Effects of Composition, Processing and Structure on Properties of Nickel and Nickel Alloys

Nickel and nickel alloys are used in the chemical processing, pollution control, power generation, electronics, and aerospace industries, which are taking advantage of their excellent corrosion, oxidation, and heat resistance.

Nickel is ductile and can be made by the conventional processing methods into cast, P/M, and various wrought products: bar/wire, plate/sheet, and tube. Commercially pure nickel has moderately high values of melting temperature (1453°C), density (8.902 g/cm³), and elastic modulus (204 GPa). It is ferromagnetic, with a Curie temperature of 358°C (676°F) and good electrical (25% IACS) and thermal conductivity (82.9 W/m K, or 48 Btu/ft h °F).

Elemental nickel is used principally as an alloying element to increase the corrosion resistance of commercial iron and copper alloys; only about 13% of annual consumption is used in nickel-base alloys. Approximately 60% is used in stainless steel production, with another 10% in alloy steels and 2.5% in copper alloys. Nickel is also used in special-purpose alloys: controlled expansion, electrical resistance, magnetic, and shape memory alloys.

Effects of Alloying Elements in Nickel Alloys

Nickel has an face-centered cubic crystal (fcc) structure, to which it owes its excellent ductility and toughness. Because nickel has extensive solid solubility for many alloying elements, the microstructure of nickel alloys consists of the fcc solid-solution austenite (γ) in which precipitate particles can form.

Nickel forms a complete solid solution with copper and has nearly complete solubility with iron. It can dissolve about 35% Cr, about 20% each of molybdenum and tungsten, and about 5 to 10% each of aluminum, titanium, manganese, and vanadium. Thus, the tough, ductile fcc matrix can dissolve extensive amounts of elements in various combinations to provide solution hardening as well as improved corrosion and oxidation resistance. The degree of solution hardening has been related to the atomic size difference between nickel and the alloying element, and therefore the ability of the solute to interfere with dislocation motion.

Tungsten, molybdenum, niobium, tantalum, and aluminum, when aluminum is left in solution, are strong solution hardeners, with tungsten, niobium, tantalum, and molybdenum also being effective at temperatures above 0.6 T m (T m = melting temperature), where diffusion-controlled creep strength is important. Iron, cobalt, titanium, chromium, and vanadium are weaker solution-hardening elements. Aluminum and titanium are usually added together to form the age-hardening precipitate, Ni₃(Al, Ti).

In addition, some alloying elements can partition to γ’, affecting the interface mismatch and precipitate-coarsening kinetics as well as contributing a solution-hardening component to strength, with titanium being the most effective at room and elevated temperatures.

However, titanium, niobium, and tantalum can influence mechanical properties still further by encouraging the formation of other similar types of precipitates. With higher titanium content, γ’ will transform to the hexagonal close-packed (hcp) η- phase, Ni₃Ti, which has an acicular or cellular morphology. With increased amounts of niobium, γ’ transforms to the commercially important metastable body-centered tetragonal (bct) phase γ". A decrease in hardening will result if the equilibrium orthorhombic phase, Ni₃Nb, is allowed to form. The actual phases precipitated and their effectiveness in hardening the micro-structure are dependent on the alloy composition, the applied heat treatments, the resulting precipitate volume fraction, and the service conditions.

Carbides. Although not a carbide former, nickel dissolves many elements that readily form the carbides seen in nickel alloys (MC, M₆C, M₇C₃, M₂₃C₆). The MC carbides (where M = W, Ta, Ti, Mo, Nb) are usually large, blocky, and undesirable. The M₆C carbides (M = Mo, W) can precipitate as small platelets in the grains or as blocky particles in boundaries useful for grain control, but deleterious for ductility and stress rupture properties. The M₇C₃ (M = Cr) can be useful when precipitated as discrete particles, but more so are grain boundary particles of M₂₃C₆ (M = Cr, Mo, W), where they can enhance creep rupture properties.

If carbides are allowed to agglomerate or form grain-boundary films during heat treatment or in service at elevated temperatures, they can seriously impair ductility and cause embrittlement. As in stainless steels, precipitation of chromium carbides at boundaries can lead to intergranular corrosion due to the chromium-depleted zone alongside the grain boundary becoming anodic to the rest of the grains.

This grain-boundary sensitization is controlled in several ways:

Nickel alloys

Nickel is alloyed to extend the good corrosion resistance and good heat resistance of elemental nickel. Even with extensive amounts of alloying elements, the tough, ductile fcc austenitic matrix is preserved.

It is convenient to describe nickel alloys by grouping them into their two broad application areas: corrosion resistance, especially in aqueous environments, and heat resistance. Naturally, this artificial separation should not be considered a rigid barrier as the corrosion-resistant alloys have good strength above room temperature and the heat-resistant alloys have good corrosion resistance. The unique, special-property alloys, many of which are also used for their good corrosion and heat resistance as well as high strength, are described separately.

Corrosion-Resistant Nickel Alloys. The commercially pure nickel grades, Nickel 200 to 205, are highly resistant to many corrosive media, especially in reducing environments, but also in oxidizing environments where they can maintain the passive nickel oxide surface film. They are used in the chemical processing and electronics industries.

