Metallic glass

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Typical laboratory sample of a metallic solid glass. The cone or wedge shape is often chosen if the maximum thickness that can be achieved is not yet precisely known.

Amorphous metals or metallic glasses are - in contrast to common window glasses, glasses or, more generally, silicate glasses , which usually show insulating behavior - metal or metal-and-non-metal alloys that are not crystalline but amorphous at the atomic level Have structure and still show metallic conductivity. The amorphous atomic arrangement, which is very unusual for metals, results in a unique combination of physical properties: Metallic glasses are generally harder, more corrosion-resistant and stronger than ordinary metals. However, the ductility characteristic of most metals is usually lacking.

A general distinction is made between conventional metallic glasses, which can only be produced as thin layers or ribbons, and the relatively new solid metallic glasses . The latter form one of the most modern material classes and are the subject of intensive research in materials science and solid-state physics. Due to the still very limited knowledge and the high price, they have so far only been represented in niche applications.

Construction and manufacture

Glasses are solid materials without a crystal structure. This means that the atoms do not form a lattice, but at first glance appear to be randomly arranged: there is no long-range , but at most a short-range order ; this structure is called amorphous .

Like all glasses, amorphous metals are also created by preventing natural crystallization. This can be done, for example, by rapidly cooling (“quenching”) the melt, so that the atoms are robbed of their mobility before they can adopt the crystal arrangement. However, it is precisely this that is particularly difficult with metals, because their special bonding mechanisms mean that in most cases unrealistically high cooling rates are required. In the case of metals that consist of only one element, it is even impossible to produce a metallic glass, since the mobility of the atoms is so high at low temperatures that they always crystallize. Only alloys made from at least two metals that can be amorphized (e.g. Au In 2 ) are known. More common are amorphous alloys made of just one metal - e.g. B. Fe - and a so-called glass former - z. B. boron or phosphorus, for example in the composition Fe 4 B. Technically relevant amorphous metals are still only special alloys (mostly close to the eutectic point ) made of several elements for which the necessary cooling rate is technically achievable. For the first metallic glasses this was up to 10 6  K / s . (For comparison: with silicates a cooling rate of about 0.1 K / s is sufficient to prevent crystallization. However, if their melt were allowed to cool down slowly enough, they would also crystallize.)

Metallic glass is created as a thin band from a melt (A), which is poured onto a cooled, rotating wheel (B) and cools down suddenly. This creates a thin band (C).

The thermal conductivity sets a physical limit to rapid cooling: No matter how quickly the ambient temperature is lowered, the heat must be transported from the inside of the material to the outside surface. This means that, depending on the required cooling rate and the thermal conductivity, only a certain sample thickness can be achieved. One method is rapid cooling between rotating copper rollers ( melt spinners ). Although this is simple and inexpensive, it only allows the production of thin strips and wires.

Thin amorphous layers and amorphous ribbons can also be obtained by chemical vapor deposition or sputter deposition .

Only in recent years knows one solid metallic glasses (ger .: bulk metallic glasses ) that enable material thicknesses greater than a millimeter (an arbitrary limit). The expectations of this new class of materials are high, even if these materials have so far been used little. They usually consist of five or more different elements, with three fundamentally different atom sizes being represented. The resulting crystal structures are so complex that cooling rates of a few Kelvin per second are sufficient to suppress crystallization. Achievable thicknesses are currently one to two centimeters, with only alloys with very expensive components (e.g. zirconium , yttrium or platinum ) reaching 25 millimeters. Only PdCuNiP comes through this mark , which has held a lonely record of more than seven centimeters since 1997. Since it to a mole fraction of 40 percent palladium is, the price is very high.

properties

Metallic glasses show u. a. the typical metallic light reflection and are indistinguishable from ordinary metals for the layman. The surface can be polished to be particularly smooth and, due to its great hardness, is not easily scratched, so a particularly beautiful and lasting shine can be achieved.

