DoITPoMS

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 3 hours at 850°C, the volume fraction of d is greater than when held for shorter periods of time.

References:

[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.