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
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:
- by avoiding the chromium-carbide aging temperature range (425 to
760°C) during processing,
- with stabilization heat treatments to tie up carbon with more stable carbide formers
(niobium, tantalum, titanium), and
- by reducing the carbon level in the base alloy.
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
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
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.