Metallic glasses are

  • harder than their crystalline counterparts and have high strength. Small deformations (≈ 1%) are purely elastic. This means that the energy absorbed is not lost as deformation energy, but is given off again when the material springs back (hence its use in golf clubs, for example ). However, the lack of ductility also makes it brittle: if the material fails, it does so suddenly and by breaking, not by bending, as with a metal.
  • The corrosion resistance is usually higher than that of metals of comparable chemical composition. This is because corrosion usually attacks the grain boundaries between the individual crystallites of a metal, which amorphous materials do not.

There are magnetic and non-magnetic amorphous metals. Some of them are (largely due to the lack of crystal defects):

Conventional metals typically contract suddenly when they solidify. Since solidification as glass is not a first-order phase transition , this jump in volume does not take place here. When the melt of a metallic glass fills a shape, it retains it when it solidifies. This is a behavior that is known from polymers, for example, and which offers great advantages in processing (e.g. injection molding). The greatest hopes for the future importance of amorphous metals are placed in this property.

history

The early history of metallic glasses is closely linked to basic research on the condition of the glass itself. As early as the 1950s, the American physicist David Turnbull predicted, as part of his pioneering work on the subcooling of melts, that in principle any liquid could be cooled into the glassy state, if only its viscosity decreased quickly enough with temperature. Metals, with their properties that are particularly unfavorable for glass formation, were considered to be the touchstone of this idea.

The first amorphous metal was made by Paul Duwez (1907–1984) at the California Institute of Technology around 1960 . He used an alloy of gold and silicon in a ratio of 3: 1, very close to the eutectic point (19% silicon). The melting point of this mixture is around 500 ° C (for comparison: pure gold melts at 1063 ° C, pure silicon at 1412 ° C). The alloy remains liquid even at relatively low temperatures, which promotes glass formation. Duwez cooled his samples at more than a million Kelvin per second, but only achieved material thicknesses of less than 50 micrometers.

In 1976 H. Liebermann and C. Graham developed a technique in which long strips of amorphous metals could be produced quickly and cheaply using cooled rollers. In 1980 this led to the commercialization of the first metallic glasses under the trade name Metglas (e.g. Metglas 2705M : 75-85% by weight of cobalt , small amounts of boron , iron , molybdenum , nickel and silicon). A very successful anti-theft system in department stores uses magnetic strips made from this material.

Due to the complex manufacturing process, the low thickness that can be achieved and the high price, metallic glasses have been a physically highly interesting, but rather academic curiosity for decades. This changed suddenly in the early 1990s when the first solid metallic glasses based on palladium (very expensive) and zirconium were discovered. The first solid metallic glass ever, consisting of palladium, nickel and phosphorus , was produced by Lindsay Greer and David Turnbull in 1982. The first commercial alloy was brought onto the market by Liquidmetal Technologies under the trade name Vitreloy1 ( Liquidmetal consisting of 41.2% Zr, 13.8% Ti, 12.5% ​​Cu, 10% Ni, and 22.5% Be) brought.

The solid metallic glasses currently commercially available are made up of relatively expensive elements and, although they have now found numerous uses, are still limited to expensive niche products. Great expectations are therefore placed on the amorphous iron-based alloys discovered in the mid-1990s. In order to underline their potential to laypeople, research groups like to use the term amorphous steel , which is supposed to create a connection to what is probably the most successful metal of our time. In fact, these alloys only consist of about 50% iron . To prevent crystallization, three fundamentally different atom sizes must be present. In addition to the medium-sized iron atoms (usually also 5 to 20% chromium and manganese ), the alloys contain significant amounts of atomically larger refractory metals (usually 10% to 20% molybdenum ), as well as the atomically small elements carbon and boron (together usually more than 20%) . The first amorphous steels were discovered by Akihisa Inoue at Tōhoku University in Japan and reached thicknesses of one to two millimeters. A breakthrough is considered to be more than ten millimeters, which was achieved in 2004 by two research groups at Oak Ridge National Laboratory in Tennessee and at the University of Virginia in Charlottesville, both in the USA . The alloys in question also contain 1% to 2% rare earth metals , usually yttrium or erbium . It has not yet been conclusively clarified whether their positive influence on glass formation is due to their extreme atomic size or their high oxygen affinity, through which the melt is cleaned of disruptive oxygen atoms.

