Mechanical Properties of the Inconel 718

Aged at 680 and 750 C

C. Slama1, M. Abdellaoui2*

1 E.S.I.E.R, Km 5, Route du Kef, 9070 Medjez El Bab,Tunisie

2 I.N.R.A.P, Ple Technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisie

* Corresponding author. E-mail:

Received : 27 January 2004; revised version accepted :14 October 2004


The relationship between the mechanical properties (microhardness, 0.2 % yield strength, elongation and ultimate tensile strength) and the microstructural state of the superalloy INCONEL 718, previously quenched from 990 C for 30 min, has been determined using transmission electron microscopy. This superalloy was subject of two thermal treatments namely at 680 C and 750 C and thus during three aging duration 4, 50 and 100 h.

For aging at 680 C for 4 h, the above mentioned mechanical properties are affected by the g' phase precipitation. Beyond this aging duration, these latter properties were governed by the abundance and the coarsening of the g" particles in a way that for aging duration lesser than 50 h, the improve of the microhardness and the 0.2 % Y.S. are due to the shearing deformation mechanism as the g" grain size is less than a critical value of 35 nm. Whereas, beyond 50 h , these above mentioned properties decrease following the bypassing deformation mechanism by formation of Orowan loops around the g" precipitates.

However, for 750 C treatment, the microhardness and the 0.2 % Y.S. are first influenced by a competition between the dissolution of the g" strengthening phase and by the formation of a bad d phase. They are secondly influenced by the increase of the g" precipitate size beyond the critical value leading to a deformation following the bypassing mechanism which induces a decrease of the microhardness and the 0.2 % Y.S. In this treatment, the decrease of the elongation was attributed to the formation of brittle MC - carbide along the grain boundaries.

In all cases, the ultimate tensile strength increases when increasing the aging duration as it depends only on the g matrix properties and in no way on the strengthening phases properties.

Keywords : Inconel 718; TEM investigation; Mechanical properties.