Inconel

Inconel is a superalloy composed mainly of nickel, chromium, and iron that is often used in extreme environments where components are subjected to high temperature, pressure, or mechanical loads. Inconel alloys are oxidation- and corrosion-resistant. When heated, Inconel forms a thick, stable passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, making it attractive for high-temperature applications in which aluminum and steel would succumb to creep as a result of thermally-induced crystal vacancies. Inconel's high-temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy.
Inconel was discovered in the early 1930s by scientists at the International Nickel Company. Common trade names for various Inconel alloys include:

Alloy 625: Inconel 625, Chronin 625, Altemp 625, Sanicro 625, Haynes 625, Nickelvac 625 Nicrofer 6020 and UNS designation N06625.
Alloy 600: NA14, BS3076, 2.4816, NiCr15Fe, NiCr15Fe, NiCr15Fe8 and UNS designation N06600.
Alloy 718: Nicrofer 5219, Superimphy 718, Haynes 718, Pyromet 718, Supermet 718, Udimet 718 and UNS designation N07718.
History

The Inconel family of alloys was first developed before December 1932, when its trademark was registered by the International Nickel Company of Delaware and New York. A significant early use was found in support of the development of the Whittle jet engine, during the 1940s by research teams at Henry Wiggin & Co of Hereford, England a subsidiary of the Mond Nickel Company, which merged with Inco in 1928. The Hereford Works and its properties including the Inconel trademark were acquired in 1998 by Special Metals Corporation.

Specific data

Composition

Inconel alloys vary widely in their compositions, but all are predominantly nickel, with chromium as the second element.

Properties

When heated, Inconel forms a thick and stable passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high-temperature applications where aluminium and steel would succumb to creep as a result of thermally induced crystal vacancies. Inconel's high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the alloy. In age-hardening or precipitation-strengthening varieties, small amounts of niobium combine with nickel to form the intermetallic compound Ni₃Nb or gamma double prime. Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures. The formation of gamma-prime crystals increases over time, especially after three hours of a heat exposure of , and continues to grow after 72 hours of exposure.

