Ni-based superalloys are essential for components operating under high stress, elevated thermal and extreme mechanical conditions in corrosive industrial environments. These materials frequently experience cyclic loading during service, and approximately half of mechanical failures originate from fatigue-related damage. A thorough understanding of how microstructure influences deformation and failure mechanisms under these conditions is crucial for developing alloys with enhanced fatigue resistance. Recent advances in characterization techniques, including slip trace analysis, electron backscatter diffraction, and high-resolution digital image correlation, have provided valuable insights into the deformation behavior of other metallic systems such as magnesium, titanium, and precipitation-hardened Ni-based superalloys. These studies reveal that plastic strain tends to localize from the first fatigue cycle near long twin boundaries where the slip systems in the parent grain parallel to the twin boundaries are favorably oriented for slip. Afterwards, these locations lead to crack initiation and highlighted how that microstructural features are critical factors limiting the fatigue life. Nevertheless, there is not a detailed analysis of the fatigue crack nucleation mechanisms in other Ni-based alloys, such as solution-hardening or novel precipitation-hardening alloys.

The thesis aims to investigates the role of the microstructural features the deformation and fatigue crack initiation mechanisms in Ni-based superalloys subjected to low-cycle fatigue at room temperature. The study compares solution-hardened alloys (Inconel 600 and Hastelloy C276 alloys) and a precipitation-hardened alloy (Haynes 244 alloy) to understand how microstructure influences cyclic plasticity and crack nucleation. Based on advanced microscopy techniques, the research identifies the specific roles of grain boundaries, annealing twin boundaries, and deformation twin boundaries in the deformation mechanisms and crack initiation places of the two types of alloys.

In solution-hardened alloys, cyclic deformation is accommodated by planar slip on {111}<110> systems, with slip transfer or blocking at GBs and TBs depending on the alignment between both traces at the boundaries. Crack nucleation occurs mainly at high-angle GBs and TBs where slip transfer is blocked, causing localized stress accumulation. Heat treatment in Inconel 600, which increases grain size and promotes carbide precipitation at GBs, enhances crack initiation along these boundaries but confirms that the grain boundary misorientation angle, rather than grain size, predominantly determines fatigue behavior.

In the precipitation-hardened Haynes 244 alloy, plastic deformation occurs primarily through deformation twins rather than dislocation slip. These twins increase in number with the fatigue cycles and maintain their small thickness. Twinning and detwinning is observed in the tension and compression part of the first cycle. Cracks nucleate at GBs, annealing TBs, and deformation TBs where twin transfer is blocked, or they propagate parallel to deformation twins, even when transfer across annealing twin boundaries occurs.