Mechanical Properties and Fretting Fatigue of SMAT-Treated Stainless Steels and Nickel-based Alloys


Student thesis: Doctoral Thesis

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  • Shan CAO


Awarding Institution
Award date9 Sept 2016


Steels are one of the most important structural materials and enable breakthroughs in various fields, such as energy, transportation, safety, and nuclear industry. In the nuclear power plant, zirconium alloy is being used as fuel rod material, however, meaning of the next generation of Fast Neutron Reactors (FNR) is to enhance the efficiency of nuclear fuel in high temperatures. Therefore, iron -based alloy will be an important nuclear material in the next decade. Stainless steel will be one of the most suitable iron-based alloys for material using in nuclear power plant. In the stainless steel group, 304 and 316 stainless steels are widely used in extreme environments such as nuclear industries for their ability to resist extremely high temperatures. Besides, nickel-based alloys, such as Alloy 690 and Alloy 182 are also universally applied as structural materials in nuclear power plant. It is generally believed that mechanical properties of the material receives large public recognition, i.e. typically bulk nano-materials showed high strength, but with disappointingly low ductility. However, there is often a trade-off between strength and ductility. In this thesis, Surface Mechanical Attrition Treatment (SMAT) was performed to develop advanced steels and Ni-based alloys with high yield strength, ultimate tensile strength, excellent hardness and favorable ductility at room temperature without changing their chemical composition. Furthermore, an optimized balanced proportions of austenitic and martensitic structure was achieved in the topmost 150µm nano-layered structures with average grain size of surface (top 150µm) grain size of about 20 nm in SMAT treated stainless steels. Therefore, we investigate the relationship between mechanical properties of steels and Ni-based alloys and their microstructure in this thesis, in order to provide guidelines to improve their strength while preserving the ductility. Our results showed that the new advanced steels and Ni-based alloys have optimal combination of strength, toughness and ductility. At last, we suggested developments prospects tackling the existing problems. In addition, we have developed different processes producing different types of nanostructures in steels and Ni-based alloys with different contents of nano-structure with both high strength and high ductility. We also investigated the merit processes of the SMAT and the new nano-structure against the fretting fatigue (FF) behavior. In addition, FF scars, life time and gradient microstructure of the material has been systematically studied by using data analysis and optical microscopy.