Slip Mode Transitions and Work Hardening of a Polycrystalline Superalloy: Effects on Striation Formation at Room Temperature

ABSTRACT The crack propagation behavior of conventionally cast and wrought INCONEL 718 has been assessed to determine the dominant advancement mechanisms under different loading regimes. Long crack fatigue testing was carried out under constant load (CL) and overload (OL) scenarios. Under CL, low stress intensity ranges (Δ K ) produced a faceted fracture dominated by planar slip bands whose spacings were found to be independent of fatigue crack growth (FCG) rate ( da / dN ), while higher Δ K induced classical striation formation with spacings in agreement with da/dN. The introduction of OL cycles prompted crack retardation, which was also found to influence slip/striation formation. The transition between advancement mechanisms was influenced by the effective ∆ K acting at the crack tip and therefore the interaction between the cyclic plastic zone (CPZ) and the microstructure. This key interaction, where the CPZ size is either smaller or larger than the size of the fracturing grain, governs the shift between planar and cooperative slip advancement mechanisms, respectively. An adaptation of an existing planar slip model is proposed to incorporate the strain hardening of slip systems which, at lower ∆ K , required multiple loading cycles to occur.