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Advances in Materials Science and Engineering
Volume 2011 (2011), Article ID 940634, 9 pages
http://dx.doi.org/10.1155/2011/940634
Research Article

TEM Study of High-Temperature Precipitation of Delta Phase in Inconel 718 Alloy

1IJL-SI2M UMR 7198 CNRS-Nancy Université-UPV Metz, École des Mines de Nancy, Parc de Saurupt, CS 14234, 54042 Nancy Cedex, France
2Institut Carnot CIRIMAT, Université de Toulouse, INP-ENSIACET, 4 allée Emile Monso, BP 44362, 31030 Toulouse Cedex 4, France

Received 31 March 2011; Accepted 12 May 2011

Academic Editor: Joseph Lai

Copyright © 2011 Moukrane Dehmas et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Inconel 718 is widely used because of its ability to retain strength at up to 650C for long periods of time through coherent metastable 𝛾 Ni3Nb precipitation associated with a smaller volume fraction of 𝛾 Ni3Al precipitates. At very long ageing times at service temperature, 𝛾 decomposes to the stable Ni3Nb 𝛿 phase. This latter phase is also present above the 𝛾 solvus and is used for grain control during forging of alloy 718. While most works available on 𝛿 precipitation have been performed at temperatures below the 𝛾 solvus, it appeared of interest to also investigate the case where 𝛿 phase precipitates directly from the fcc matrix free of 𝛾 precipitates. This was studied by X-ray diffraction and transmission electron microscopy (TEM). TEM observations confirmed the presence of rotation-ordered domains in 𝛿 plates, and some unexpected contrast could be explained by double diffraction due to overlapping phases.