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Title: Residual stress and microstructure depth gradients in nitrided iron-based alloys revealed by dynamical cross-sectional transmission X-ray microdiffraction
Authors: Kurz S.J.B.
Meka, Sai Ramudu
Schell N.
Ecker W.
Keckes J.
Mittemeijer E.J.
Published in: Acta Materialia
Abstract: Thermochemical surface treatments such as nitriding result in the formation of complex near-surface gradients of phase fractions, residual stress and microstructure, which influence the structural and functional properties of the material decisively. In this work, a novel cross-sectional synchrotron microdiffraction method is used to characterize such gradients by analyzing cross-sections of nitrided Fe-Al and Fe-V alloy specimens down to a depth of 500 μm, with a depth resolution of less than 10 μm. The Debye-Scherrer diffraction data collected from the individual sample depths document very different nitride-precipitation mechanisms and resulting stress gradients. In nitrided Fe-Al samples, the delayed precipitation of largely incoherent AlN particles leads initially to the development of internal microcracks, followed by a pronounced increase of compressive stress until plastic deformation sets in, which finally results in the formation of regions with tensile and compressive stress. In contrast, the VN precipitation during nitriding of Fe-V alloys occurs very quickly and generates a desired high compressive stress at the surface of the nitrided part. The tiny and coherent VN precipitates increase the yield strength of the nitrided zone significantly. The evaluated corresponding ferrite-lattice parameter depth profiles can be quantitatively described as the outcome of (competing) effects of solute (Al, V) depletion, (excess) nitrogen dissolution and the emergence of a hydrostatic strain due to elastic accommodation of the precipitate/matrix misfit. The novel technique to expose depth gradients in real space, with micrometer resolution, opens the way to understand the development of microstructure and stress upon (thermochemical) surface treatment. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Citation: Acta Materialia, 87: 100-110
Issue Date: 2015
Publisher: Elsevier Ltd
Keywords: Depth profiles
High-energy X-ray diffraction
Residual stress
ISSN: 13596454
Author Scopus IDs: 55636332900
Author Affiliations: Kurz, S.J.B., Max Planck Institute for Intelligent Systems, Germany
Meka, S.R., Max Planck Institute for Intelligent Systems, Germany
Schell, N., Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Germany
Ecker, W., Materials Center Leoben Forschung GmbH, Leoben, A-8700, Austria
Keckes, J., Department of Materials Physics, Montanuniversität Leoben, Austria
Mittemeijer, E.J., Max Planck Institute for Intelligent Systems, Germany, Institute for Materials Science, University of Stuttgart, Germany
Funding Details: The authors are grateful to Mr Meisner and Mr Weible for specimen preparation, Mr Kress and Mr Engelhardt for nitriding experiments, Mr Werner for chemical analysis, Mrs Haug for EPMA measurements and Dr Hosmani for providing the Fe–V alloy specimens (all within the MPI for Intelligent Systems, Stuttgart, Germany). W.E. and J.K. appreciate financial support from the Austrian Federal Government (in particular from Bundesministerium für Verkehr, Innovation und Technologie and Bundesministerium für Wirtschaft, Familie und Jugend) represented by Österreichische Forschungsförderungsgesellschaft mbH and the Styrian and the Tyrolean Provincial Government, represented by Steirische Wirtschaftsförderungsgesellschaft mbH and Standortagentur Tirol, within the framework of the COMET Funding Program. Österreichische Forschungsförderungsgesellschaft, FFG; Bundesministerium für Verkehr, Innovation und Technologie, BMVIT; Steirische Wirtschaftsförderungsgesellschaft, SFG
Corresponding Author: Kurz, S.J.B.; Max Planck Institute for Intelligent SystemsGermany
Appears in Collections:Journal Publications [MT]

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