Inconel 718
| Inconel 718 [1][2] | |
|---|---|
 A billet of Inconel 718 | |
| Synonym | IN718, Werkstoff 2.4668 |
| Material type | Alloy |
| Alloy properties | |
| UNS identifier | N07718 |
| Alloy type | Nickel-based superalloy |
| Composition |
|
| Physical properties | |
| Density (ρ) | 8.2 g/cm3 |
| Mechanical properties | |
| Young's modulus (E) | 216-99 GPa @ −308–2,000 °F (−189–1,093 °C) |
| Tensile strength (σt) | Rod, bar, plate: 142–160 ksi (979–1,103 MPa) (peak aged) |
| Elongation (ε) at break | Rod, bar, plate: 18-25% (peak aged) |
| Poisson's ratio (ν) | 0.25-0.402 @ −308–2,000 °F (−189–1,093 °C) |
| Thermal properties | |
| Melting temperature (Tm) | 2,300–2,440 °F (1,260–1,338 °C) |
| Thermal conductivity (k) | 77 BTU/(hr·ft⋅°F) @ 70 °F (21 °C) – 196 BTU/(hr·ft⋅°F) @ 2,000 °F (1,093 °C) |
| Specific heat capacity (c) | 0.104 BTU/(lb⋅°F) (0.435 J/g⋅°C) |
Inconel Alloy 718 (UNS designation N07718) 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.[3]
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.[4] Because of its excellent properties, IN718 is commonly used in aerospace, petrochemical, and power generation industries alike.[3]
Microstructure
[edit]
Strengthening Phases
[edit]Inconel 718 is an age-hardenable austenitic alloy. The microstructure of IN718 is made up of a face-centered cubic (FCC) matrix with large amounts of strengthening second phases. The most important of these are the γ' and γ'' nanoscale intermetallics - FCC Ni3(Al, Ti, Nb) and body-centered tetragonal (BCT) Ni3Nb, respectively.[3] 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.[5] 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.[5][6] Unlike many other Ni-based superalloys, the M23C6 carbide is not formed.[7]
Deleterious Phases
[edit]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 (also Ni3Nb), 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.[8] 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.[8] 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.[7]
The Laves phase has the composition (Ni, Fe, Cr)2(Nb, Mo, Ti) 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.[9] 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.[7][9][10]
Applications
[edit]Inconel 718 has a wide array of uses, including, but not limited to:[11][12][13]
- Aircraft turbine engines
- Steam-line piping
- Petrochemical fasteners, valves, and springs
- Rocket engines
- Nuclear power generation
- Aircraft compressor airfoils
- Suppressors
See also
[edit]References
[edit]- ^ "Special Metals - INCONEL Alloy 718" (PDF). Special Metals. 2007.
- ^ "Special Metals INCONEL® Alloy 718". ASM (Aerospace Specification Metals) Inc.
- ^ a b c Hosseini, E.; Popovich, V. A. (2019-12-01). "A review of mechanical properties of additively manufactured Inconel 718". Additive Manufacturing. 30 100877. doi:10.1016/j.addma.2019.100877. ISSN 2214-8604.
- ^ Eiselstein, H. L.; Tillack, D. J. (1991). "The Invention and Definition of Alloy 625". Superalloys: 1–14. doi:10.7449/1991/SUPERALLOYS_1991_1_14. ISBN 0-87339-173-X.
- ^ a b Lee, Gang Ho; Kim, Byoungkoo; Jeon, Jong Bae; Park, Minha; Noh, Sanghoon; Kim, Byung Jun (2025-02-01). "Precipitate phase behavior and mechanical properties of Inconel 718 according to aging heat treatment time". Materials Science and Engineering: A. 924 147776. doi:10.1016/j.msea.2024.147776. ISSN 0921-5093.
- ^ Mitchell, A. (2010), "Primary Carbides in Alloy 718", Superalloy 718 and Derivatives, John Wiley & Sons, Ltd, pp. 161–167, doi:10.1002/9781118495223.ch11, ISBN 978-1-118-49522-3, retrieved 2025-11-02
- ^ a b c Radavich, John (1989). "The Physical Metallurgy of Cast and Wrought Alloy 718". Superalloy 718 - Metallurgy and Applications: 229–240. doi:10.7449/1989/superalloys_1989_229_240. ISBN 0-87339-097-0.
- ^ a b Azadian, Saied; Wei, Liu-Ying; Warren, Richard (2004-09-01). "Delta phase precipitation in Inconel 718". Materials Characterization. 53 (1): 7–16. doi:10.1016/j.matchar.2004.07.004. ISSN 1044-5803.
- ^ a b Schirra, John; Caless, Robert; Hatala, Robert (1991). "The Effect of Laves Phase on the Mechanical Properties of Wrought and Cast + HIP Inconel 718". Superalloys 718, 625 and Various Derivatives: 375–388. doi:10.7449/1991/superalloys_1991_375_388. ISBN 0-87339-173-X.
- ^ Devendranath Ramkumar, K.; Jagat Sai, R.; Sridhar, G.; Santhosh Reddy, V.; Prabaharan, P.; Arivazhagan, N.; Sivashanmugham, N. (2015-02-01). "Influence of Filler Metals in the Control of Deleterious Phases During the Multi-pass Welding of Inconel 718 Plates". Acta Metallurgica Sinica (English Letters). 28 (2): 196–207. doi:10.1007/s40195-014-0185-5. ISSN 2194-1289.
- ^ deBarbadillo, J.j.; Mannan, S.k. (2010), "Alloy 718 for Oilfield Applications", Superalloy 718 and Derivatives, John Wiley & Sons, Ltd, pp. 579–593, doi:10.1002/9781118495223.ch45, ISBN 978-1-118-49522-3, retrieved 2025-11-03
- ^ Jewett, R. P.; Halchak, J. A (1991). "The Use of Alloy 718 in the Space Shuttle Main Engine". Superalloys 718, 625 and Various Derivatives: 749–760. doi:10.7449/1991/superalloys_1991_749_760. ISBN 0-87339-173-X.
- ^ Schafrik, Robert; Ward, Douglas; Groh, Jon (2001). "Application of Alloy 718 in GE Aircraft Engines: Past, Present and Next Five Years" (PDF). Superalloys 718, 625, 706 and Various Derivatives: 1–11. doi:10.7449/2001/Superalloys_2001_1_11. ISBN 0-87339-510-7.