Understanding Alloy 718: Composition, Properties and Heat Treatment Explained

Alloy 718, often known by the trademark Inconel® 718, is a material that sits at the intersection of strength, stability, and corrosion resistance - a rare combination that has made it indispensable in aerospace, energy, and advanced engineering environments. While our Alloy 718 page offers a concise overview of its availability and basic performance metrics, this article aims to go deeper by offering engineers, metallurgists and technical buyers a more comprehensive understanding of its behaviour, processing, and application logic.

In this guide, we explore the composition of Alloy 718, its nuanced heat treatment behaviour, its mechanical property evolution, and the technical rationale behind when and why to select Alloy 718.

What Makes Alloy 718 Unique?

Alloy 718 belongs to a class of nickel-based superalloys that derive their strength from controlled precipitation hardening. Unlike other austenitic alloys, Alloy 718 maintains its mechanical integrity in conditions involving:

• Long-term exposure to high temperatures (up to 650°C)
• Repeated thermal cycling
• High mechanical loading (tension, fatigue, and creep)
• Aggressive oxidising or corrosive atmospheres

Its exceptional performance in such environments is the result of meticulously balanced alloying and complex phase transformations during heat treatment. This synergy is what sets Alloy 718 apart from similar high-performance materials.

A Closer Look at Alloy 718 Composition

At a glance, Alloy 718 is a Ni-Cr-Fe-Mo-Nb alloy, but its real strength lies in how these elements interact during thermal exposure. While your product datasheet provides exact percentages, the following details why each element is present and what role it plays metallurgically.

Niobium (Nb) & Precipitation Hardening

The addition of niobium is arguably the most defining feature of Alloy 718. When aged, Nb forms a coherent γ″ phase (Ni₃Nb), which is the primary strengthening phase. This disc-shaped intermetallic structure impedes dislocation motion, providing Alloy 718 with its outstanding yield strength and creep resistance.

Unlike the γ′ phase (Ni₃(Al,Ti)), which is more dominant in alloys like Inconel 625 or René 41, the γ″ phase in Alloy 718 provides:

• Slower coarsening at high temperatures
• Improved resistance to thermal softening
• Better stability over longer time exposures

Molybdenum (Mo)

Molybdenum improves solid-solution strengthening, increases resistance to pitting corrosion, and enhances the alloy’s creep strength.

Titanium (Ti) and Aluminium (Al)

These elements assist in forming the secondary γ′ phase, which works in tandem with γ″ to refine the alloy’s hardness and microstructural stability - especially during prolonged service.

Iron (Fe) as Matrix Filler

Iron balances the cost while supporting the alloy’s structural integrity. Unlike higher-purity superalloys, the Fe content in Alloy 718 allows it to be both high-performance and relatively affordable, making it suitable for broader industrial use beyond aerospace.

The Science Behind Alloy 718 Heat Treatment

What makes Alloy 718 heat treatment so technically fascinating is how it walks a fine line between microstructural refinement and stability. The goal is to optimise strength without sacrificing ductility or weldability.

Standard Treatment Cycle (AMS 5662/5663)

Most high-strength applications use a double-ageing treatment:

• Solution Anneal at ~980–1010°C — dissolves previous precipitates and homogenises the structure
• Age 1: ~720°C for 8 hours, slow cool
• Age 2: ~620°C for 8 hours — precipitates fine γ″ and γ′ phases

Controlled Precipitation

The dual-phase strengthening is not just about reaching peak hardness. It’s about managing precipitate morphology and ensuring uniform dispersion, avoiding grain boundary embrittlement, and ensuring long-term structural integrity, even under creep or thermal fatigue.

Welding Considerations

One of Alloy 718’s greatest advantages over similar alloys is its weldability. The heat-affected zone (HAZ) does not suffer from significant microfissuring due to the slow precipitation kinetics of γ″, unlike alloys heavily reliant on γ′.

