Invar
An alloy that barely changes size with temperature — a coincidence of magnetism that turned out to be worth a Nobel Prize.
Invar is iron with about 36 per cent nickel, and it expands roughly a tenth as much as ordinary steel when heated. Over a useful range around room temperature its thermal expansion is close to zero.
The cause is genuinely strange and is why the discovery mattered as physics as much as engineering. The alloy is ferromagnetic, and its magnetic ordering produces a volume expansion that happens to fall as temperature rises at almost exactly the rate the ordinary thermal expansion rises. The two cancel. Above the Curie temperature the magnetic contribution disappears and Invar expands like any other alloy — the property is not a property of the material so much as an accidental balance between two effects, and it holds only where the balance does.
Charles Édouard Guillaume received the 1920 Nobel Prize in Physics for the discovery, which is unusual for what is essentially a metallurgical observation, and reflects how important precise measurement was to the physics of the period.
Processing
Melted, rolled and machined conventionally, and it is soft and gummy to cut. Its dimensional stability depends on careful heat treatment and stress relief: residual stress from machining relaxes over time and moves the part, which defeats the entire purpose, so precision Invar components are stabilised by thermal cycling and left to age before final machining.
Uses
Precision instruments, surveying tape and the pendulum rods and balance wheels of accurate clocks, which is what it was invented for. Length standards. The shadow masks of colour cathode ray televisions, historically an enormous use that vanished entirely with the technology.
Membrane tanks in LNG carriers, where a structure holds liquid at −162 °C and must not contract away from its supports. Composite mould tooling, where the tool must not change size relative to the carbon fibre part curing on it. Optical and satellite structures, and the seals between metal and glass in scientific instruments.
History
Discovered by Guillaume in 1896 at the International Bureau of Weights and Measures, while looking for a cheaper substitute for the platinum-iridium of the metre standards. He found something better than a substitute.
Its effect on timekeeping was immediate. A pendulum clock's rate depends on the pendulum's length, and temperature compensation had previously required ingenious mechanical arrangements of two metals; an Invar rod made the problem disappear.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
is an input to
- Heat treatment — process · stress relieved and thermally cycled, because residual machining stress relaxes over time and moves the part, which defeats the entire purpose
is an alternative to
- Carbon fibre — material · as mould tooling that must not change size relative to the composite curing on it — Invar is heavy and predictable, carbon tooling is light and matches the part exactly
is produced by
- Alloying and melting — process · thirty-six per cent nickel into iron, at a composition where thermal expansion very nearly cancels
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)