In-situ observation of short crack growth behavior for powder superalloys with different gradient grain microstructures

The fatigue crack initiation and short crack propagation behavior of a 3rd generation powder superalloy with the different microstructures were studied by in-situ scanning electron microscope (SEM) observation. The effects of microstructures at the wheel rim (coarse grain structure), wheel centre (fine grain structure), and grain transition zone (gradient grain tructure) of the dual performance powder turbine disk on the fatigue short crack propagation were investigated. The results show that, the crack prefers to nucleate from the first hardening phase at the notch. The gradient microstructure exhibits the multiple cracking at the grain boundaries. For the specimens with the gradient microstructure, the short crack growth behavior at room temperature is significantly affected by the microstructures, and the fatigue crack growth rate shows the large fluctuation. At the lower stress intensity factor range, the crack propagation rate of the coarse grains is higher than that of the fine grains. With the increase of stress intensity factor range, the crack propagation rate of the fine grains is increased faster and finally higher than that of the coarse grains, which mainly attributes to the competition mechanism of the long slip path, the enhanced slip reversibility, the few grain boundary barriers, and the reduced discontinuity.