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      Predicting dwell fatigue life in titanium alloys using modelling and experiment

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          Abstract

          Fatigue is a difficult multi-scale modelling problem nucleating from localised plasticity at the scale of dislocations and microstructure with significant engineering safety implications. Cold dwell fatigue is a phenomenon in titanium where stress holds at moderate temperatures lead to substantial reductions in cyclic life, and has been implicated in service failures. Using discrete dislocation plasticity modelling complemented by transmission electron microscopy, we successfully predict lifetimes for ‘worst case’ microstructures representative of jet engine spin tests. Fatigue loading above a threshold stress is found to produce slip in soft grains, leading to strong dislocation pile-ups at boundaries with hard grains. Pile-up stresses generated are high enough to nucleate hard grain basal dislocations, as observed experimentally. Reduction of applied cyclic load alongside a temperature excursion during the cycle lead to much lower densities of prism dislocations in soft grains and, sometimes, the elimination of basal dislocations in hard grains altogether.

          Abstract

          Material fatigue is the most common source behind failures of mechanical structures. Here the authors combine transmission electron microscopy, high-resolution electron backscatter diffraction and discrete dislocation plasticity modeling to study the underlying mechanism of dwell fatigue in titanium alloys.

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          Most cited references 49

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          XLI. The equilibrium of linear arrays of dislocations.

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            High-resolution elastic strain measurement from electron backscatter diffraction patterns: new levels of sensitivity.

            In this paper, we demonstrate that the shift between similar features in two electron backscatter diffraction (EBSD) patterns can be measured using cross-correlation based methods to +/- 0.05 pixels. For a scintillator screen positioned to capture the usual large solid angle employed in EBSD orientation mapping this shift corresponds to only approximately 8.5 x 10(-5)rad at the pattern centre. For wide-angled EBSD patterns, the variation in the entire strain and rotation tensor can be determined from single patterns. Repeated measurements of small rotations applied to a single-crystal sample, determined using the shifts at four widely separated parts of the EBSD patterns, showed a standard deviation of 1.3 x 10(-4) averaged over components of the displacement gradient tensor. Variations in strains and rotations were measured across the interface in a cross-sectioned Si1-x Gex epilayer on a Si substrate. Expansion of the epilayer close to the section surface is accommodated by tensile strains and lattice curvature that extend a considerable distance into the substrate. Smaller and more localised shear strains are observed close to the substrate-layer interface. EBSD provides an impressive and unique combination of high strain sensitivity, high spatial resolution and ease of use.
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              Discrete dislocation plasticity: a simple planar model

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                Author and article information

                Contributors
                yilun.xu@imperial.ac.uk
                fionn.dunne@imperial.ac.uk
                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group UK (London )
                2041-1723
                17 November 2020
                17 November 2020
                2020
                : 11
                Affiliations
                [1 ]GRID grid.7445.2, ISNI 0000 0001 2113 8111, Department of Materials, , Imperial College, ; London, SW7 2AZ UK
                [2 ]GRID grid.4991.5, ISNI 0000 0004 1936 8948, Department of Materials, , University of Oxford, ; Oxford, OX1 3PJ UK
                [3 ]GRID grid.1121.3, ISNI 0000000403961069, Rolls-Royce plc, ; Derby, DE24 8BJ UK
                Article
                19470
                10.1038/s41467-020-19470-w
                7672227
                33203830
                615bd6fb-6b3d-4abc-b2af-c0ee50cfccbd
                © The Author(s) 2020

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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                engineering, materials science

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