Nickel-base superalloy 718 microstructure characterization

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Carpenter Technology&#;s 718 is a precipitation hardenable (PH) nickel-base superalloy. It is made of a γ matrix strengthened by a combination of Ni3(Ti,Al)-γ&#; (gamma prime) and Ni3Nb-γ&#;&#; (gamma double prime) precipitates and also contains carbides and the Nb-rich δ phase. It is a workhorse alloy in the aerospace and oil and gas industries because of its exceptional high-temperature strength and creep resistance, as well as good corrosion resistance. Critical to manufacturing an alloy with such high performance is understanding the processing-properties-microstructure relationship, and materials characterization sits at the center of that understanding.

Figure 1. The processing-properties-microstructure relationship can be envisioned as a pyramid supporting material performance at the apex.

Carpenter Technology&#;s materials characterization expertise is centered in the R&D Characterization Laboratory. Here, samples of 718 are prepared and polished for either light optical observation (LOM) or scanning electron microscopy (SEM). For LOM, samples are usually etched with Waterless Kallings or Lucas&#; Reagent, specialty etchants to reveal the grain size and microstructure. Some features are too fine for optical microscopy and must be observed by SEM. These observations about the alloy&#;s microstructure reveal critical information about its processing and properties, and the link between them.

Grain structure

A fully recrystallized microstructure with a fine uniform grain is necessary to yield good mechanical properties. When grains are too fine, the alloy strength suffers, and when grains are too coarse, the stress rupture properties suffer. Large un-recrystallized grains (URGs) have a much larger size than the average (Figure 2) and indicate that the solution temperature was too low to achieve full recrystallization. Unrecrystallized grains severally affect stress rupture properties, but their presence in the billet can be identified during sonic inspection.

Figure 2. Unrecrystallized grain (URG) within an otherwise uniform grain structure, etched with Waterless Kallings.

718 phases

Gamma double prime (γ&#;&#;)
Unlike most nickel-base superalloys that are γ&#; strengthened, 718 is mainly strengthened by γ&#;&#; precipitates in the peak age condition. γ&#;&#; has an ordered DO22 structure and a Ni3Nb composition and appears as nanometer scale discs under the transmission electron microscope (TEM). (Figure 3).

Figure 3. TEM dark field image of gamma double prime.

Gamma prime (γ&#;)
γ&#; plays a minor role in the strengthening of 718. It has a Ni3(Al,Ti) composition, is spherical in shape, and appears as small dots within the matrix.

Delta Phase (δ)
δ is a Nb-rich phase with an ordered orthorhombic structure and appears as long needles on grain boundaries, twin boundaries, or within grains (Figure 4-6). A small amount of delta phase is beneficial to control grain growth during solution and aging and can deliberately be precipitated during a heat treatment called &#;delta dump.&#; However, too much delta phase is detrimental and decreases the fracture toughness, strength, and creep resistance.

Figure 4. No visible delta phase, etched with Waterless Kallings.

Figure 5. Some grain boundary delta phase on the grain boundaries, etched with Waterless Kallings.

Figure 6. Abundant delta phase, etched with Waterless Kallings.

Carbides
In addition to the formation of γ&#;&#; and γ&#; phases during aging, small secondary carbides precipitate and grow, increasing 718 hardness. Small secondary carbides are visible using the SEM. In addition to the small secondary carbides, large blocky carbonitrides are present in the microstructure. Ti-rich carbonitrides appear peach in color, while Nb-rich carbonitrides are grey. These large, blocky carbides can act as crack initiation sites, reducing fatigue performance. Under the right heat treat conditions, these carbonitrides can precipitate out as a grain boundary film that degrades ductility.

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The superalloy of choice

Fifty-five years after its invention, 718 remains the predominant superalloy choice for many high temperature and extreme environment applications. In gas turbines and jet engines, where increased operating temperatures mean increased engine efficiency, 718 is used extensively in parts that are subject to high temperature fatigue, stress rupture, and creep loading. There is also a large market for 718 in the oil and gas industries where the high pressure and high temperature corrosive fluids used in drilling require high strength and corrosion resistance. With its high performance under extreme environments, 718 remains relevant and integral to power generation, aerospace, and oil and gas industries.

Get in touch with the alloy experts at Carpenter Technology to ask any questions about your unique application!

Nickel Alloys We offer:

200/201 &#; Nickel 200 is commercially pure which means that it is at least 99.6% nickel. It boasts great resistance to chemicals and corrosive elements. Therefore, it is one of the best nickel based alloys for maintaining purity, 201 is the low carbon version of Nickel 200.

400/405 &#; Cold rolling usually hardens Nickel 400, which is a solid solution nickel alloy. Various industries, such as marine and chemical processing, widely use this alloy.

600/601 &#; Nickel 600 is 72% nickel and primarily used for situations that require high resistance to corrosion and heat. Therefore, this is a great balance between being easy to work with while also being highly durable and sturdy.

625 &#; Nickel 625 is 58% percent nickel by default with the other majority metal being Chromium at almost 20%. Furthermore, this nickel alloy is created to withstand cryogenic and ~ degree temperatures and has high corrosion resistance and excellent fabricability.

718 &#; Nickel 718 is a combination of nickel and cobalt and must be a minimum of 50% nickel. In fact, we know 718 for its ability to resist post-weld cracking.

800H &#; Nickel 800H is only 30-35% nickel by default and is an amalgamation between Chromium and Iron, which has a minimum composition of 39.5%. Initially designed with the intention to marry the benefits of all of the listed metals, this nickel steel alloy guarantees high-temperature strength, maintaining chemical properties, resistance to oxidation and high temperatures as well as high-temperature corrosion.

825 &#; Nickel 825 comprises a minimum of 38% nickel and ~20% chromium and a minimum of 22% chromium. Furthermore, this alloy is designed to resist corrosive elements and provides high resistance for stress corrosion cracking.

We Also Offer

901 &#; Nickel 901 comprises 42.5% nickel and only 12.5% chromium with a balancing element of Iron. Whereas, the fluctuation of iron suggests that relying solely on iron as the dominant factor over the others listed in the article is not advisable.

K500 &#; Nickel K500 is 63% nickel minimum and is an excellent corrosion resistance piece of nickel alloy steel. Aluminum and titanium are added to the base. Therefore, it allows it to work well under heat intense situations.

L605 &#; Nickel L605 is a mixed metal by being 9% nickel, 14% Tungsten, and 19% chromium at their minimums. This is why it&#;s a solid nickel alloy with excellent high-temperature strength able to resist up to degrees. This is the most ideal for gas turbines such as rings, blades, and combustion chambers.

Alloy C276 &#; This mixed metal is an &#;aggressive&#; media metal. It is mostly chromium, molybdenum, iron, tungsten, and nickel. Localized corrosion resistance is high, and low carbon minimizes precipitation during welding.

Alloy X-750 &#; Alloy X-750 is 70% nickel minimum with the other top elements being chromium and iron. This nickel alloy is ideal for welding because it hardens during weld precipitation.

Alloy A286 &#; Alloy A286 has significant oxidation resistance and strength as well as being able to withstand temperatures near degrees. Made of 24% nickel, 13% chromium and elements of titanium, it makes it a very versatile metal.

Alloy 188 &#; Alloy 188 is another chromium-nickel-tungsten nickel alloy mixture that has a resistance of up to degrees and is adamant under such vicious conditions.

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