Surface intergranular cracking in large strain fatigue

The formation of surface intergranular cracks has been investigated with a coarse-grained polycrystal of nickel, deformed in low-cycle fatigue at 573 K. The evolution of the cracks was followed as a function of fatigue life fractions, and the factors favoring their formation were identified. It was found that in air, surface intergranular cracking occurs early in fatigue life and is induced by the impinging slip traces at the interface. Grain boundaries other than coherent twin boundaries and those with Σ < 5 are susceptible to such cracking. Depending on the boundary plane orientation and on the geometry of the operative slip vectors relative to the specimen surface, the grain boundary cracks may or may not grow to any appreciable extent. Crack growth is accelerated if the boundary plane makes a large angle with the stress axis and if the differential out-of-surface component of the operative slip vectors in the adjoining grains is large. In vacuum, slip is dispersed and surface rumplings become effective in grain boundary crack nucleation. The evolution of surface intergranular cracks, however, is delayed as opposed to tests conducted in air. The results are interpreted in terms of the interaction between crystal dislocations and grain boundaries and on the state of stress at the grain boundaries.