Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Micrograph Library Full Record for Micrograph 717

Full Record for Micrograph 717

Link to full size image of micrograph 717
[197 KB]

View micrograph
.. in new window

View micrograph and record
.. in new window

You can also view and download the micrographs on Flickr
Micrograph no
Brief description
IN718 nickel-based superalloy held for 6 hours at 850°C
alloy Link to MATTER Glossary entry for alloy, metal, nickel, nickel-based superalloy, niobium, precipitation Link to MATTER Glossary entry for precipitation
Metal or alloy
Ni 53, Fe 19, Cr 18, Nb 5 (wt% approx) + small amounts of Ti, Mo, Co, Al
Standard codes
Held for 6 hours at 850°C.
High temperature turbines.
Sample preparation
Grind on 240 grit SiC paper followed by polish on 9<span class="symbol">m</span>m, 6<span class="symbol">m</span>m, 1<span class="symbol">m</span>m, and OPS.
Reflected light microscopy, cross-polarised, with Nomarski filter
Length bar
25 μm
Further information
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, g, and the precipitates, g', g'', and d.

The primary strengthening phase is g'' [1], the composition of which is Ni3Nb. It has a body-centred tetragonal structure, and forms semi-coherently as disc-shaped platelets within the g 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 g', Ni3Al (between 650°C and 850°C), and d, the same composition as g'' (between 750°C and 1000°C). It has been commented that, at large volume fractions and when it forms continuously along grain boundaries, d is detrimental to both strength and toughness [2].

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

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

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

The micrograph shows the microstructure after being held for 6 hours at 850°C, the volume fraction of d is greater than when held for shorter periods of time.


[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.

R Guest
Rolls-Royce University Technology Centre, Department of Materials Science and Metallurgy, University of Cambridge
Licence for re-use
Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales
Related micrographs