve the corrosion properties. Developing stable nano-oxides dispersed into the ferritic matrix is provided by Y2O3, Ti, and Zr addition. These nano-precipitates block the movement of dislocations, improving the mechanical behaviour under high temperatures conditions.

    Traditionally, Y2O3 was the main compound used to produce nano-oxides. However, during the last years, Ti was employed to provide a particle size refinement, developing non-stoichiometric Y-Ti-O nanoclusters whose nature and size increased the performance of the ODS alloy.

    The objective of this Thesis is to develop an ODS ferritic steel alloyed with Al, whose microstructure has greater mechanical properties stability than current microstructures. To achieve this, the Zr addition has been considered as an element that would refine the dispersion and prevent the incorporation of Al into the nano-oxides.

    Therefore, Zr will improve the creep behaviour thanks to the formation of high thermal stable Y-Zr-O. However, facing Al, the precipitates composition varies forming: Y-Al-O, Y-Ti-O, Y-Zr-Al-O, Y-Zr-Al-Ti-O, Y-Zr-Ti-O, W-Ti-Zr, Zr-Ti .

    Trying to avoid the competition established between the different oxide formers and to control the oxides species formation, a unique nano-oxide (YTiZrO), synthesized by co-precipitation, is used.

    Indeed, the processed steels developed in this Thesis are divided in F-ODS-I, in which the different oxides formers are inserted as Y2O3, Zr and Ti and in F-ODS-II where the oxides formers are added as a complex nano-oxide YTiZrO.

    Traditionally, ODS steels are manufactured by mechanical alloying following by hot isostatic pressing (HIP) or hot extrusion (HE) whose thermal activation could lead to a noticeable grain growth. Therefore, the use of field assisted sintering techniques (FAST), including spark plasma sintering (SPS) is postulated as an alternative on the densification of ferritic ODS steel. In this research, it has been demonstrated how the use of faster heating rate together with short sintering times, allows to achieve accurate density values without losing some microstructure features reached during the milling step.

    2 Moreover, the nano-precipitate nature was studied by transmission electron microscopy (TEM). The nano-precipitates thermal stability was evaluated by In-situ TEM annealing. This study allows to assess the microstructure stability under high temperature conditions.

    The mechanical behaviour at room temperature was analysed by microhardness and microtensile tests, comparing the response with the same material without Zr. To analyse the material response at high temperature, small punch tests were carried out. Eventually, the different strengthening mechanisms were studied through a mathematical model, to estimate a theoretical value of the Yield strength (YS).

    The produced materials (F-ODS-I and F-ODS-II) presented an interesting stability at high temperature along with highlighted mechanical properties, being consequently an interesting candidate for its final application on GENIV reactors.