Amorphous metal

Amorphous metals or metallic glasses - in contrast to the conventional windows, spectacles or general silicate glasses generally exhibit insulating properties - are alloys having no crystalline, but has an amorphous structure at an atomic level and yet exhibit metallic conductivity. The very unusual for metals amorphous atomic arrangement has a unique combination of physical properties result in: Metallic glasses are generally harder, corrosion resistant and stronger than ordinary metals. However, characteristic of most metals commonly lacks ductility.

A distinction is generally conventional metallic glasses which can be prepared only as a thin layer or bands, and the relatively new massive metallic glasses. The latter form one of the most modern material classes and are subject of intense research in materials science and solid state physics. Due to the very limited knowledge and the high price they are currently represented only in niche applications.

Design and production

Glasses are solid materials without crystal structure. This means that the atoms do not form a lattice, but are randomly arranged at first glance: There is no remote, but at best a short-range order, this structure is referred to as amorphous.

Like all glasses are also formed amorphous metals by the natural crystallization is prevented. This can, for example, by rapid cooling ( " quenching" ) done the melt so that the atoms of the mobility is stolen before they can take the crystal array. However, currently this is particularly difficult with metals, because due to their specific binding mechanisms in most cases require unrealistically high cooling rates. For the metals which consist of only one element, it is even impossible to produce a metallic glass, as the mobility of the atoms down to low temperatures is so high that they always crystallize. It is only known alloys of at least two metals which are amorphisierbar (eg AuIn2 ). More common are amorphous alloys of one metal - eg Fe - and a so-called glass formers - eg, boron or phosphorus, such as in the composition Fe80B20. Industrially relevant amorphous metals are to this day even only special alloys (usually close to the eutectic point ) consists of several elements for which the necessary cooling rate is technically feasible. This was the first metallic glasses even up to 106 K / s (For comparison: . Silicates at a sufficient cooling rate of about 0.1 K / s to prevent crystallization If one, however, let her melt to cool slowly enough so they too would crystallize. )

The thermal conductivity is a physical limit to the rapid cooling no matter how fast the ambient temperature is lowered, the heat must be transported from the interior of the material to the outer surface. This means that depending on the required cooling rate, and the thermal conductivity only a certain thickness of the sample can be achieved. One method is the rapid cooling between rotating copper rollers. Although this is simple and inexpensive, but 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 since a few years knows one massive metallic glasses (English: bulk metallic glasses ), which allow material thicknesses greater than one millimeter ( an arbitrary limit). The expectations of this new class of materials are high, even if they are been used very little. They consist usually of five or more different elements, usually three fundamentally different atomic sizes are represented. The resulting crystal structures are so complex that already sufficient cooling rates of a few Kelvin per second in order to suppress the crystallization. Achievable thicknesses are currently between one and two centimeters, only alloys with very expensive components (eg zirconium, yttrium, or platinum) reach 25 millimeters. About this brand comes only PdCuNiP holding a clear record of more than seven centimeters since 1997. Since there is a mole fraction of 40 per cent of palladium, the price is very high.

Properties

Metallic glasses show, inter alia, the typical metallic light reflection and are indistinguishable to the layman of ordinary metals. The surface can be polished very smooth and will not scratch due to the high hardness so easily, so can achieve a particularly beautiful and lasting shine.

Metallic glasses are

• Harder than their crystalline counterparts and have high strength. Small deformations ( ≈ 1%) are purely elastic. That is, the absorbed energy is not lost as the deformation energy, but is again fully discharged when spring-back of the material (hence the use of, for example, in golf clubs ). The lack of ductility but also makes them brittle: If the material fails, then abruptly and by breaking, not by bending, as in a metal.
• The corrosion resistance is usually higher than metals comparable chemical composition. This is because most corrosion attack at grain boundaries between the Einzelkristalliten a metal which does not exist in amorphous materials.

