In this dissertation, the thermal fatigue behaviour of DD8 single crystal nickel-base superalloy has been investigated and the comparison of the thermal fatigue property of this alloy with DZ38G directionally solidified superalloy has been made. It has been found that the thermal fatigue life of DD8 single crystal alloy which is related to the upper cyclic temperature, the solidification rate and the crystal orientation has been improved significantly compared with that of DZ38G alloy. The density of dislocations increases with the increasing thermal fatigue cycles. The fracture appearance is characterized by cleavage. The high cyclic fatigue crack propagation behaviour of the alloy has also been studied. The results reveal that there is no secondary crack observed on the fracture surface and the crack propagation rate is not influenced by the solidification rate when ΔK, the stress intensity factor range, is lower than 17MPa m~(1/2) or larger than 40MPa m~(1/2). However, when 17MPa m~(1/2) < ΔK < 40MPa m~(1/2), the secondary cracks are easily observed in the specimens with low solidification rate. In this case, the main crack propagation rate is small. A series of observation show that higher stress ratio results in higher fatigue crack growth rate and lower stress intensity factor threshold value. The crystal orientation has little effect on the fatigue crack growth behaviour. The fracture surface is characterized by cleavage and the fatigue striation is observed occasionally.
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