In.var and re.var

INVAR Effect- after 100 years finally understood

In 1897 the Swiss physicist Charles Edouard Guillaume discovered that fcc Fe-Ni alloys with a Ni concentration around 35 atomic %, now called INVAR, exhibit anomalously low, almost zero, thermal expansion over a wide temperature range. This discovery immediately found widespread application in the construction of calibrated, high-precision mechanical instruments, such as seismographs and hair springs in watches. Today, Invar alloys are used in temperature-sensitive devises, such as shadow masks for television and computer screens. In 1920 Guillaume was awarded the Nobel Prize in Physics for the discovery of these Fe-Ni alloys.

It was realized early on that the explanation of the Invar effect is related to magnetism. Yet, though it has been 100 years since this effect was discovered, it was not understood. In a recent article published in Nature ''Origin of the Invar effect in iron-nickel alloys (Nature 400, 46 (1999)), I. Abrikosov and B. Johansson from Uppsala node of the Network, in collaboration with Mark van Schilfgaarde from Sandia National Laboratories, Livermore, USA, presented results of ab initio calculations of volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random fcc Fe-Ni Invar alloy, where they allowed for noncollinear spin alignments, i.e. where the spins may be canted with respect to the average magnetization direction. They have found that the evolution of the magnetic structure already at zero temperature is characterized principally by a continuous transition from the ferromagnetic state at high volumes to a disordered noncollinear configuration at low volumes, and that there is an additional, comparable contribution to the net magnetization from the changes in the amplitudes of the local magnetic moments. The noncollinearity gave rise to an anomalous volume dependence of the binding energy curve, and this allowed Mark van Schilfgaarde, I. Abrikosov and B. Johansson to explain the well-known peculiarities of Invar systems.

http://psi-k.dl.ac.uk/TMR1/summary_report.htm
The result on INVAR has been obtained within EU TMR
Network Ab-Initio Calculations of Magnetic Properties of Surfaces, Interfaces and Multilayers guided by prof. Walter from Daresbury Laboratory, Warrington, UK
We thank for the permission.

Memory-shape flaps do not require huge hydraulic actuators but only heating wires. They are used in USA military aviation from sixties Other applications of memory-shape alloys span from robot actuators, hydraulic fittings  to medical protesis.

The most common shape-memory metals are Nickel-titanium 50-50 alloys or copper alloys, like  CuZnAl, and CuAlNi, but even Pt is used. The wire, twisted at low temperature, when heated will come back to the original shape. And the “original” shape? It is fixed bending the wire at 500ºC. It can be bent and unbent a million times

In shape-memory alloys two sub-phases coexist: hard, high-T austenite and low-T, plastic martensite. Cooling brings all austenite to martensite. Subsequent deformation keeps the martensite structure intact. It return to the original austenite after heating.

“Normal” material are subject to a permanent plastic deformation, once stress exceeds the limit of elasticity. In shape-memory alloys, a thermal treatment, after the stress has been removed, brings the object to the original dimensions.

After: K. Ireland, University of Wollongong, Material Engineering

This (left) picture from Transmission Electron Microscopy shows coexistence of martensite (long needles) and austenite (patches) phases. In pure titanium only one phase exists (Scanning EM).

This Fe-Cr-Ni-Mo dual-phase stainless steel elongates better than the chewing gum