Image icon  IN718 nickel-based superalloy held for 72 hours at 850°C

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Description: IN718 is a nickel-based superalloy composed of approximately 53 wt% Ni, 19 wt% Fe, 18 wt% Cr, 5 wt% Nb, and small amounts of Ti, Mo, Co, and Al. The alloy has a number of distinct phases present in its microstructure. These are namely the matrix, γ, and the precipitates, γ', γ'', and δ.

The primary strengthening phase is γ'' [1], the composition of which is Ni3Nb. It has a body-centred tetragonal structure, and forms semi-coherently as disc-shaped platelets within the γ matrix, having three variants lying on the {100} planes. It is stable for over 10,000 hours at 600°C; however, above this temperature it decomposes to form γ', Ni3Al (between 650°C and 850°C), and δ, the same composition as γ'' (between 750°C and 1000°C). It has been commented that, at large volume fractions and when it forms continuously along grain boundaries, δ is detrimental to both strength and toughness [2].

The δ phase that forms is more stable than the γ'' phase, and has an orthorhombic structure. During cooling, δ phase precipitates begin to form along the {111} planes in the matrix, nucleating at grain boundaries at approximately 1010°C.

The γ' precipitates, of L12 structure, are seen within the γ matrix when it has been depleted of niobium due to the formation of γ'' and δ.

The Time-Temperature-Transformation (TTT) diagram for IN718 shows that δ precipitates at higher temperatures than the γ'', and that over long periods of time both δ and γ' are more stable than γ''.

The micrograph shows the microstructure after being held for 72 hours at 850°C, the long precipitates are seen to have become more spheroidal than after shorter periods. This occurs because the spherical shapes have a lower surface area to volume ratio than the needles and are, therefore, more thermodynamically stable.


[1] J. W. Brooks and P. J. Bridges. Metallurgical Stability of Inconel Alloy 718. Superalloys '88, pages 33-42, 1988.
[2] B. Gleeson. High-Temperature Corrosion of Metallic Alloys and Coatings: Volume II. In M. Schütze, editor, Corrosion and Environmental Degradation, volume 19 of Materials Science and Technology, chapter 5, pages 173 - 228. Wiley, 2000.

Keywords: alloy • metal • nickel • nickel-based superalloy • niobium • precipitation
Categories: Science approaches > Microstructure
Materials > Metals & alloys > Nickel & alloys
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: DoITPoMS, University of Cambridge
R Guest, Rolls-Royce University Technology Centre, Department of Materials Science and Metallurgy, University of Cambridge
Published by: DoITPoMS, University of Cambridge
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Date created: 28 January 2003
Date added: 21 August 2009
Resource ID: 1749