To address this issue, a series of studies have been carried out to investigate the effects of inclusions under various conditions of loading, but it remains clear that crack nucleation occurring at agglomerate inclusions remains a significant technological and scientific challenge, and this paper addresses the mechanistic basis by which it occurs in the nickel superalloys. However, although great efforts have been made to reduce contamination introduced during manufacturing steps, non-metallic inclusions are inevitable in these alloys, resulting in degradation of mechanical properties and scatter in fatigue life. Modern superalloys with exceptional high temperature properties are produced via a powder metallurgy (PM) route in order to minimize micro/macrochemical segregation. Nickel-based superalloys are widely used for turbine disc applications. 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. A number of fatigue crack nucleation indicators are also assessed against the experimental results. Crack nucleation by interfacial decohesion at the nickel matrix/agglomerate inclusion boundaries is observed experimentally, and systematic modelling studies enable the mechanistic basis of the nucleation to be established. HR-EBSD and HR-DIC elastic and total strain measurements demonstrate qualitative and quantitative agreement with crystal plasticity results.
The full thermal and mechanical loading history was reproduced within the model. Experimentally, this sample has been subjected to heat treatment leading to the establishment of residual (elastic) strains local to the agglomerate and subsequently loaded under conditions of low cyclic fatigue. The methodology is to use the crystal plasticity model in conjunction with direct measurement at the microscale using high (angular) resolution-electron backscatter diffraction (HR-EBSD) and high (spatial) resolution-digital image correlation (HR-DIC) strain measurement techniques. A crystal plasticity finite-element model, which explicitly and directly represents the complex microstructures of a non-metallic agglomerate inclusion within polycrystal nickel alloy, has been developed to study the mechanistic basis of fatigue crack nucleation.