“Towards superalloys with increased fatigue life: Microstructure optimization via advanced processing routes, driven by the minimization of strain localization”
Wrought polycrystalline Nickel base superalloys are widely used in the aerospace and energy industries, owing to their excellent strength at high temperature, oxidation and creep resistance. To optimize fatigue properties (fatigue crack initiation, resistance to crack propagation, fatigue life), a deep understanding of how strain localizes in the early stages of plasticity is required. Newer generation alloys are relatively clean of manufacturing defects (pores, inclusions, pre-existing cracks) and strain localization is thus directly related to the microstructure which involves the size, orientation and shape of individual grains and precipitates. The spatial arrangement of these features is the result of the thermo-mechanical conditions in which the material was processed. Strain localization occurs under the form of slip bands which can readily be mapped using digital correlation in the scanning electron microscope (SEM-DIC). In this study, high resolution SEM-DIC is employed to probe the location of thousands of slip bands over a large region of interest in a widely used superalloy, Inconel 718. Multi-modal data merging techniques and computer vision are implemented to automatically and statistically investigate the location of slip bands as a function of the 3D microstructure, captured using electron backscatter diffraction (3D-EBSD). Strong correlations are obtained between the location of slip bands and particular microstructure objects: annealing twin boundaries and triple junctions. Micro-mechanical reasons will be discussed. The weak points of the microstructure being identified, new forging routes are designed in the aim to produce microstructures with reduced strain localization. The efficiency of such routes will be discussed.