INVAR: The amazing super-low thermal expansion alloy you’ve never heard of – that’s been around for over 100 years.

§ While discovered more than 100 years ago the thermal expansion characteristics of Invar are prevalent in everything from tape measures to multibillion-dollar satellites.

§ The Invar alloy has spawned a whole family of similar materials with varying characteristics.

§ Invar’s ability to maintain a rigid structure under a range of temperatures is invaluable in the world of mechanics and precision devices.

§ The discovery of Invar by Charles-Edouard Guillaume led to a Nobel Prize in 1920 which says everything.

While it is highly likely that many people have never heard of Invar this is an alloy which has been round for more than 100 years. The importance of this material must not be underestimated because it is a low expansion alloy which is commonplace in many everyday items. We need to go back to 1896 for the discovery of Invar (a name derived from the word “invariable”) for which Swiss physicist Charles-Edouard Guillaume won the Nobel Prize in 1920. The very fact that he is the one and only scientist in history to be honoured with a Nobel Prize for a metallurgical achievement perhaps puts his discovery into context.

While Invar is still used in its most basic form it has also spawned an array of other low expansion nickel iron alloys with similar characteristics.

Guillaume’s quest

The story of Invar began when Charles-Edouard Guillaume was given the task of finding a metal which could be used in tapes and wires which would not change in length (i.e. would not expand). When you think about it, there is no point having for example a metal measuring tape which expands and contracts dependent upon the temperature. So, in his quest to find the answer he decided to try a number of nickel/iron alloy mixes.

While it took some time, it became apparent that the perfect mix of iron and nickel was 64% iron and 36% nickel. It was this particular mix of metals which gave the least amount of thermal expansion (the material is best known for its low coefficient of thermal expansion) and hey presto, the discovery of Invar. Whether Charles-Edouard Guillaume quite realised the impact that Invar would have on the world of mechanics and mechanical engineering at the time is debatable.

High and low temperatures

While Invar, and its related materials, are used in situations where high dimensional stability is required one interesting anomaly surrounds what is known as the Curie temperature. Below the Curie temperature the iron/nickel mix is what is known as ferromagnetic. This means that the dimensions of the alloy hardly move with temperature changes at least until we reach above and beyond the Curie temperature.

In an unexplained phenomenon, once Invar experiences heat above the Curie temperature it will begin to expand at a high rate. This is because the product is no longer ferromagnetic and unable to hold the stability seen at lower temperatures. It is somewhat strange that a physicist was able to create the alloy, we use it in everyday situations but the world of science does need not yet fully understand the impact the Curie temperature has on Invar.

Early applications for Invar

If you sit back, you can probably think of at least a dozen ways in which the low coefficient of thermal expansion associated with Invar would prove very useful. The number of applications has mushroomed since the 1920s when it became more widely used, often replacing more expensive materials.

In the relatively early years Invar was used in the following products with varying mixes of nickel:-

§ Survey tapes and wire

§ Pendulums for grandfather clocks

§ Glass sealing wire

§ Light bulbs

§ Electric vacuum tubes for use by the electronics industry

§ Thermostats

§ Incandescent lighting

§ Precision machined parts

§ Circuit breakers

§ Aerospace and automotive controls

§ Heating and air conditioning systems

The more that physicists understood the characteristics and performance of Invar (and its associated mixes) the more uses came to light. More recent applications include:-

§ Transporting liquid natural gas on tankers – Invar minimises the cryogenic shrinkage

§ Shadow masks in high definition television tubes

§ Precision laser and optical measuring systems

§ Wave guide tubes

§ Microscopes

§ Giant space telescopes

§ Mounted lenses

§ Orbiting satellites

§ Ring laser gyroscopes

While the 36% nickel content in Invar offers the lowest coefficient of thermal expansion it doesn’t offer the lowest Curie temperature. As we touched on above, the Curie temperature is the level at which the ferromagnetic condition which holds the material so stable begins to weaken. The Curie temperature for Invar is 280°C (536°F) where there is a 36% nickel mix. If we look towards a 50% nickel mix this increases the Curie temperature up to 565°C (1050°F) but there is more natural expansion with this mix even before reaching its Curie temperature.

So in summary, a higher nickel mix increases the Curie temperature at which the material expands at a higher rate. However, the optimal 36% nickel mix may have a lower Curie temperature than the 50% mix but it is far more rigid in its natural state.

A vital alloy

When you dig a little deeper into everyday materials and everyday products you will see the impact that discoveries such as Invar have had more than 100 years after its initial discovery. These materials are not necessarily well-known outside of the engineering community but the impact they have had on our everyday lives should not be underestimated.

If you consider this material in its many different forms (known as the Invar family) is helping with such simple tools as measuring tapes right through to the most complicated and expensive satellites, it does put it all into perspective. The discovery of a product back in 1896 which remained rigid under a large temperature range would not necessarily have caught the headlines. However, it has literally changed the world we live in today, our ability to create precision tools and measuring devices capable of superhuman accuracy.

Summary

The original Invar mix of 64% iron and 36% nickel creates the perfect material for many situations. It is extremely rigid under a range of temperatures – which is vital for many of the products and services we have mentioned above. The only downside is the “relatively low” Curie temperature which is 280°C (536°F) with a 36% mix. Above this temperature the product will expand at a relatively high rate which makes it incompatible for many high-temperature environments.

Increasing the nickel mix does increase the Curie temperature, after which point the material will expand more rapidly, but also increases the underlying coefficient of thermal expansion. For many everyday situations the 36% nickel mix will be more than enough but this can be varied for different requirements.