Understanding fatigue crack nucleation from inclusions in a powder nickel alloy using micromechanics

This PhD project is concerned with fatigue crack nucleation from inclusions in a powder metallurgy (PM) nickel-based superalloy. The inclusions are introduced during the manufacturing due to the use of ceramic crucibles and result in scatter in fatigue life. Development of predicative capability of fatigue crack nucleation from inclusions requires mechanistic understanding of deformation at controlling length scales. Compositional variations near the inclusion were captured using wavelength dispersive X-ray spectroscopy (WDX), energy dispersive X-ray spectroscopy (EDX), and focused ion beam-secondary ion mass spectroscopy (FIB-SIMS). Local microstructural hetrogeneities were characterized by secondary electron microscopy (SEM) and related to the change in chemistry. Establishment of thermal residual elastic strains, lattice rotations, and dislocations at inclusion/nickel interfaces were quantified using electron backscatter diffraction (EBSD). A microstructurally faithfull crystal plasticity finite element (CPFE) model was developed. Detailed comparisons were made with the experimental measurements and good agreement was achieved. Patterns of plastic strains, residual stresses, and dislocation densities in a cyclically deformed nickel polycrystal were measured by digital image correlation (DIC) and EBSD. The mechanistic basis for crack nucleation via inclusion/nickel interface decohesion and particle cracking involved slip localization, establishment of high dislocation densities and local stress. A microstructurally representative CPFE model was developed for mechanistic study of inclusion/nickel interface decohesion. Decohesion was found to be driven by interface tensile normal stress alone, and the interfacial strength was determined to be in the range of 1270-1480 MPa.