Effect of grain size on fatigue crack initiation
For fine - grained nickel - based alloys, the number of grain boundaries per unit volume increases significantly. Grain boundaries act as effective barriers to dislocation movement: when cyclic stress is applied, dislocations accumulate at grain boundaries instead of forming large - scale slip bands (which are the main sources of fatigue crack initiation). Meanwhile, fine grains can disperse local stress concentration, reducing the probability of crack nucleation and thus improving the fatigue limit and high - cycle fatigue (HCF) resistance of the alloy.
For coarse - grained nickel - based alloys, the small number of grain boundaries leads to easy formation of continuous slip bands inside grains under cyclic loading. These slip bands will cause plastic deformation and micro - cracking on the alloy surface, accelerating fatigue crack initiation. As a result, coarse - grained alloys generally exhibit lower high - cycle fatigue strength.
In stage I, for fine - grained alloys, cracks need to continuously change propagation paths when crossing multiple grain boundaries, which consumes a large amount of fracture energy and slows down the propagation rate. For coarse - grained alloys, cracks can propagate along the slip direction inside large grains with less resistance, leading to faster propagation.
In stage II, the propagation of fatigue cracks is dominated by the stress intensity factor. At this time, the influence of grain size is weakened, but fine grains still have a certain advantage in slowing down crack propagation compared with coarse grains. However, if the grain size is too small, the proportion of grain boundaries increases, which may lead to intergranular brittleness under certain stress conditions and offset part of the fatigue performance improvement.
Fine - grained alloys are prone to grain boundary sliding at high temperatures, which accelerates fatigue failure.
Moderately coarse - grained alloys have better resistance to grain boundary sliding, so their low - cycle fatigue performance is superior to that of fine - grained alloys in this scenario.
For components subjected to low - stress and high - cycle fatigue (e.g., structural parts of gas turbines), a fine - grained structure (grain size grade 6–8) is preferred to maximize the fatigue limit.
For components subjected to high - temperature and low - cycle fatigue (e.g., turbine blades), a moderately coarse - grained structure (grain size grade 2–4) is more suitable to balance high - temperature strength and fatigue crack propagation resistance.
