Invar is a very low coefficient of thermal expansion metal alloy. When temperature varies of 1° C, an Invar rod 10 kilometres long varies its length of 0.8 - 2 centimetres, depending on how it has been worked. Indeed, a steel rod in the same conditions would vary of 11 cm., a brass rod of 19 and an aluminium rod of 25.5. 10 kilometres correspond to one million centimetres, so expansions are usually expressed in parts per million per C degree of temperature variation. There is an alloy called Super Invar with a smaller coefficient of thermal expansion, less than 0.63 parts per million (PPM) and in some cases, negative: the alloy reduces its length when temperature increases. This can happen by no more than 0.1 PPM per C degree of temperature variation (1 millimetres per 10 kilometres per C degree of temperature variation). Invar is composed of steel with about 36% nickel, and smaller quantities of other elements, added to increase machinability or remained from the production process, where its difficult to obtain absolutely pure alloys. Three kinds of Invar can be purchased, at least from our sources.
1) Invar 36, seldom used because very difficult to machine. Its coefficient of thermal expansion is about 1.6 PPM. This is its detailed chemical composition:
2) FM Invar (Free Machining Invar), containing tiny amounts of selenium and more manganese than Invar 36. Unfortunately, it contains twice the carbon contained in Invar 36 and this causes its long-term structural instability. In other words, it suffers from small spontaneous deformations during years.
As we said, one of the problems arising from the use of Invar is given by its structure instability. The length of a pendulum rod made with this alloy can vary with time, having a bad influence on the clock stability.
A famous case is Shortt No. 4, a very accurate (and expensive) clock, which suffered from this problem. Using old Invar can be a good solution: most of deformations should have already happened.
In recent years, NASA developed a particular kind of Invar, HP (High Purity) Invar 36. It has a much-improved coefficient of thermal expansion and structure stability than common Invar 36. A small quantity has been made to be used on the Cassini spacecraft camera. Pure iron and nickel powders have been weighed, mixed, pressed into a mould and sintered in controlled atmosphere. Half of the resulting product was extruded and half was hot hammered.
The exceptional properties were attributed to the high purity of the alloy, especially to the very low carbon content (under 0.01%). Unfortunately, it cannot be purchased!
[From the NASA Technical Support Package "Temporally and Thermally Stable Iron/Nickel Alloy" for the August 1995 issue of NASA Tech Briefs]