Current research focuses on the still problematic fracture behavior of amorphous metals. A higher plastic deformability would be desirable, so that the material yields somewhat under high loads rather than breaking immediately. While solid-state physics tries to clarify fundamental questions about the fracture mechanisms, materials scientists are currently striving to prevent these mechanisms. Possible approaches are embedding foreign particles ( carbon fibers , nanotubes etc.) or deliberately allowing the formation of small crystallites in the amorphous phase. The result would be a composite that offers the benefits of metallic glasses without suffering from the disadvantages.

Another problem is that amorphous steels in particular usually have to be manufactured under laboratory conditions (for example in a vacuum). Here too, progress is currently being made.

Applications

Conventional metallic glasses, which can be manufactured relatively inexpensively as thin strips, have been used since the 1980s mainly in the following areas of electrical engineering due to their special soft magnetic properties :

  • as cores for sensors (current transformer, FI switch).
  • as cores for transformers with particularly low no-load losses. These are mainly used in the USA.
  • in harmonious and acousto-magnetic security labels .

Solid metallic glasses have a unique combination of material properties, but are relatively expensive. They are therefore mainly used in luxury items or high-tech applications (also in the military sector), where the high price plays a subordinate role. The commercially available solid metallic glasses often compete with titanium . The pioneer is Liquidmetal Technologies , which mainly offers zirconium-based glasses. Other commercial suppliers of solid metallic glasses are YKK and Advanced Metal Technology .

Aerospace
Since high material prices do not play a role in these areas due to the generally high costs and the top priority of safety, metallic glasses are considered here wherever their special properties could play a role. Parts of the solar wind collectors of the Genesis probe were made of amorphous metal.
Material refinement for industrial applications
The surface properties of conventional materials can be made harder, more resistant and more wear-resistant by coating with amorphous metals (commercial example: Liquidmetal-Armacor Coating ).
medicine
Scalpels (especially ophthalmic ) made of amorphous metal are already available , which because of their great hardness are sharper than those made of stainless steel and keep their sharpness longer. Due to the biocompatibility , the high strength with relatively low weight and the resistance to wear and tear , consideration is given to surgical implants.
military
Numerous development projects, especially those of the US Department of Defense, are testing the use of amorphous metals for various applications. Tungsten- based metallic glasses, for example, are supposed to replace conventional tungsten alloys and depleted uranium in armor-piercing bullets because of their great hardness and self-sharpening behavior . In military aviation, amorphous metal coatings are said to increase the hardness and corrosion resistance of light metals such as aluminum and titanium.
Jewellery
Some metallic glasses are made of precious metals (e.g. platinum ), but are much harder than these and therefore do not scratch. In addition, the special processing options make it possible to create shapes that are difficult to achieve with conventional metals.
Sports and leisure articles
Golf club in 1998 one of the first commercial products of amorphous metal and were within the scope of large-scale advertising campaigns (including the PGA - professional golfer Paul Azinger ) used by the company Liquidmetal for the launch of the material. Golf clubs particularly benefit from the unrivaled elasticity of amorphous metals. In development (though not yet commercialized in part) are tennis and baseball bats , fishing equipment, skis , snowboards , bicycles and sport rifles.
Consumer electronics
The smooth, shimmering and scratch-resistant surface of metallic glasses has led to its use for the housing of exclusive cell phones , MP3 players and USB sticks . The high strength (better than titanium ) allows thinner walls, thus even lower weight and even more miniaturization. Processing in injection molding allows more freedom in design and cheaper processing than stainless steel or titanium, which have to be forged. Delicate mobile phone hinges, where great forces act on the smallest components, benefit from the superior mechanical properties of metallic glasses.

