Invar

Alloy of nickel and iron with low coefficient of thermal expansion

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Samples of Invar

The coefficient of thermal expansion of nickel/iron alloys is plotted here against the nickel percentage (on a mass basis) in the alloy. The sharp minimum occurs at the Invar ratio of 36% Ni.

Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel-iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes, and is a registered trademark of ArcelorMittal.

The discovery of the alloy was made in 1895 by Swiss physicist Charles Édouard Guillaume for which he received the Nobel Prize in Physics in 1920. It enabled improvements in scientific instruments.

Properties

Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy. In one commercial version, it consists of approximately 36% nickel and 64% iron. The invar range was described by Westinghouse scientists in 1961 as "30–45 atom percent nickel".

Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20°C and 100°C) of about 1.2 × 10⁻⁶ K⁻¹ (1.2 ppm/°C), while ordinary steels have values of around 11–15 ppm/°C. Extra-pure grades (<0.1% Co) can produce values as low as 0.62–0.65 ppm/°C. Some formulations display negative thermal expansion (NTE) characteristics. Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep.

Applications

Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, color-television tube shadow-mask frames, valves in engines, and large aerostructure molds.

One of its first applications was in watch balance wheels and pendulum rods for precision regulator clocks. When the clock pendulum was first invented, the limit to timekeeping accuracy was due to thermal variations in the length of clock pendulums. The Riefler regulator clock, developed in 1898 by Clemens Riefler, was the first clock to use an Invar pendulum. It had an accuracy of 10 milliseconds per day and served as the primary time standard in naval observatories and for national time services until the 1930s.

In land surveying, when first-order (high-precision) elevation leveling is performed, the leveling rod used is made of Invar instead of wood, fiberglass, or other metals. Invar struts were used in some pistons to limit their thermal expansion inside their cylinders. In the manufacture of large composite material structures for aerospace carbon fiber layup molds, Invar is used to facilitate the manufacture of parts to extremely tight tolerances.

In the astronomical field, Invar is used as the structural components that support dimension-sensitive optics of astronomical telescopes. Superior dimensional stability of Invar allows the astronomical telescopes to significantly improve observation precision and accuracy.

Variations

There are variations of the original Invar material that have slightly different coefficients of thermal expansion, such as:

• Inovco, which is Fe–33Ni–4.5Co and has an α of 0.55 ppm/°C (from 20 to 100°C).
• FeNi42 (e.g., NILO alloy 42), which has a nickel content of 42% and α ≈ 5.3 ppm/°C, matching that of silicon. It is widely used as lead frame material for integrated circuits. For more information, please visit Nickel Base Alloy.
• FeNiCo alloys—named Kovar or Dilver P—that have the same expansion behavior (~5 ppm/°C) and form strong bonds with molten borosilicate glass. These are used for glass-to-metal seals and to support optical parts in a wide range of temperatures and applications, such as satellites.

Explanation of Anomalous Properties

A detailed explanation of Invar's anomalously low CTE has proven elusive for physicists.

All the iron-rich face-centered cubic Fe–Ni alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy compositions. Scientists had proposed that Invar's behavior was a direct consequence of a high-magnetic-moment to low-magnetic-moment transition occurring in the face-centered cubic Fe–Ni series; however, this theory was proven incorrect. Instead, it appears that the low-moment/high-moment transition is preceded by a high-magnetic-moment frustrated ferromagnetic state in which the Fe–Fe magnetic exchange bonds have a large magneto-volume effect of the right sign and magnitude to create the observed thermal expansion anomaly.

Wang et al. considered the statistical mixture between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs) in Fe3Pt, with the free energies of FM and SFCs predicted from first-principles calculations, and were able to predict the temperature ranges of negative thermal expansion under various pressures. It was shown that all individual FM and SFCs have positive thermal expansion, and the negative thermal expansion originates from the increasing populations of SFCs with smaller volumes than that of FM.

See Also

• Constantan and Manganin, alloys with relatively constant electrical resistivity
• Elinvar, an alloy with relatively constant elasticity over a range of temperatures
• Sitall and Zerodur, ceramic materials with a relatively low thermal expansion
• Borosilicate glass and Ultra-low expansion glass, low expansion glasses resistant to thermal shock

References

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Invar is a nickel-iron, low expansion alloy and is commonly used in applications that require high dimensional stability. As it is able to maintain almost constant dimensions between temperatures of -100°C and 260°C, it is utilized in a variety of industries. Today we’ve put together a few examples of uses of Invar.

Some uses of Invar include:

1. Clock Pendulums

Clock pendulums were one of the first uses of Invar due to its near zero coefficient of linear thermal expansion enabling accurate timekeeping. When the clock pendulum was first invented, accuracy was compromised due to the varying temperatures throughout the seasons. However, by using Invar, the length of the pendulum didn’t change and therefore the time was always correct.

2. Optical Engineering & Precision Instruments

The high dimensional stability and low coefficient of thermal expansion is also beneficial for a number of optical engineering and precision instruments. For lasers, thermostats, and waveguide tubes, heat is a considerable factor during use, so Invar’s ability to maintain its dimensions and structure is essential. Invar is also incredibly durable, making it perfect for precision instruments such as measuring and positioning devices, and a variety of scientific instruments like microscopes and telescopes.

3. Large Aerostructure Molds

The modern generation of aircraft require Invar for large composite material structures and molds. This alloy keeps tight dimensional tolerances while advanced components are used at moderately high temperatures. Invar is also a vital material for the future of aerospace engineering as the evolution of technology is leading to orbit satellites, ring laser gyroscopes, and other high-tech applications.

Although more costly than, for example, molds made from stainless steels, Invar is becoming the first-choice alloy for mold tooling. Its molds or tools offer far better stability and have a much greater life expectancy, making it a more cost-effective solution on high production demands. In today’s quality-focused world, Invar’s CTEs offer much better dimensional tolerances for a finished part, which is often critical in aerospace or high-tech markets and applications.

4. Transportation of Liquid Natural Gas

With the ability to minimize cryogenic shrinkage, Invar has been used in the construction of containers to transport liquid natural gas. With its near zero coefficient of linear thermal expansion, Invar can provide the significant insulation needed to keep it in its liquid form.

As you can see, Invar’s low coefficient of thermal expansion and guaranteed dimensional stability has made it one of the most common and desirable materials for many industries. At City Special Metals, Invar 36 is one of our signature specialist alloys. With a unique range of Invar stock and over 20 years’ experience, we can help and advise you on whether it is right for your application.

To learn more about Invar, visit our Invar 36 product page or talk to one of our experts by contacting us.

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