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Abstract
ALLVAC 718Plus is a new commercial superalloy derived from Inconel 718, but possessing a higher temperature capability whilst employing the same philosophy regarding the microstructure. Many articles have been published describing various heat treatments exploiting the precipitation of intermetallic phases at grain boundaries to optimize the mechanical properties over a range of testing conditions. The requirement to further improve the mechanical properties of this alloy drives our interest in the precipitation mechanism of the delta and eta phases found in this alloy. We report the presence of finely layered structures composed of two phases, delta and eta, with distinct structures and chemistries. Possible pathways to explain this precipitation in 718Plus are considered as follows: (i) the sequential formation of the delta from eta phase and (ii) the simultaneous precipitation of both eta and delta facilitated via solute rejection. Both can result in the formation of those small delta layers observed in HRSTEM. We discuss which is most likely by comparing the relative alignment of the phases by image processing and the analysis of the HRSTEM images, and propose formation mechanisms consistent with the distinctive dislocation structures observed at the interface.
Acknowledgements
The authors would like to acknowledge the efforts of the EPSRC, the Department of Materials Science and Metallurgy of University of Cambridge and Rolls-Royce plc for their financial support in the completion of this work (EPSRC – Rolls-Royce plc Strategic Partnership: EP/H500375/1 and EP/H022309/1 and TSB project ‘Siloet’ TP number: AB266C/4). The Department of Materials Science & Metallurgy wishes to acknowledge the financial support of the School of Physical Sciences, University of Cambridge for the Titan electron microscope. Rolls-Royce plc is also gratefully acknowledged for supplying the material. We acknowledge CEA-LETI (Grenoble, France) and FEI company for access to the FEI Tecnai Osiris electron microscope.