Fatigue Crack Initiation in Alloy 718 at Elevated Temperature

In order to investigate the mechanism of fatigue crack initiation in Alloy 718 at elevated temperature, rotating bending fatigue tests were carried out at room temperature and 500℃ for the alloy with different grain sizes of 18 and 88μm. Fatigue strength in the long life region decreased markedly with increase in grain size at both temperatures. Non-propagating cracks of one or two grain sizes were observed in both fine and coarse grained specimens under stress levels near a horizontal line in each S-N curve, though the S-N curve of fine grained specimens showed a duplex one at 500℃. All of fractures occurred from surface in the life range up to 108cycles except for those originated from the subsurface of fine grained specimens at 500℃ beyond∼107cycles. Cracks nucleated from not only slip bands but also annealing twin boundaries, irrespective of grain size and fracture type. The frequency for twin boundary cracking increased with increases in grain size and temperature and with decrease in stress.