3. Thesis (Viola PAUL)

An improved combination of strength and ductility is generally a trade-off relationship, and it remains a major research topic in the field of structural materials. A bimodal grained microstructure, consisting of a coarse-grained “core” region and a surrounding fine-grained “shell” region, exhibits a good balance between strength and ductility. Therefore, the strengthening mechanism that contributed to the excellent mechanical properties of the harmonic structure was investigated. Furthermore, the deformation and fracture behavior were also studied. The individual strengthening factor in the Hall–Petch model was investigated using nanoindentation technique to understand the strengthening mechanisms in the SUS304L stainless steel harmonic structure. This is because of the limitation of the conventional evaluation methods for mechanical properties, such as macroscopic tensile or compression tests, which cannot measure each factor separately. Therefore, nanoindentation technique was applied locally, such as in the “grain interior” to evaluate the matrix strength (𝜎!) and “on grain boundary” and “near grain boundary” to assess the grain boundary effect (𝑘) for each region of the shell and core. Important parameters such as the pop-in behavior on the initial loading curve and nanohardness were analyzed.