Strengthening mechanisms

The most prevalent hardening mechanisms for Inconel alloys are precipitate strengthening and solid solution strengthening. In Inconel alloys, one of the two often dominates. For alloys like Inconel 718, precipitate strengthening is the main strengthening mechanism. The majority of strengthening comes from the presence of gamma double prime precipitates. Inconel alloys have a γ matrix phase with a face-centered cubic structure. γ″ precipitates are made of Ni and Nb, specifically with a Ni₃Nb composition. These precipitates are fine, coherent, disk-shaped, intermetallic particles with a tetragonal structure. This is phase will be the most important to stabilize, so Inconel alloys are strengthened by limiting coarsening of γ″ precipitates and limiting the γ″ to delta phase transition.
Secondary precipitate strengthening comes from gamma prime precipitates. The γ' phase can appear in multiple compositions such as Ni₃. The precipitate phase is coherent and has an fcc structure, like the γ matrix; The γ' phase is much less prevalent than γ″. The volume fraction of the γ″ and γ' phases are approximately 15% and 4% after precipitation, respectively. Because of the coherency between the γ matrix and the γ' and γ″ precipitates, strain fields exist that obstruct the motion of dislocations. The prevalence of carbides with MX compositions also helps to strengthen the material. For precipitate strengthening, elements like niobium, titanium, and tantalum play a crucial role.
Because the γ″ phase is metastable, over-aging can result in the transformation of γ″ phase precipitates to δ phase precipitates, their stable counterparts. The δ phase has an orthorhombic structure, a Ni₃ composition, and is incoherent. As a result, the transformation of γ″ to δ in Inconel alloys leads to the loss of coherency strengthening, making for a weaker material. That being said, in appropriate quantities, the δ phase is responsible for grain boundary pinning and strengthening. Composition of Inconel alloys are tuned to maximize the stability of this metastable γ″ phase and to slow this transition. This results in a narrow range of Nb, Al, Ti, in Inconel alloys. These elements are also prone to forming other undesirable phases which segregate at grain boundaries and deplete the matrix of important elements.
Another common phase in Inconel alloys is the Laves intermetallic phase. Its compositions are _x_y and Ni_yNb, it is brittle, and its presence can be detrimental to the mechanical behavior of Inconel alloys. Sites with large amounts of Laves phase are prone to crack propagation because of their higher potential for stress concentration. Additionally, due to its high Nb, Mo, and Ti content, the Laves phase can exhaust the matrix of these elements, ultimately limiting the γ″ phase formation and resulting in decreased precipitate and solid-solution strengthening more difficult.
During ageing, Niobium Carbide precipitation in Alloy 718 occurs predominantly at grain boundaries and proceeds from isolated precipitates to film-like boundary layers along the grain boundaries as temperature and hold time increase. The principal grain-boundary carbide is a niobium-rich MC type. Boundaries containing MC carbides are often bordered by distinct regions lacking γ″ and, to a lesser extent, γ' precipitates. These precipitate-free zones arise from local depletion of solute elements and vacancies near the grain boundary. At around 700–750 °C, the fraction of carbide-covered boundaries grows with time; at higher temperatures, co-precipitation with the δ phase is frequently observed. Progressive boundary coverage by carbides correlates with a shift in fracture mode from transgranular to intergranular, with microcracks often initiating at matrix–carbide interfaces where deformation bands impinge on the boundary.
The precipitation behavior of Inconel 718 is governed by its time–temperature–transformation characteristics, which describe how the microstructure evolves under different heat-treatment conditions. After solution annealing near 1165 °C, most phases such as γ', γ″, δ, and Laves are dissolved back into the face-centered-cubic γ matrix. Controlled aging treatments are then used to selectively precipitate strengthening or stabilizing phases. Short aging at approximately 960 °C encourages the formation of the delta phase along grain boundaries, which refines the grain structure but can reduce the niobium available for γ″ precipitation. This is not favorable as γ″ is the primary precipitate responsible for strengthening. Subsequent aging at about 760 °C followed by a lower-temperature stage near 680 °C precipitates a fine dispersion of coherent gamma prime and γ″ particles throughout the matrix.
At longer times or higher temperatures, the metastable γ″ phase gradually transforms into δ, and coarse niobium-rich Laves or niobium carbide particles can form at grain boundaries. Such transformations consume matrix niobium and lead to over-aging, which decreases strength. Precise control of temperature and hold time during multi-step aging determines the balance between strengthening and microstructural stability in Inconel 718.
For alloys like Inconel 625, solid solution hardening is the main strengthening mechanism. In Inconel 625, the elevated Mo and Cr levels serve as strong solid‐solution matrix stiffeners that enhance creep resistance and high‐temperature strength without relying on the conventional superalloy age-hardening precipitates. Nb and Ta can also contribute to solid solution strengthening to a lesser extent. In solid solution strengthening, Mo atoms are substituted into the γ matrix of Inconel alloys. Because Mo atoms have a significantly larger radius than those of Ni, the substitution creates strain fields in the lattice, which hinder the motion of dislocations, ultimately strengthening the material.
The combination of elemental composition and strengthening mechanisms is why Inconel alloys can maintain their favorable mechanical and physical properties, such as high strength and fatigue resistance, at elevated temperatures, specifically those up to.

Machining

Inconel is a difficult metal to shape and to machine using traditional cold forming techniques due to rapid work hardening. After the first machining pass, work hardening tends to plastically deform either the workpiece or the tool on subsequent passes. For this reason, age-hardened Inconels such as 718 are typically machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required. Alternatively, the majority of the machining can be performed with the workpiece in a "solutionized" form, with only the final steps being performed after age hardening. However some claim that Inconel can be machined extremely quickly with very fast spindle speeds using a multifluted ceramic tool with small width of cut at high feed rates as this causes localized heating and softening in front of the flute.
External threads are machined using a lathe to "single-point" the threads or by rolling the threads in the solution treated condition using a screw machine. Inconel 718 can also be roll-threaded after full aging by using induction heat to without increasing the grain size. Holes with internal threads are made by threadmilling. Internal threads can also be formed using a sinker electrical discharge machining.