Nickel based alloys (e.g. Fe-Cr-Ni(Mo) alloys) alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to low temperatures. Nickel and its alloys are highly resistant to corrosion in many environments, especially those that are basic (alkaline). Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base element for superalloys. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials.
Types of Nickel Alloys
Nickel-based Superalloys
superalloys - inconel - turbine
bladeNickel-base superalloys currently constitute over 50% of the weight of advanced aircraft engines. Nickel-base superalloys include solid-solution-strengthened alloys and age-hardenable alloys.
Age-hardenable alloys consist of an austenitic (fcc) matrix dispersed with coherent precipitation of an Ni3(Al,Ti) intermetallic with an fcc structure. Ni-based superalloys are alloys with nickel as the primary alloying element are preferred as blade material in the previously discussed applications, rather than Co- or Fe-based superalloys. What is significant for Ni-based superalloys is their high strength, creep and corrosion resistance at high temperatures. It is common to cast turbine blades in directionally solidified form or single-crystal form. Single-crystal blades are mainly used in the first row in the turbine stage.
For example, Inconel is a registered trademark of Special Metals for a family of austenitic nickel-chromium-based superalloys. Inconel 718 is a nickel-based superalloy that possesses high strength properties and resistance to elevated temperatures. It also demonstrates remarkable protection against corrosion and oxidation. Inconel’s high temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy. Inconel 718 is composed of 55% nickel, 21% chromium, 6% iron, and small amounts of manganese, carbon, and copper.
Nickel Silver
Nickel silver, known also as German silver, nickel brass or alpacca, is a copper alloy with nickel and often zinc. For example,
UNS C75700 nickel silver 65-12 copper alloy has good corrosion and tarnish-resistance, and high formability. Nickel silver is named due to its silvery appearance, but it contains no elemental silver unless plated.
Invar
Invar is a group of low thermal expansion nickel-iron alloys consisting primarily of nickel and iron (e.g. FeNi36). The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes. The Invar alloy is ductile and easily weldable, and machinability is similar to austenitic stainless steel.
Invar is used where high dimensional stability is required, such as precision instruments, clocks . Alloys with low coefficients of expansion form the essential part of bimetals and thermostats. Invar itself is still used today in vast numbers of household appliances, from electric irons and toasters to gas cookers and fire safety cutoffs. Invars can be also used in glass-to-metal seals, and electronic and radio components. Almost all variable condensers are made of Invar. Struts on jet engines are made of Invar to ensure rigidity with temperature changes.
Applications and Characteristics of Nickel Alloys
Nickel and nickel alloys are used for a wide variety of applications, the majority of which involve corrosion resistance and/or heat resistance. Some of these include:
- Aircraft gas turbines
- Steam turbine power plants
- Medical applications
- Nuclear power systems
- Chemical and petrochemical industries
A number of other applications for nickel alloys involve the unique physical properties of special-purpose nickel-base or high-nickel alloys. These include:
- Low-expansion alloys
- Electrical resistance alloys
- Soft magnetic alloys
- Shape memory alloys
Heat-Resistant Applications. Nickel-base alloys are used in many applications where they are subjected to harsh environments at high temperatures. Nickel-chromium alloys or alloys that contain more than about 15% Cr are used to provide both oxidation and carburization resistance at temperatures exceeding 760°C.
Corrosion Resistance. Nickel-base alloys offer excellent corrosion resistance to a wide range of corrosive media. However, as with all types of corrosion, many factors influence the rate of attack. The corrosive media itself is the most important factor governing corrosion of a particular metal.
Low-Expansion Alloys Nickel was found to have a profound effect on the thermal expansion of iron. Alloys can be designed to have a very low thermal expansion or display uniform and predictable expansion over certain temperature ranges.
Iron-36% Ni alloy (Invar) has the lowest expansion of the Fe-Ni alloys and maintains nearly constant dimensions during normal variations in atmospheric temperature.
Electrical Resistance Alloys. Several alloy systems based on nickel or containing high nickel contents are used in instruments and control equipment to measure and regulate electrical characteristics (resistance alloys) or are used in furnaces and appliances to generate heat (heating alloys).
Types of resistance alloys containing nickel include:
- Cu-Ni alloys containing 2 to 45% Ni
- Ni-Cr-Al alloys containing 35 to 95% Ni
- Ni-Cr-Fe alloys containing 35 to 60% Ni
- Ni-Cr-Si alloys containing 70 to 80% Ni
Types of resistance heating alloys con-taining nickel include:
- Ni-Cr alloys containing 65 to 80% Ni with 1.5% Si
- Ni-Cr-Fe alloys containing 35 to 70% Ni with 1.5% Si + l% Nb
Soft Magnetic Alloys. Two broad classes of magnetically soft materials have been developed in the Fe-Ni system. The
high-nickel alloys (about 79% Ni with 4 to 5% Mo; bal Fe) have high initial permeability and low saturation induction.
Shape Memory Alloys.Metallic materials that demonstrate the ability to return to their previously defined shape when subjected to the appropriate heating schedule are referred to as shape memory alloys. Nickel-titanium alloys (50Ni-50Ti) are one of the few commercially important shape memory alloys.
Properties of Nickel Alloys
Material properties are intensive properties, that means they are independent of the amount of mass and may vary from place to place within the system at any moment. The basis of materials science involves studying the structure of materials, and relating them to their properties (mechanical, electrical etc.). Once a materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of the structure of a material and thus of its properties are its constituent
chemical elements and the way in which it has been processed into its final form.
Mechanical Properties of Nickel Alloys
Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial and engineers must take them into account.
Yield Strength
Yield strength of of constantan – 45Ni-55Cu depends greatly on the heat treatment procedure, but for annealed alloy is about 150 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins.
Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behaviour termed a yield point phenomenon. Yield strengths vary from 35 MPa for a low-strength aluminum to greater than 1400 MPa for very high-strength steels.
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