Amorphous steels are expected to meet high expectations once they are ready for the market. In contrast to the already commercialized metallic glasses, the material costs would be low enough to make them a full-fledged structural material that is also suitable for larger components. If the existing technical problems are solved and amorphous steels are ready for the market, they would primarily compete with titanium and stainless steel and score with their higher corrosion resistance and better processability.

literature

  • Werner Schatt, Hartmut Worch, Horst Blumenauer: Materials science. 8th edition. German Verl. For basic industry, Stuttgart 1996, ISBN 3-342-00675-7 .
  • Karl Nitzsche, Hans-Jürgen Ullrich, Jürgen Bauch: Functional materials in electrical engineering and electronics. 2nd Edition. German For basic industry, Leipzig 1993, ISBN 3-342-00524-6 .
  • M. Barrico (Ed.): Advanced Engineering Materials. 9, Special Issue: Bulk Metallic Glasses, 2007, doi: 10.1002 / adem.200790013 .
  • AI Salimon, MF Ashby, Y. Bréchet, AL Greer: Bulk metallic glasses: What are they good for? In: Materials Science and Engineering A. 375-377, No. 1-2, 2004, pp. 385-388, doi: 10.1016 / j.msea.2003.10.167 .
  • J. Schroers, N. Paton: Amorphous metal alloys form like plastics. In: Advanced Materials and Processes. 2006, pp. 61-63.
  • WH Wang; C. Dong; CH Shek: Bulk metallic glasses . In: Materials Science and Engineering R: Reports. 44, No. 2–3, 2004, pp. 45–89, doi: 10.1016 / j.mser.2004.03.001 .
  • T. Hartmann, D. Nuetzel: New Amorphous Brazing Foils For Exhaust Gas Applications . In: Anatol Rabinkin (Ed.): Brazing and soldering: Proceedings of the 4th International Brazing an Soldering Conference, April 26-29, 2009, Orlando, Florida, USA . Miami, ISBN 978-0-87171-751-1 , pp. 110-117 ( PDF ).
  • G. Herzer: Amorphous and nanocrystalline soft magnets . In: George C. Hadjipanayis (Ed.): Proceedings of the NATO Advanced Study Institute on Magnetic Hysteresis in Novel Materials, Mykonos, Greece, 1-12 July 1996 . tape 338 . Kluwer Academic Publishers, Dordrecht / Boston / London 1997, ISBN 0-7923-4604-1 , p. 711-730 ( PDF ).

Web links

Commons : Metallic Glasses  - collection of images, videos and audio files

References and comments

  1. Only if with extremely strong "amorphicity" the typical energies of the spatial potential fluctuations were much greater than the involved characteristic thermal excitation energies of the metallic glasses - which is not the case - only then would a special quantum mechanical effect, the so-called Anderson- Localization , the electronic state of the system is not metallic, but insulating.
  2. ^ Akihisa Inoue: Bulk Glassy Alloys: Historical Development and Current Research . In: Engineering . tape 1 , no. 2 , 2015, p. 185-191 , doi : 10.15302 / j-eng-2015038 .
  3. AJ Drehman, AL Greer, D. Turnbull: Bulk formation of a metallic glass: Pd40Ni40P20 . In: Applied Physics Letters . tape 41 , 1982, pp. 716-717 , doi : 10.1063 / 1.93645 .
  4. ^ ZP Lu, CT Liu, JR Thompson, WD Porter: Structural Amorphous Steels . In: Physical Review Letters . tape 92 , no. 24 , 2004, pp. 245503 , doi : 10.1103 / PhysRevLett.92.245503 .
  5. VR Ramanan, M. Carlen, ABB, Distribution goes green PDF  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / www.lead-central.com