There are magnetic and non- magnetic amorphous metals. Some of them are (mainly because of the lack of " crystal defects " )

• The best commercially available soft magnetic materials: The amorphous alloys of the " glass - formers " boron, silicon and phosphorus and the metals iron, cobalt and / or nickel are magnetic, normal ( ie non- dominance of cobalt), " soft-magnetic ", ie low-coercivity, and have the same
• A high electrical resistance (usually metallic, although the conductivity is, however, of the same order as in the molten metal just above the melting point ). This leads to low " electric eddy current loss " (English: losses by "eddy currents " ), which makes the materials for transformers interesting ( see below).

Conventional metals typically draw upon solidification abruptly together. Since the solidification is as glass, no phase transition of first order, this volume will jump here not take place. If the melt of the metallic glass fills a mold, it retains this during solidification. This is a behavior which is known for example, from polymers, where (e.g., injection molding) has great advantages in processing. In this property, therefore, the highest hopes for the future importance of amorphous metals are set.

History

The early history of metallic glasses is closely linked to the basic research on the glassy state itself. Already in the 1950s, said the American physicist David Turnbull ahead as part of his pioneering work on undercooling of melts, that in principle, any liquid could be cooled into the glassy state, if only its viscosity would decrease fast enough with the temperature. Metals with their particularly unfavorable for the glass -forming properties were considered to spearhead this idea.

The first amorphous metal was created around 1960 by Paul Duwez ( 1907-1984 ) at the California Institute of Technology. He used an alloy of gold and silicon in the ratio 3:1, very close to the eutectic point (19 % silicon). The melting point of this mixture is approximately 500 ° C (compared to pure gold melts at 1063 ° C, pure silicon at 1412 ° C). Thus, the alloy remains a liquid even at relatively low temperatures, which favors the formation of glass. Duwez cooled from its samples with more than a million degrees Kelvin per second, but achieved only a material thickness of less than 50 microns.

1976 developed H. Lieberman and C. Graham, a technique in which fast and cheap long by means of cooled rollers tapes were made ​​of amorphous metals. This resulted in 1980 to the commercialization of the first metallic glasses under the trade name of Metglas (e.g. Metglas 2705M: 75-85 % by weight of cobalt, small amounts of boron, iron, molybdenum, nickel, and silicon). A very successful system to prevent theft in stores uses magnetic strip of this material.

Due to the elaborate manufacturing, low -reach thicknesses and the high price of metallic glasses were indeed a physically very interesting but rather academic curiosity for decades. This changed in the early 1990s abruptly when the first bulk metallic glasses were discovered on the basis of palladium (very expensive) and zirconium. The first solid metallic glass at all consisting of palladium, nickel and phosphorus was established in 1982 by Lindsay Greer and David Turnbull. The first commercial alloy was purchased from Liquid Metal Technologies under the trade name Vitleroy1 (comprising 41.2% Zr, 13.8% Ti, 12.5% ​​Cu, 10% Ni, and 22.5 % loading ) on the market.

The current commercially available bulk metallic glasses consist of relatively expensive elements and, although they have since found numerous applications, still limited to expensive niche products. Great expectations are therefore directed discovered the mid-1990s, amorphous iron-based alloys. To over lay to underline their potential use research groups like the term amorphous steel, which is to perceive a link to the most successful metal of our time. Actually, however, these alloys consist of only approximately 50% of iron. To prevent crystallization, three fundamentally different atomic sizes must be present. In addition to the medium-sized iron atoms (usually also vary from 5 to 20 % chromium, and manganese) containing alloys, significant amounts atomically more refractory metals (usually 10 to 20% molybdenum), and atomically small elements carbon and boron ( together usually more than 20 %). The first amorphous steels were discovered by Inoue Akihisa the Tohoku University in Japan, reaching a thickness of one to two millimeters. As a breakthrough applies the achievement of more than ten millimeters, which in 2004 by two research groups at Oak Ridge National Laboratory in Tennessee and the University of Virginia in Charlottesville, both were achieved in the U.S.. The alloys concerned additionally contain 1 to 2% rare earth metals, usually yttrium or erbium. It is not yet clear whether the positive influence on the formation of glass lies at the extreme atom size or to their high oxygen affinity, by which the melt is purified of harmful oxygen atoms.

