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|PRODUCT DATA of 03: Nickel and its Alloys|
|Material||03: Nickel and its Alloys|
|General Information||As a family, the Ni-based alloys are used in many engineering fields for their corrosion resistance and high-temperature performance.Some alloysare used in electrical applications (e.g. heating elements). The magnetic characteristics of certain alloys are utilized in transformer components. A few alloys have controlled-expansion and constant-modulus properties (bimetals, thermostats, glass sealing, precision equipment). Others were developed for specific applications (hydrogen storage) or to exploit a particular peculiarity (shape-memory effect). There are also a number of alloys used as welding and brazing filler materials. Some Ni-based materials are applied as coatings or hard facings to other materials to provide wear or corrosion resistance.|
NOTE: Ni-alloys are often known by trade names, rather than by their specification code numbers.
|Use in Spacecraft||Nickel plating appears in many applications (e.g. electronics, thermal control and corrosion protection). |
Ni-alloys are applied to subsystems requiring corrosion resistance (storage and delivery systems); high-temperature performance, often combined with oxidation resistance (propulsion units -- gas turbines and rocket motors, power generation, heat-exchangers and turbines); high-reliability, high-strength fasteners.
Magnetic alloys find a limited but important role. "Memory alloys" can be used as actuators.
|Main Categories||Nickel-based materials can be grouped by principal alloying additions. However, alloys within one composition grouping can be used in more than one general application group. For example: the majority of nickel-iron-chromium alloys in the Inconel and Incoloy series are now applied to elevated-temperature service, except two which are primarily used for their corrosion resistance.
The main use of commercially pure nickel is in platings (by electro- or electroless deposition) to provide corrosion protection to the underlying substrate materials. Electroless nickel can be hardened to provide abrasion resistance whilst retaining corrosion resistance .Nickel provides elevated-temperature corrosion resistance to many acids. As it is ferromagnetic, care is needed in its use in some applications (electronics).
The resistance of Ni-alloys to a particular corrosive media largely depends on the composition.
Heat-resistant alloys tend to form two, not entirely independent, groups. They were developed to:
Almost all heat-resistant Ni-alloys are developments of the basic 80Ni - 20Cr composition. Modifications to this include variations in the Cr content and the addition of other alloying elements. Ni-Fe-Cr (usually with 15 %-25 % Cr) alloys are used at service temperatures up to about 1100 ºC in oxidizing, carburizing, sulphidizing environments and also are resistant to other forms of chemical attack. Under thermal cycling, the protective oxide layer can crack and spall.
Creep-resistant alloys (nickel-based superalloys) probably have the most complex compositions of any engineering alloys and have similarly complex microstructures. The alloying additions are designed to exploit many “micro structural engineering” techniques, such as phase stabilization, precipitation hardening, dispersion strengthening, grain-boundary pinning and solid-solution strengthening as well as give corrosion resistance. Alloying increases the strength and temperature capability but reduces the processability (since the alloys are specifically designed to resist deformation at temperature). This limits the product forms available. Sheet and complex forgings can only be made in lower-alloy variants and their temperature resistance is correspondingly lower.Creep-resistant alloys can be grouped by application:
Nickel-based superalloys possess good combinations of high-temperature mechanical properties and oxidation resistance up to approximately 550 ºC. Many of these alloys also have excellent cryogenic temperature properties.
Continued alloy development has produced materials specifically designed for processing in particular ways: directional solidification and single-crystal castings; powder metallurgy and associated consolidation techniques. These materials optimize mechanical properties in selected directions, and so increase creep resistance in the dominant direction experienced in service.
Magnetic alloys generally have a high magnetic permeability in low or moderate strength magnetising fields, or exhibit particular magnetic hysterisis characteristics. The resulting magnetic properties depend on careful control of specialized processing methods. They are mainly used in telecommunications or for electronic transformer components. Pure nickel and some high nickel content-Co alloys have magnetorestrictive characteristics used in transducers. Ni-Fe alloys containing about 30 %Ni and nickel-30 %copper alloys have permeabilities that vary rapidly with temperature at “normal” temperatures and find uses in temperature compensation devices.
With careful control of composition and processing techniques, the thermal expansion coefficient of some Ni-Fe alloys can be low or be matched to the CTE of non-metallic materials such as glasses and ceramics. Some alloys can, by composition modifications, be strengthened, making them suitable for load-bearing applications. Uses include vacuum equipment, metrology and chronometry. Some Ni-Fe alloys exhibit positive temperature coefficients of elastic modulus (most other metallic materials have negative values). These materials find specialist uses in springs and vibrating devices to ensure stability during changes of temperature.
Ni-Ti memory alloys are based around the 50:50 composition. They can be deformed below a specific temperature, then, on heating above a higher temperature (these systems show some thermal hysterisis), they return to the original shape. The cold deformation produces micro structural phase changes which accommodate the reshaping without permanent material flow. On heating these microstructural changes are reversed and the shape returns to the original. Applications include temperature sensitive actuators, fixing and gripping devices (often in inaccessible locations).
|Processing and Assembly||The chemical composition largely dictates the processing methods applicable to a particular alloy. In addition to casting, normally under vacuum, and forging, powder metallurgy techniques are used to produce highly-alloyed or dispersion strengthened materials from metal powders. Similar processes, e.g. hot isostatic pressing, can be used for the consolidation (porosity elimination) of cast components. All processes should be strictly controlled and the specifications applied to aircraft and other critical industry applications (power generation) are used.|
|Precautions||In electronic assemblies, brass terminals can be plated with a barrier layer of nickel provided that its magnetic properties are acceptable in the final assembly.
NOTE Nickel can have poor solderability compared with copper platings.
The precise operating environment shall be carefully evaluated to ensure that the correct alloy is selected (e.g. resistance to a particular chemical at service temperatures; combined temperature, hot-corrosion and oxidation resistance; electrical and magnetic requirements or constraints).
Thermal cycling can affect oxidation and hot-corrosion resistance by affecting the surface composition of alloys. Spalling of the protective layer increases attack by corrosive media. Depletion of alloying elements in precipitation hardening superalloys can occur in high-temperature oxidizing environments. This is especially important for thin materials, since a slight depletion effect can represent a considerable proportion of the effective material cross-section.
A full evaluation of service conditions and interfacial effects (e.g. thermal mismatch and diffusion) shall be carried out when selecting and using coatings for oxidation or corrosion resistance. Barrier, ceramic-type coatings can crack and spall during thermal cycling and elements of metal coatings can diffuse into the substrate at prolonged elevated temperatures.
As a class, alloys with a high nickel content are resistant to stress corrosion cracking. Alloys that were evaluated are listed as high-resistance. For non-listed alloys a SCC evaluation shall be obtained prior to use.
|Hazardous and Precluded||Alloys with a high nickel content are susceptible to sulphur embrittlement. Sulphur is a common constituent of industrial oils, greases and cutting lubricants, so careful cleaning of components is necessary prior to heat-treatments or prior to use in high-temperature environments|
|Effects of Space environment|
|Some Representative Products||Ni-alloys are often known by their trade names, rather than by their specification code numbers. They form a family of materials that have developed over the last 70-odd years and been modified to enhance certain properties over others. Consequently there exist the main trademarked suppliers products, but many variants of these are available under different proprietary names. The following list is not comprehensive of all that is available across Europe.
Nickel alloys that were evaluated and shown to have a high resistance to stress corrosion cracking are listed in Table A-3 (from ECSS-Q-ST-70-36)