Mechanical Property Evolution Across Service Temperatures

While our Alloy 719 page lists nominal strength values, it's important to understand the mechanical evolution of Alloy 718 across service conditions:

Tensile and Yield Strength

Even after 1,000 hours at 649°C, Alloy 718 maintains yield strength in the range of 85 ksi, which is exceptional for nickel-based alloys in prolonged exposure.

Fatigue and Creep

The combined γ′/γ″ strengthening ensures:

• Excellent low-cycle fatigue resistance in turbine blades and rotating hardware
• Reliable creep rupture life - especially under steady loads and thermal cycling

Cryogenic Performance

What’s often overlooked is that Alloy 718 also performs well at cryogenic temperatures (down to -253°C), where it maintains high fracture toughness and ductility, making it ideal for cryogenic storage and rocket hardware.

Oxidation and Corrosion Resistance Behaviour

Alloy 718’s corrosion resistance plays a significant role in its suitability for demanding industrial environments. Beyond its strength and fatigue performance, it offers reliable protection against both oxidative and aqueous corrosion, making it ideal for aerospace, marine, chemical processing, and energy sectors.

In oxidising environments, such as jet engine combustion zones or power generation equipment, the alloy forms a stable, adherent oxide layer that limits further scaling. This passive oxide film helps preserve surface integrity and structural performance over time.

Alloy 718 also exhibits strong resistance to:

• Chloride stress corrosion cracking (SCC): A common failure mode in high-chloride, high-temperature environments
• Seawater and marine atmospheres: Suitable for offshore and naval applications
• Sour gas (H₂S) environments: Common in oil & gas production; Alloy 718 remains stable under these conditions
• Acidic media: Resistant to nitric acid and other aggressive oxidisers, thanks to its chromium content

This performance profile contributes significantly to material selection decisions where both mechanical and chemical durability are required.

Machinability and Forming Characteristics

Despite its high strength, Alloy 718 can be successfully machined and formed with the right tooling and processing strategy. These characteristics make it suitable for complex component geometries and high-precision parts in aerospace, energy, and medical fields.

Machining Alloy 718:

Machining is best performed in the annealed condition, prior to final heat treatment. Due to its work-hardening nature and high strength, the use of carbide tools, rigid fixturing, and low feed/speed rates is strongly recommended.

Typical guidance includes:

Use of sharp, coated carbide inserts Low cutting speeds with high feed rates to reduce tool wear Use of coolant and chip breakers to manage heat

Forming Alloy 718:

Cold forming is possible with careful attention to intermediate stress-relieving anneals. The alloy exhibits good ductility in the solution-annealed state, allowing for rolling, bending, and deep drawing operations prior to final ageing treatments.

For critical parts, hot working between 980°C and 1100°C ensures improved workability without compromising grain structure.

These practical considerations make Alloy 718 a flexible yet high-performing material, suitable for CNC machining, aerospace welding, pressure vessels, and formed structural parts.

When to Use Alloy 718: Material Selection Logic

When deciding whether Alloy 718 is the right material, engineers should ask:

If the application involves multiple stresses - mechanical, thermal, and corrosive - over long durations, Alloy 718 often emerges as the most robust and cost-effective choice among superalloys.

Supporting Specifications & Further Reading

While data sheets provide numbers and composition charts, understanding the interplay of phase transformations, microstructure, and alloying strategy is essential for truly optimising Alloy 718 in your application.

NeoNickel is proud to support this level of technical depth with:

• Complete Alloy 718 Product Range
• Expert consultation on form selection, processing, and performance parameters

Alloy 718 is far more than a high-strength metal. It’s a technically complex, phase-stable, multi-purpose material built for environments where other materials fail. Its unique ability to be machined, welded, aged, and deployed across an unusually broad temperature and loading spectrum makes it a staple of high-performance engineering.

Whether you're designing turbine hardware, downhole tools, or cryogenic assemblies, Alloy 718 offers a rare mix of reliability, strength, and endurance.

Get in touch to speak to a specialist or request a quote tailored to your application.