The current research focuses on the still problematic fracture behavior of amorphous metals. Desirable would be a higher plastic deformation, so that the material at high loads rather gives something, instead of breaking the same. While the solid-state physics attempts to address fundamental questions about the fracture mechanisms, materials scientists are currently working afterwards to prevent these mechanisms. Possible approaches are, embedding of foreign particles ( carbon fibers, nanotubes, etc.) or intentional allowing the formation of small crystallites in the amorphous phase. The result would be a composite material, which offers the advantages of metallic glasses, without suffering from the disadvantages.

Another problem is that, in particular, the amorphous steels usually still have to be produced under laboratory conditions (for example in vacuo ). Again, progress currently being made ​​.

Applications

Conventional metallic glasses, which can be relatively inexpensively as thin bands, are used because of their special soft magnetic properties since the 1980s, mainly in the following application areas of electrical engineering:

• As nuclei for sensors ( current transformer RCD).
• As cores for transformers, with particularly low no-load losses. These are used primarily in the United States.
• In harmonious and acousto-magnetic EAS labels.

Pioneer and leader in both cases is the company Metglas with their eponymous alloys.

Massive metallic glasses having a unique combination of material properties, however, are relatively expensive. You can find so your application mainly in luxury goods or high-tech applications (including military), where the high price plays a subordinate role. The commercially available bulk metallic glasses are often in competition with titanium. Pioneer is the company Liquidmetal Technologies, which offers mainly zirconium -based glasses. Other commercial suppliers of bulk metallic glasses are YKK and Advanced Metal Technology.

• Aerospace: As in these areas high material prices do not play a role due to the generally high costs and the top priority of safety, metallic glasses are everywhere considered here, where their special properties could play a role. Parts of the solar wind collectors of Genesis probe consisted of amorphous metal.
• Material processing for industrial applications: The surface properties of conventional materials can be made harder, more resistant and wear-resistant by coating with amorphous metals ( commercial example: Liquid Metal Coating Armacor ).
• Medicine: Already available (especially ophthalmic ) scalpels made ​​of amorphous metal, which are sharper because of the high hardness such as stainless steel and retain their sharpness longer. Due to the biocompatibility, high strength at relatively low weight and resistance to wear is thinking about surgical implants.
• Military: Many development projects, particularly the U.S. Department of Defense to test the use of amorphous metals for various applications. How to tungsten -based metallic glasses replacement due to high hardness and self-sharpening behavior of conventional tungsten alloys and depleted uranium in armor-piercing kinetic energy projectiles. In military aviation, amorphous metal coatings to increase the hardness and corrosion resistance, lightweight metals such as aluminum and titanium.
• Decorations: Some metallic glasses are made of precious metals (eg platinum ), but are much harder than this and therefore do not scratch. In addition, can generate forms that are difficult to achieve with conventional metals due to the special processing options.
• Sports and leisure products: golf clubs in 1998 were one of the first commercial products of amorphous metal and were used as part of large-scale advertising campaigns (including the PGA Tour golfer Paul Azinger ) of the company Liquidmetal for the launch of the material. Golf clubs especially benefit from the unrivaled elasticity of amorphous metals. In development (albeit partially not yet commercialized ) are tennis and baseball bats, fishing equipment, skis, snowboards, bicycles and sporting rifles.
• Consumer electronics: The smooth, shining and scratch-resistant surface of metallic glasses has led to the use for the housing of exclusive mobile phones, MP3 players and USB sticks. The high strength (better than titanium) allows for thinner wall thicknesses, so that even lower weight and more miniaturization. The processing in the injection molding allows more freedom in design and less expensive processing as stainless steel or titanium, which have to be forged. Petite mobile phone hinges, where large forces are applied to the smallest components, to benefit from the superior mechanical properties of metallic glasses.

High expectations are met with the amorphous steels, should they reach the market. In contrast to the commercialized metallic glasses, the material cost would be low enough to make it a full-fledged structural material, which is also suitable for larger components. Should the existing technical problems are solved and obtain amorphous steels market, they would come to titanium and stainless steel, especially in competition and score points by their higher corrosion resistance and better processability.