They are hot worked at 650 to 1230 °C, annealed at 700 to 925 °C, and are hardened by cold working. For processed sheet, for example, the tensile properties in the annealed condition (460 MPa, tensile strength; 148 MPa, yield strength; and 47% elongation) can be increased by cold rolling up to 760 MPa tensile strength, 635 MPa yield strength, and 8% elongation.

Because of its nominal 0.08% C content (0.15% max), Nickel alloy 200 (UNS No 2200) should not be used above 315°C, since embritlement results from the precipitation of graphite in the temperature range 425 to 650°C. Higher-purity nickel is commercially available for various electrical applications.

The low-alloy nickels. These alloys contain 94% min Ni. The 5% Mn solid-solution addition in Nickel 211 protects against sulfur in service environments. As little as 0.005% S can cause liquid embrittlement at unalloyed nickel grain boundaries in the range between 640 and 740°C.

Duranickel, alloy 301 (Ni-4.5Al-0.6Ti), offers the corrosion resistance of commercially pure nickel with the strengthening provided by the precipitation of γ’. There is sufficient alloying additions in alloy 301 to lower the Curie temperature, making the alloy weakly ferromagnetic at room temperature.

The nickel-copper alloys are strong and tough, offering corrosion resistance in various environments, including brine and sulfuric and other acids, and showing immunity to chloride-ion stress corrosion. They are used in chemical processing and pollution control equipment. Capable of precipitating γ’, Ni₃ (Al, Ti), with its 2.7Al - 0.6Ti alloy addition, alloy K-500 adds an age-hardening component to the good solution strengthening and work-hardening characteristics already available with the nominal 30% Cu in alloy 400. The composition of these alloys can be adjusted to decrease the Curie temperature to below room temperature.

The Ni-Cr-Fe (-Mo) alloys might simply be thought of as nickel-base analogs of the iron-base austenitic stainless steel alloys, with an interchange of the iron and nickel contents. In these commercially important alloys the chromium content in general ranges from 14 to 30% and iron from 3 to 20%. With a well-maintained Cr₂O₃ surface film, these alloys offer excellent corrosion resistance in many severe environments, showing immunity to chloride-ion stress-corrosion cracking. They also offer good oxidation and sulfidation resistance with good strength at elevated temperatures. These nickel-rich Ni-Cr-Fe alloys have maximum operating temperatures in the neighborhood of 1200°C.

The Ni-Cr-(Fe)-Mo alloys consist of a large family of alloys that are used in the chemical processing, pollution control, and waste treatment industries to utilize their excellent heat and corrosion resistance. Alloys in this commercially important family, such as C-276 and alloy 625, are made even more versatile by their excellent welding characteristics and the corrosion resistance of welded structures.

The molybdenum additions to these alloys improve resistance to pitting and crevice corrosion. Aluminum improves the protective surface oxide film, and the carbide formers titanium and niobium are used to stabilize the alloys against chromium-carbide sensitization. Even with the low-level additions of aluminum and titanium to alloy 800, for example, small amounts of γ’ can form in service during exposure to elevated temperatures. The high molybdenum and silicon additions in Hastelloy B and D promote good corrosion resistance during in the presence of hydrochloric and sulfuric acids.

Heat-Resistant Nickel Alloys. These nickel-containing materials include nickel-, iron-nickel-, or cobalt-base alloys. They can be made by wrought and P/M methods, and also with castings produced with carefully controlled conditions to provide the desired polycrystal, or elongated (directionally solidified), or single-crystal grain structure for improved elevated-temperature mechanical properties. The majority of the nickel-base superalloys utilize the combined strengthening of a solution-hardened austenite matrix with γ’ precipitation.

The iron-base Fe-Ni-Cr heat-resistant alloys are extensions of the iron-base stainless steels with higher nickel and additions of other alloying elements. Retaining the fcc iron-nickel austenite matrix, these alloys (alloys A-286 and 901, for example) are workable into various wrought forms and are capable of precipitation hardening with γ’.

Alloys 903 and 909 are controlled thermal expansion Fe-Ni-Co-base alloys that are capable of age hardening with Ni₃(Nb, Ti) precipitation and are designed to have high strength and low coefficient of thermal expansion for applications in gas turbine rings and seals up to 650°C.

These alloys are hot worked at about 870 to 1120°C and solution heat treated at 815 to 980 °C. The standard aging treatment consists of 720 °C for 8 h, furnace cool at 55°C/h to 620°C for 8 h, followed by air-cooling. Alloy 909 in the as-hardened condition, for example, retains much of its room-temperature yield strength (1070 MPa) at 540°C, namely, 895 MPa.

Specialty Nickel Alloys. Unique combinations of properties are available with other nickel-base alloys for special applications. While some of these properties are also available to some extent with alloys described above, the alloys described below were developed to promote their rather unique properties.

There are many electrical resistance alloys used for resistance heating elements. They can contain 35 to 85% Ni, but invariably contain greater than 15% Cr to form an adherent surface oxide to protect against oxidation and carburization at temperatures up to 1000 to 1200°C in air.