Effect of surface defects on the high cycle fatigue properties of a single crystal superalloy

A nickel-based single crystal superalloy was casted using spiral crystal selection in the directional solidification furnace. The standard cylindrical specimens for rotary bending high cycle fatigue test were machined after standard heat treatment. The void with different size was made by an electro-spark machine in the middle of the fatigue sample to simulate blade surface defects. The effect of surface void on high fatigue property of the alloy at 980 ℃ with stress 400 MPa and 500 MPa was investigated respectively. SEM was used to examine the fracture surface of fatigue specimens. The results show that the fatigue lives of the sample with surface void decreased to some extent compared with standard sample and it decreased with the rising size of surface void. SEM observations show that the crack initiation site, the fatigue crack propagation area and the rapid fatigue fracture area can be observed. Compared with standard sample, the fatigue cracks of the sample with surface void initiated at void including surface. All the samples were multi-source fatigue fracture. The rotary bending high cycle fatigue at high temperature all exhibited qusi-cleavage fracture which is different from tensile and stress rupture at same temperature.