Microscopic heterogeneity of low cyclic fatigue damage in Ni-based single crystal superalloy DD413

The heterogeneous low cyclic fatigue damage level in the Ni-based single crystal superalloy DD413 was quantitatively assessed under the strain control of ±0.8% and ± 1.1%. The specimens subjected to ±0.8% strain amplitude exhibited an almost pure elastic deformation response throughout the entire cyclic loading period. In contrast, the ±1.1% strain-controlled deformed specimen exhibited a gradual increase in plastic strain, rising from 0.046% in the 1st cycle to 0.072% by the 268th cycle. The brittle carbides caused a significant mechanical incompatibility with the matrix. This was observed through the orientation dispersal over 16° in the {001} pole figure of the ±1.1% specimen, which was substantially broader than the ±0.8% specimen where the dispersion was only 1.2°. The large orientation gradient in the interdendrite (IR) triggered the dispersion of the microstructure-averaged orientation of the ±1.1% specimen, as evidenced by a mean grain reference orientation deviation (GROD) angle three times higher in the IR compared to the dendrite (DR). While the DR and IR of ±0.8% specimen showed similar mean GROD angles. This substantial orientation gradient was sustained by the high density of geometrically necessary dislocations (GNDs) in the IR, which was double that of the DR. Differently, the DR and IR in the ±0.8% specimen exhibited similar mean densities of GNDs. The study suggests that minimizing microstructural heterogeneity at the dendritic scale may enhance the durability of DD413 alloy components when exposed to cyclical stress with greater strain amplitudes.