This study provides direct observation of the crack closure mechanism of a naturally occurring, tortuous, 3D microstructurally small fatigue crack (SFC) in additively manufactured Inconel 718. In conclusion, the intergranular stresses from the CPFEM model and far-field HEDM measurements up to incipient yield are shown to be in good agreement, and implications for application of such an integrated computational/experimental approach to phenomena such as fatigue and crack propagation is discussed. The predicted intergranular stresses for 39 internal grains are then directly compared to data from 4 far-field measurements taken between ~4% and ~80% of the macroscopic yield strength. The initial microstructure of the central slab of the gage section, measured via near-field HEDM, is used to inform a CPFEM model.
Here, we utilize an HEDM dataset from an alphatitanium alloy (Ti-7Al), collected at the Advanced Photon Source, Argonne National Laboratory, under in situ tensile deformation. While there have been extensive studies validating homogenized CPFEM response at a macroscopic level, a lack of detailed data measured at the level of the microstructure has hindered more stringent model validation efforts. Crystal Plasticity Finite Element Models (CPFEM) are one such class more » of micro-mechanical models. The ability to measure these data during deformation in situ makes HEDM an ideal tool for validating micro-mechanical deformation models that make their predictions at the scale of individual grains. The near-field and far-field configurations provide complementary information orientation maps computed from the near-field measurements provide grain morphologies, while the high angular resolution of the far-field measurements provide intergranular strain tensors. High-Energy Diffraction Microscopy (HEDM) is a 3-d x-ray characterization method that is uniquely suited to measuring the evolving micromechanical state and microstructure of polycrystalline materials during in situ processing. (ANL), Argonne, IL (United States) Sponsoring Org.: USDOE Office of Science (SC), Basic Energy Sciences (BES) OSTI Identifier: 1563914 Alternate Identifier(s): OSTI ID: 1562979 Grant/Contract Number: AC02-06CH11357 Resource Type: Journal Article: Accepted Manuscript Journal Name: Acta Materialia Additional Journal Information: Journal Volume: 179 Journal Issue: C Journal ID: ISSN 1359-6454 Publisher: Elsevier Country of Publication: United States Language: English Subject: 36 MATERIALS SCIENCE Fatigue crack growth High-energy X-ray diffraction microscopy Microstructure Polycrystalline material Reciprocal space mapping = ,
Publication Date: Research Org.: Argonne National Lab. (AFRL), Wright-Patterson AFB, OH (United States). Purdue Univ., West Lafayette, IN (United States).Further, this detailed dataset, produced by a suite of X-ray characterization techniques, can provide the necessary validation, at the appropriate length-scale, for SFC models. The findings suggest that both the slip system level stresses and microplasticity events within grains are necessary to get a complete description of the SFC progression. Specifically, the most active slip system in a grain, determined by the maximum resolved more » shear stress, aligns with the crack growth direction and the degree of microplasticity ahead of the crack tip helps to identify directions for potential occurrences of crack arrest or propagation. Analysis of the local micromechanical state in the grains ahead of the crack front is used to rationalize the advancing small crack path and growth rate.
Reciprocal space analysis is used to further examine the deformation state of grains that plasticize in the vicinity of the crack. Cyclic loading is periodically interrupted to conduct far-field HEDM to determine the centroid position, average orientation, and average lattice strain tensor for each grain within the volume of interest. Initial near-field high-energy X-ray diffraction microscopy (HEDM) is used for high-resolution characterization of the grain structure, elucidating grain orientations, shapes, and boundaries. Absorption contrast tomography is used to resolve the intricate 3D crack morphology and spatial position of the crack front. Here in this study, we employ a suite of techniques, based on high-energy synchrotron-based X-ray experiments that allow us to track a nucleated crack, propagating through the bulk of a Ni-based superalloy specimen during cyclic loading. The small fatigue crack (SFC) growth regime in polycrystalline alloys is complex due to the heterogeneity in the local micromechanical fields, which result in high variability in crack propagation directions and growth rates.
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