Inconel 718


Inconel Alloy 718 is one of the most commonly used nickel-based superalloys, an alloy class defined by high strength and resistance to elevated temperatures, corrosion, and oxidation. IN718 is especially designed for fatigue and creep resistance at temperatures up to 700°C.
Inconel 718 was incidentally developed in the 1960s during INCO's development of Inconel 625. The purpose of its development was to create material that could be used for steam-line piping. Because of its excellent properties, IN718 is commonly used in aerospace, petrochemical, and power generation industries alike.

Microstructure

Strengthening Phases

Inconel 718 is an age-hardenable austenitic alloy. The microstructure of IN718 is made up of a face-centered cubic matrix with large amounts of strengthening second phases. The most important of these are the γ' and γ nanoscale intermetallics - FCC Ni3 and body-centered tetragonal Ni3Nb, respectively. The γ' phase is fully coherent in the FCC matrix, making it highly stable, even with extended exposure to elevated temperatures. The γ' phase acts as a barrier to dislocation motion, and forms anti-phase boundaries when sheared, thus increasing the energy required to plastically deform the alloy. The γ phase offers even greater strengthening, as the FCC-BCT lattice mismatch imparts a large coherency hardening effect. Carbides are also an important strengthening phase, primarily in the form MC, but also as M2C and M7C3, where M is a major alloying element. These carbides typically form on the alloy's grain boundaries, thus pinning them and inhibiting grain boundary sliding, a process required for low temperature diffusional creep. Unlike many other Ni-based superalloys, the M23C6 carbide is not formed.

Deleterious Phases

In addition to the strengthening phases described above, there are multiple phases that weaken IN718, including the δ phase and the Laves phase. The δ phase has the composition Ni3Nb and an orthorhombic crystal structure. It forms between the temperatures of 700°C and 1000°C. with a peak precipitation rate at ~900°C. The δ phase is more stable than the γ phase, though it precipitates very sluggishly. Hence, the δ phase will not precipitate until it is kinetically favorable, and γ will be lost as a result. The δ phase typically nucleates at grain boundaries and grows as thin plates into the grain. The presence of δ indicates a loss of γ and thus lessens the alloy's hardenability. Additionally, it has been shown that the δ phase is associated with an increased susceptibility to hot cracking. The δ phase can, however, be used to strengthen the alloy during processing. Because it nucleates at grain boundaries, it can be used to pin then during forging, thus controlling grain size.
The Laves phase has the composition 2 and a hexagonal topologically close-packed structure. It forms when the alloy is subjected to temperatures above ~1000°C. The Laves phase forms in large globular aggregates within the IN718 matrix. Because the Laves phase is significantly richer in Nb than any of the strengthening phases, its presence requires a depletion of γ' and γ, and thus weakens the material significantly. Additionally, the Laves phase is very brittle and thus reduces the toughness by acting as a crack nucleation site for fracture. Furthermore, it can reduce IN718's mechanical properties through melting and microfissuring.

Applications

Inconel 718 has a wide array of uses, including, but not limited to: