The Effects of the Surface Nanocrystallization and Hardening (SNH) Process on Fatigue Resistance

Nanocrystalline and coarse-grained materials have shown high resistances for the fatigue-crack initiation and growth, respectively. In this thesis, the surface- mechanical-attrition (SMA) treatment has been employed for the surface- nanocrystallization and hardening (SNH) process to produce engineered materials with coarse grains in the interior and grain-refinement progressively down to the nano-size on the surface.

The effects of the different processing parameters of the SMA treatment, including the ball size, materials, and treatment time, on the SNH process of the Ti-6Al- 4V and Ni-based Hastelloy® C-2000 Alloys were investigated from the viewpoints of the microstructure, microhardness, surface roughness, and fatigue behavior, by means of the optical microscopy, scanning-electron microscopy, transmission-electron microscopy, X-ray diffraction, Vickers-microhardness indentation, three-dimensional (3D) non-contact scanning white light interferometry profilometer, as well as four- point-bend tests.

The surface-grain refinement was found in all the samples. The surface hardness was improved in the Ni-based Hastelloy® C-2000 superalloys. After the treatment, the surface roughness definitely increased. The fatigue results and fractographes showed that the SMA treatment could improve the fatigue lives. However, the longer-time plus greater-size-balls treatment has the adverse effect on the fatigue lives. The fatigue behaviors of the treated samples were highly influenced by the adhesion-fracture region on the surfaces due to the interaction between the balls and the sample surface during the treatment.

Further research activities were suggested in this thesis.