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Ultrasonic surface deep rolling (USDR), oxygen boost diffusion (OBD), and their combination (USDR-OBD) were all used to improve the surface hardening of pure titanium. The microstructure, microhardness, and fatigue life of pure titanium treated by USDR, OBD, and USDR-OBD methods were analyzed. USDR treatment induced a severe deformation area, while OBD treatment produced a brittle oxygen diffusion zone. The USDR-OBD treated samples approached the highest hardness in comparison with other treated samples. The fatigue lives of USDR treated samples were improved, which was due to the high compressive residual stress and refined grains. However, the fatigue lives of both OBD treated samples and USDR-OBD treated samples were decreased due to premature crack initiation and rapid propagation in the oxygen diffusion zone. Finally, the fatigue fracture mechanisms of different samples were proposed.

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

The micromechanical deformation behaviour of the commercial Ni-base polycrystalline Ni-base superalloy GH4169 has been investigated in terms of the volume fraction and grain boundary distribution of the δ (Ni3Nb) phase. The main objective of this work was to gain an understanding about the mechanisms of deformation of the superalloy at the grain scale. To that purpose, experimental and microstructural observations are relied upon to construct realistic models of the polycrystalline superalloy microstructure using crystal plasticity concepts. It was found that a volume fraction of the grain boundary δ phase in excess of approximately 3.0% results in a weakening of the room temperature strength, and the detrimental effect increases with increasing the δ phase volume fraction. This study also revealed that the grain boundary δ phase, being stronger than the surrounding grains, impedes the transmission of plastic slip through the grain boundaries, thus enhancing the localisation of plastic slip within the grains. However, the strengthening effect associated with such slip blocking mechanism by the δ phase is considerably lower than the weakening of the deformation resistance of the grains due to the depletion of the meta-stable γ″ precipitates within the grains resulting from the stable δ phase formation.

Small fatigue crack initiation behaviour in a series of specimens containing different sizes of micro-notch has been observed through RepliSet system during interrupted low cycle fatigue tests. From the replicating observations, very initial single crack or multiple cracks near the micro-notch area can be captured. Since then, the crack initiation life for each specimen can be determined. Computationally, a numerical procedure with a combination of crystal plasticity theory and finite element implementation has been constructed for predicting small fatigue crack initiation life as well as identifying the crack initiation sites near the micro-notches. With the help of fatigue indicator parameter represented by accumulated plastic slip, the predicted numbers of cycles for the submodels with various micro-notch sizes agreed with the experimental ones. In addition, the simulation results can successfully reveal the mechanism in crack initiation sites. The stress concentration effect was found to be responsible for corner crack initiation, while the combined effects of stress concentration and grain orientation around micro-notch area showed great influence on edge crack initiation.