Special-purpose Nickel Alloys

Magnetically soft nickel alloys

In terms of magnetic properties, metals can be divided into five categories:

• Paramagnets
• Ferromagnets
• Ferrimagnets
• Antiferromagnets
• Antiferromagnets

The magnetic properties of metals are influenced by temperature. Among all metals only iron, nickel and cobalt show ferromagnetic properties at room temperature.

The rise of temperature causes ferromagnets to lose their magnetic susceptibility. When the temperature reaches a certain value, the ferromagnetic material becomes a paramagnetic one. This value is called the Curie Temperature of a metal.

The magnetic permeability of ferromagnetic metals depends on the intensity of the external magnetic field. When such a field is removed, ferromagnets still display residual magnetism. By means of this force, permanent magnets attract or repel other metals or magnets.

A magnetized ferromagnetic alloy still exhibits residual magnetism, which wends the same direction as the field did. Demagnetization requires an oppositely oriented field of a certain strength, called coercion. If this field is stronger than said strength, the alloy gets magnetized -directed the opposite way then the field.

Each remagnetization involves irreversible dissipation of energy that’s called hysteresis energy loss.

Magnetically soft nickel alloys – properties

• High initial permeability µ
• High max relative permeability
• Constant permeability in changing temperatures
• High saturation magnetostriction
• An immediate reaction to the magnetic field's change
• Low Curie Temperature
• High resistivity
• Low eddy-current loss in alternating flux
• Narrow hysteresis loop (magnetization – demagnetization loop)
• Low hysteresis-energy loss

Shortly put- magnetically soft alloys are easy to magnetize, demagnetize and remagnetize.

Nickel-containing magnetically soft alloys – applications

Nickel-containing magnetically soft alloys are used extensively in a variety of applications, including:

• Audio-related transformers
• Magnetic amplifiers
• Transducers
• Radar pulse transformers
• Synchronous motors and torque motors
• Loading coils
• Radiofrequency shields (RF shields)
• Temperature compensators

Permalloy is a unique magnetically soft alloy. Its feature is an almost rectangular shape of the hysteresis loop and two remanence values.

Magnetically soft amorphous metals

Amorphous metals (metallic glass) are characterized by high resistance, following low hysteresis loss and the ability to work at high frequencies.

Nickel-containing, magnetically soft amorphous metals are:

• Metglas 40Fe-40Ni-14P-6B
• Metglas 40Fe-38Ni-4Mo-18B

Magnetically hard nickel-containing alloys

Magnetically hard alloys are used to produce permanent magnets that exhibit strong residual magnetism and aren’t easy to demagnetize and remagnetize.

Alnico alloys are used to craft permanent magnets.

Low-expansion nickel alloys

The thermal expansion means that bodies change (almost always increase) their length or volume in response to a rise in temperature. Every substance is more or less subject to this phenomenon.

The coefficient of thermal expansion is the relative expansion (also called strain) of a material divided by the change in temperature (usually 1K). It's calculated by comparing the accurate measurements before and after heating.

This coefficient generally varies with temperature. It means that the expansion of a metal rod heated from 0 to 20 °C is different from the expansion of the same rod heated from 200 to 220 °C.

The coefficient of thermal expansion for most of metals and alloys is relatively large and ranges from about 5 to 25 μm/m ⋅ K in the room temperature. Expansion joints in road bridges or railway tracks may serve as an example of counteracting high thermal expansion of iron and steel alloys.

Low-expansion nickel alloys – application

Applications of low-expansion nickel alloys usually require:

• Very low thermal expansion (0 to 2 μm/m ⋅ K) over a certain temperature range, or
• Uniform and predictable thermal expansion over a certain temperature range.

Low-expansion alloys are used for example as/in:

• Geodetic instruments – rods and tape measures
• Clocks and timers – compensation pendulums, flywheels
• Engines – internal combustion engines' pistons
• Measuring devices – duplex metals, bimetal strips
• Glass-metal sealings – electron tubes
• Storage and transport of liquefied natural gas - valves and pipes
• Power industry – superconductive systems
• Electronics – integrated circuits' frames, elements of radio devices, TV masking frames
• Laser and optical measuring devices – housings and structural elements

Alloying additions

Low-expansion nickel alloys are mainly made of nickel and iron. With the exception of super-invar alloy that shows small expansivity only in the very narrow range of temperature, the minimal coefficient of thermal expansion occurs at the ratio 64Fe-36Ni (such an alloy is called invar). This low expansivity prevails only in a certain range of temperature.

Change in nickel amount moves this range of temperature toward higher or lower temperature but to change the nickel amount and keep the low coefficient: certain alloying additions are needed. While keeping the coefficient of thermal expansion low, such additions increase it a bit nonetheless.

In summary:

• Pure iron-nickel alloy – Lowest coefficient of thermal expansion over a certain temperature range.
• Chromium, Manganese, Wolfram, Molybdenum – Enable to increase the nickel content, which shifts the minimum expansivity range toward higher temperatures. Increase the coefficient of thermal expansion vs invar.
• Copper, Coal, Cobalt – Enable to decrease nickel content, which shifts the minimum expansivity range toward lower temperatures. Increase the coefficient of thermal expansion vs invar.

Alloys for seals

Direct, hermetic sealings of two non-elastic materials with different thermal expansion factor can not withstand big temperature changes. Either a gap opens between those materials, or large forces damage one of them.

Therefore, some alloys are engineered to have thermal expansion corresponding to certain other materials over a specified temperature range. For example: Kovar alloys or glass-sealing alloys expand exactly like many types of glass and ceramics. Platinate alloy responds to a change in temperature just like platinum.

These alloys enable production of hermetic metal-glass seals. Such connections are very solid and don't require any additional o-rings. In the past direct metal-glass seals were very common in radio technology- in electron tubes.

Such a precise coefficient of thermal expansion of an alloy is determined by the amount of nickel and cobalt in the iron matrix.

Shape memory nickel-titanium alloys

Some metals demonstrate the ability to return to a previously defined shape or size, according to the shape memory effect.

Shape memory effect states that an alloy shaped in a certain temperature and then reshaped in another: will return to the original shape when brought back to the initial (shaping) temperature.

Applications of shape memory alloys

Applications for such alloys can be divided into:

• Actuation devices – coffe pot thermostats, medical instruments like clot filters in blood veins.
• Constrained recovery devices – hydraulic pipes fittings.
• Superelastic devices – eyeglass frames, orthodontic archwires.
• Martensitic devices – vibration dampers, open heart surgery tools, fatigue resistant wires.

Shape memory nickel-titanium alloys - composition

The base for such alloys is more or less 50-50 intermetallic compound of nickel and titanium. Other commonly used alloying additions are:

• Excess nickel – strongly lowers the martensitic transformation temperature and increases the yield strength of the austenite.
• Iron – lowers the transformation temperature.
• Chromium – lowers the transformation temperature.
• Copper – decreases the hysteresis and lowers the deformation stress of the martensite.