Some Effects of Microstructure and Environment on Fatigue Crack Propagation

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Abstract
Fatigue crack growth rates have been determined in nickel-base alloys both after a normal heat treatment and after heating for a further 10 000 h. The latter treatment causes coarsening of the γ‵-particles so that they are no longer sheared by dislocations when the material is deformed. Differences in cracking rates at both ambient and elevated temperatures may be understood in terms of slip reversibility and oxide penetration when deformation occurs by shearing of precipitates. A further result of the prolonged aging period is a considerable reduction in the cyclic fracture toughness of a cast alloy. This can be traced to a transition to intergranular cracking along regions of coarsened precipitation adjacent to the grain boundaries. A comparison of cracking rates in air and vacuum shows that the presence of air can lead to an increase in growth rate and to a situation where no clear threshold is apparent. Crystallographic cracking (analogous to Stage I fatigue) is observed in both air and vacuum. One effect of oxygen in tests at high temperatures is to cause dissolution of the γ‵-particles in the region of the crack tip. Under high-temperature corrosive conditions (air/SC2/SO3 atmosphere) an increase in scatter and in average propagation rate can be correlated with the formation of liquid sulphide films at the crack tip.