Fe-Ni Invar alloys: A review

Fe-Ni alloys with face centred cubic structure having Ni concentration around 36% exhibit extremely low or no coefficient of thermal expansion over a wide range of temperatures around room temperature which is called Invar behaviour. The Invar behaviour was first observed in Fe-Ni alloys by Charles Édouard Guillaume in the year 1896 and was awarded Nobel prize in physics in the year 1920. The observed Invar behaviour was puzzling and elusive for physicists. The origin of Invar phenomena has been the subject of serious concern for physicists since the observation of the phenomenon. The first model explained the Invar behaviour is called two gamma state model suggested by Weiss, considers two magnetic states of Fe in Invar alloys one with high spin high volume and another with low spin low volume states. When the temperature is increased low spin low volume states get populated at the cost of high spin high volume states and thus compensate the expected thermal expansion. However, this was shown to be incorrect and it seems that high moment to low moment transition is preceded by a frustrated ferromagnetic state. The moment volume instabilities in Invar alloys also lead to anomalous elastic properties. The Invar alloys find applications in the fabrication of watches, cryogenic storage dewars and aerospace engineering parts.