Nickel Alloy 718

The Nickel base alloy 718 is a frequently used precipitation hardenable material that was designed for high high yield strength, high strain hardening, high tensile strength and excellent creep-rupture properties at temperatures up to 730°C. The relatively slow age-hardening response of this alloy allows heat treatment and welding procedures without spontaneous hardening during heating and cooling. This alloy has excellent weldability when compared to the conventional nickel-base superalloys hardened by aluminum and titanium. Alloy 718 is often used for jet engine and high-speed airframe parts such as wheels, buckets, spacers, and high temperature bolts and fasteners.

The Nickel-based alloy 718 is recently also widely used for critical applications under extreme conditions such as sour gas environments in the oil and gas industry, due to its room temperature strength and high aqueous corrosion resistance.

Alloy 718 is a precipitation-hardened material strengthened by an ordered body-centered tetragonal (BCT) phase (primary strengthening precipitate), and an ordered face-centered cubic (FCC) phase. Beside these two coherent matrix-strengthening phases, incoherent precipitates of delta particles also tend to form at grain boundaries.

 

Element

Min

Max

Carbon

--

0.08

Manganese

--

0.35

Silicon

--

0.35

Phosphorus

--

0.015

Sulfur

--

0.015

Nickel + Cobalt

50.0

55.0

Chromium

17.0

21.0

Cobalt

--

1.00

Iron

Balance

Aluminum

0.35

0.80

Molybdenum

2.80

3.30

Titanium

0.65

1.15

Boron

0.001

0.006

Copper

--

0.15

Cb + Ta

4.75

5.50

 

 

 

Multi-scale and spatially resolved hydrogen mapping in a NieNb model alloy reveals the role of the d phase in hydrogen embrittlement of alloy 718
Acta Materialia 109 (2016) 69:
Here we investigated the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary NieNb model alloy to study the role of d phase in hydrogen embrittlement of alloy 718. We focus on two aspects, namely, (1) mapping the hydrogen distribution with spatial resolution enabling the observation of the relations between desorption profiles and desorption sites; and (2) correlating these observations with mechanical testing results to reveal the degradation mechanisms.
Acta Materialia 109 (2016) 69 Multiscale[...]
PDF-Dokument [2.6 MB]
Hydrogen-assisted failure in Ni-based superalloy 718 studied under in situ hydrogen charging: The role of localized deformation in crack propagation
Acta Materialia 128 (2017) 365-374
Acta Materialia 128 (2017) 365 Hydrogen-[...]
PDF-Dokument [5.3 MB]

In this project we investigated hydrogen embrittlement in Ni-based superalloy 718 by tensile testing at slow strain rate under continuous electrochemical hydrogen charging. Hydrogen-assisted cracking mechanisms were studied via electron backscatter diffraction (EBSD) analysis and electron channeling contrast
imaging (ECCI). In order to elucidate the effects of stress or strain in the cracking mechanisms, material conditions with different strength levels were investigated, including samples in solution annealed (as water quenched) and 780°C age-hardened states. The microstructure observations in the vicinity of the cracks enabled us to establish correlations between the microstructure, crack initiation sites, and crack propagation pathways. Fracture in the hydrogen-charged samples was dominated by localized plastic deformation. Strain-controlled transgranular cracking was caused by shear localization due to hydrogen-enhanced localized plasticity (HELP) and void nucleation and coalescence along {111} slip planes in both, the solution annealed and age-hardened materials. Stress-assisted intergranular cracking in the presence of hydrogen was only observed in the high strength age-hardened material, due to slip localization at
grain boundaries, grain boundary triple junction cracking, and delta/gamma-matrix interface cracking. To investigate the effect of delta-phase in crack propagation along grain boundaries, the over-aged state (aged at 870°C) with different precipitation conditions for the d-phase was also investigated. Observations confirmed that presence of delta-phase promotes hydrogen-induced intergranular failure by initializing micro-cracks from delta/gamma interfaces.

 

 

Acta Mat. 2011, 59, p. 364