O n t h e T r a c k o f M o d e r n P h y s i c s

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.htmThe 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 Temmerman from Daresbury Laboratory, Warrington, UK We thank for the permission.Some material do not expand with temperature, some of them even shrink, like Germanium at low temperatures. Sometimes, they expand in one dimension but shrink in another direction.

Negative expansion is often connected to the presence of some sub-crystal structures, moving and rotating independently.An axample is silver-copper oxide, of the cuprite symmetry structure [2].

Some material shrink with temperature,

another come back to their old shape.

We call them “shape-memory” alloys.

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Germanium

Thermal expansion coefficient

Photon energy h

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Two inter-penetrating networks of corner sharing M4O tetrahedra with O-M-O linear coordination

Ag2O & Cu2O

We can see these tiny changes of interatomic distances with an even smaller sonde (X-rays). One of the techniques is called “Extended Fine Structure X-Ray Absorption” It works in the following manner:

1. X-ray (synchrotron radiation) is tuned to the energy of an internal-electron level of the probe atom.

2. An absorbed X-ray quantum releases an electron from this atom.

3. This electron (i.e. the quantum wave objects) interfers

with itself, getting scattered on neighborourhood atoms.

4. A fine structure, depending on the interatomic diustances is observed in the X-ray absorption. In this mode the neighbourhood of the X-ray absorping atom is exploited.

Figures are from prof. P. Fornasini, University of Trento, Physics Department,Thanks!

Do you remember old spring-moved clocks?

Their pendulum was attached to thin, high-precision spings, made of INVAR.

Old material, but new discoveries!

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.

USAF Aircraft Pictures - http://sun.vmi.edu/hall/afpics.htm

Memory-shape flaps do not require hugehydraulic 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.

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

www.fz-juelich.de/iwv/iwv1/datapool/page/9/fgl2.jpg Forschungszentrum Jülich

Nitinol Devices and Componentshttp://www.nitinol.com/images/3slide3.gif

http://www.ifm.eng.cam.ac.uk/people/sc444/ Innovative Manufacturing Research Centre Cambridge University Engineering Department

After: K. Ireland, University of Wollongong, Material Engineeringhttp://www.uow.edu.au/eng/mm/matl/ShapeMemoryAlloys.pdf

plastic deformation

Hook’s law

loading

unloading

stress

plastic deformation

Hook’s law

loading

unloading

stress

Shape memory

“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.

deformation deformation

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.

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).

Invar is Fe-Ni, 65-35% alloy.

Ab-initio calculations [1] showed that magnetic

moments of nickel (blue here) are aligned, while

those of iron (red) seem to be chaotic. And this

disorder-in- order assures the minimum volume

(and energy) Read below.

[1] M. van Schilfgaarde, I. A. Abrikosov, B. Johansson, Origin of the Invar effect in iron–nickel alloys, Nature 400 (1999) 46 [2] S. a Beccara, G. Dalba, P. Fornasini, R. Grisenti, A. Sanson, and F. Rocca, Local thermal expansion in a cuprite structure: the case of Ag2O, Phys. Rev. Lett. 89, 25503 (2002)

And this “cosmic” rubber is soft, if torn slowly, but springs, if hit.

The Material Science came to the playground!

Nature 400 (1999) 46