Download Title: Co-precipitation analysis of Inconel 718 superalloy by X

Title: Co-precipitation analysis of Inconel 718 superalloy by X-ray mapping
in scanning transmission electron microscopy (STEM)
Authors: Luiz Henrique de Almeida1, Lena Valle1, Carla Brandão Woyames1, Jean Dille2
1. Universidade Federal do Rio de Janeiro, Rio de Janeiro, Rio de Janeiro, Brazil
2. Université Libre de Bruxelles, Brussels, Belgium
Inconel 718 is a nickel-iron-chromium superalloy, widely used in aerospace as well as nuclear
and oil industries. The γ’ and γ” phases are responsible for the high temperature strength of
Inconel 718. The coherent ” phase (Ni3Nb; bct structure, elongated disc-like morphology) is
the primary strengthening phase, and the coherent ’ phase (Ni3(Al,Ti); fcc structure, spherical
morphology) is less effective for strengthening [1]. If the alloy is kept between approximately
700 and 900°C, dissolution of the metastable ” phase may occur, allowing the precipitation of
the stable δ phase (Ni3Nb; orthorhombic structure, platelet morphology) [2]. Although the
literature reports that the δ phase precipitation is responsible for the loss in the mechanical
strength of the Inconel 718, previous work [3] showed that this phase does not influence the
tensile strength, yield strength, and hardness values at room temperature. There is only a
slight influence on the ductility of this alloy. The γ’ and γ” phases precipitate simultaneously or
sequentially, depending on the chemical composition of the alloy, between 600 and 900oC. The
sequence of precipitation of γ′ and γ″ phases is governed by the relative concentration of Al, Ti
and Nb. In some cases, γ’ phase precedes the γ” precipitation, resulting on the formation of
the co-precipitate γ’/γ” [4,5]. While these phases are together in the same precipitate, there is
a relative difficulty in identifying them separately. For this reason, the material used in this
work was subjected to an 800°C isothermal aging heat treatment for 6 hours, in order to
precipitate γ’ and γ” phases.
The samples were prepared in a twin-jet electropolisher in a chemical solution of 10 %
perchloric acid and 90 % ethanol at 30V and -20°C.
The use of scanning transmission electron microscopy (STEM) images, obtained with a FEI
Titan G2 instrument at 200 kV, coupled to X-ray mapping with Bruker ChemSTEM technology,
allowed to distinguish γ’ from γ” (figure 1). These phases were clearly identified by the
predominant colors that indicates high concentration of aluminum and niobium, for γ’ and γ”,
respectively. The study showed that this is the most fast and precise technique to identify each
phase individually and it is certainly important to quantify the phases present in this alloy.
References:
[1] COZAR, R.; PINEAU, A., Metallurgical Transactions, v. 4, p. 47-59, 1973.
[2] AZADIAN,S., WEI,L.Y., WARREN,R., Materials Characterization. 53, 2004, p 7-16.
[3] VALLE, L. C. M. et al., Journal of Materials Engineering and Performance, v. 22, p. 15121518, 2013.
[4]NALAWADE, S. A. et al., Materials Science and Engineering A, v. 527, p. 2906-2909, 2010.
[5] PING, D. H. et al., Materials Science and Engineering A, v. 456, p. 99-102, 2007.
Acknowledgements: Finep, Fapperj and Cnpq for financial support.
Co-precipitate γ’/γ”
20 nm
Figure 1. MET micrograph of alloy Inconel 718, showing the present of the γ’ (spherical
morphology) and γ” (elongated disc morphology). These phases were clearly identified
individually by the predominant colors that indicates high concentration of aluminum (yellow)
and niobium (green), for γ’ and γ”, respectively. This showed the co-precipitation of γ’/γ”.