The Invar - Family of functional alloys

In nature, as well as in man-made products, materials such as rock, glass, wood, concrete, water, plastics, metals and alloys, etc., have a propensity to change length under the effect of thermal variations linked either to climate or to the operation of installations; they expand if the temperature rises, or contract if it falls. A remarkable example is mercury, whose high thermal expansion (0.18 10 ^–3 / ^o C) enabled the manufacture of the first thermometers. In solid materials, these dimensional variations cause temporary or even permanent deformations, which can have unfortunate consequences. In some cases, these consequences are remedied by the use of expansion joints or bellows, depending on the sector in question. Just as researchers are looking for materials that are non-corrodible (in a given medium), they have long been keen to develop materials that are non-dilatable (at least over a certain temperature range). This objective became particularly important in metrology when length standards and, in particular, secondary standards were needed. The discovery of Invar®, an iron alloy with 36% nickel, made it possible to meet this objective. The name Invar was given to this alloy because of its very low coefficient of linear expansion over a fairly wide temperature range, giving it dimensional invariance.

The name Invar conjures up a beautiful metallurgical story; in this name are confused the Fe-Ni 36 alloy and the physical effect that gives the alloy its dimensional stability. In fact, the discovery of Invar, which according to some historians of science was due to chance, would not have been possible without the persevering and rigorous work of a physicist and metrologist, Charles-Édouard Guillaume, who succeeded in combining science and industry in a harmonious and complementary way (cf. Historical overview, § 1). While, in the words of Pierre Chevenard, "it would be futile to ignore the role of chance, the great purveyor of inventions", the next step was to ensure the fruitful development of the Invar discovery. The discovery of Invar's very low coefficient of expansion (around 10 ^–6 / °C at 20 °C) made it possible to realize every metrologist's dream: to obtain a standard for measuring length at a much lower price than the platinum standard with 10% iridium. While the latter was suitable for obtaining the primary length standard, it was not suitable for obtaining secondary standards, which had to be produced in large numbers.

After chance, it was time to study and understand the physics behind this dilatometric anomaly in Invar or similar alloys with 30% nickel. The idea was to provide a solution to a given problem, namely...