Amorphous metal

An amorphous metal (also known as metallic glass or glassy metal) is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a highly ordered arrangement of atoms. Amorphous metals are non-crystalline, and have a glass-like structure. But unlike common glasses, such as window glass, which are typically electrical insulators, amorphous metals have good electrical conductivity. There are several ways in which amorphous metals can be produced, including extremely rapid cooling, physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying.

In the past, small batches of amorphous metals have been produced through a variety of quick-cooling methods. For instance, amorphous metal ribbons have been produced by sputtering molten metal onto a spinning metal disk (melt spinning). The rapid cooling, on the order of millions of degrees Celsius a second, is too fast for crystals to form and the material is “locked” in a glassy state. More recently a number of alloys with critical cooling rates low enough to allow formation of amorphous structure in thick layers (over 1 millimeter) have been produced; these are known as bulk metallic glasses (BMG). More recently, batches of amorphous steel with three times the strength of conventional steel alloys have been produced.

The first reported metallic glass was an alloy (Au75Si25) produced at Caltech by W. Klement (Jr.), Willens and Duwez in 1960. This and other early glass-forming alloys had to be cooled extremely rapidly (on the order of one megakelvin per second, 106 K/s) to avoid crystallization. An important consequence of this was that metallic glasses could only be produced in a limited number of forms (typically ribbons, foils, or wires) in which one dimension was small so that heat could be extracted quickly enough to achieve the necessary cooling rate. As a result, metallic glass specimens (with a few exceptions) were limited to thicknesses of less than one hundred micrometers.

In 1969, an alloy of 77.5% palladium, 6% copper, and 16.5% silicon was found to have critical cooling rate between 100 and 1000 K/s.

In 1976, H. Liebermann and C. Graham developed a new method of manufacturing thin ribbons of amorphous metal on a supercooled fast-spinning wheel. This was an alloy of iron, nickel, phosphorus and boron. The material, known as Metglas, was commercialized in the early 1980s and is used for low-loss power distribution transformers (Amorphous metal transformer). Metglas-2605 is composed of 80% iron and 20% boron, has Curie temperature of 373 °C and a room temperature saturation magnetization of 1.56 teslas.

In the early 1980s, glassy ingots with 5 mm diameter were produced from the alloy of 55% palladium, 22.5% lead, and 22.5% antimony, by surface etching followed with heating-cooling cycles. Using boron oxide flux, the achievable thickness was increased to a centimeter.

Research in Tohoku University and Caltech yielded multicomponent alloys based on lanthanum, magnesium, zirconium, palladium, iron, copper, and titanium, with critical cooling rate between 1 K/s to 100 K/s, comparable to oxide glasses.

In 1988, alloys of lanthanum, aluminium, and copper ore were found to be highly glass-forming. Al-based metallic glasses containing Scandium exhibited a record-type tensile mechanical strength of about 1500 MPa.

In the 1990s new alloys were developed that form glasses at cooling rates as low as one kelvin per second. These cooling rates can be achieved by simple casting into metallic molds. These “bulk” amorphous alloys can be cast into parts of up to several centimeters in thickness (the maximum thickness depending on the alloy) while retaining an amorphous structure. The best glass-forming alloys are based on zirconium and palladium, but alloys based on iron, titanium, copper, magnesium, and other metals are also known. Many amorphous alloys are formed by exploiting a phenomenon called the “confusion” effect. Such alloys contain so many different elements (often four or more) that upon cooling at sufficiently fast rates, the constituent atoms simply cannot coordinate themselves into the equilibrium crystalline state before their mobility is stopped. In this way, the random disordered state of the atoms is “locked in”.

In 1992, the commercial amorphous alloy, Vitreloy 1 (41.2% Zr, 13.8% Ti, 12.5% Cu, 10% Ni, and 22.5% Be), was developed at Caltech, as a part of Department of Energy and NASA research of new aerospace materials. More variants followed.

In 2004, bulk amorphous steel was successfully produced by two groups: one at Oak Ridge National Laboratory, who refers to their product as “glassy steel”, and the other at the University of Virginia, calling theirs “DARVA-Glass 101”. The product is non-magnetic at room temperature and significantly stronger than conventional steel, though a long research and development process remains before the introduction of the material into public or military use.

In 2018 a team at SLAC National Accelerator Laboratory, the National Institute of Standards and Technology (NIST) and Northwestern University reported the use of artificial intelligence to predict and evaluate samples of 20,000 different likely metallic glass alloys in a year. Their methods promise to speed up research and time to market for new amorphous metals alloys.

Construction and production
Glasses are solid materials without crystal structure. That is, the atoms do not form a lattice, but are arranged randomly at first glance: there is no distance, but at most a close order, this structure is called amorphous.

Like all glasses, amorphous metals are created by preventing natural crystallization. This can be done, for example, by rapid cooling (“quenching”) of the melt so that the atoms are robbed of the mobility before they can take the crystal arrangement. However, this is particularly difficult for metals, since it requires unrealistically high cooling rates in most cases due to their special binding mechanisms. With metals that consist of only one element, it is even impossible to produce a metallic glass, because the mobility of the atoms down to low temperatures is so high that they always crystallize. Only alloys of at least two metals that are amorphisable are known (eg, AuIn2). More common are amorphous alloys of only one metal Fe – and a so-called glass former -. B. boron or phosphorus, such as in the composition Fe4 B.. Technically relevant amorphous metals are even today only special alloys (usually close to the eutectic point) of several elements for which the necessary cooling rate is technically achievable. This was still up to 10 6 K / s for the first metallic glasses. (For comparison: in the case of silicates, a cooling rate of about 0.1 K / s suffices to prevent crystallization, but if they were allowed to cool down slowly enough, they too would crystallize.)

The thermal conductivity places a physical limit on rapid cooling: no matter how quickly the ambient temperature is lowered, the heat has to 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 the rapid cooling between rotating copper rollers (melt spinning). This is simple and inexpensive, but only allows the production of thin strips and wires.

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

Only a few years ago, massive metallic glasses (Bulk metallic glasses) are known, which allow material thicknesses of more than one millimeter (an arbitrarily chosen limit). Expectations for this new class of materials are high, even though they have been little used so far. They usually consist of five or more different elements, with usually three fundamentally different atomic sizes are represented. The resulting crystal structures are so complex that even cooling rates of a few Kelvin per second are sufficient to suppress the crystallization. Achievable thicknesses are currently one to two centimeters, whereby only alloys with very expensive components (eg zirconium, yttriumor platinum) reach 25 millimeters. About this brand comes only PdCuNiP, which holds since 1997 a lone record of more than seven centimeters. Since there is a mole fraction of 40 percent palladium, the price is very high.

Amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The viscosity prevents the atoms moving enough to form an ordered lattice. The material structure also results in low shrinkage during cooling, and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wearand corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics.

Amorphous metal 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 a deformation energy, but is fully released when springing back the material (hence, for example, in golf clubs). However, the lack of ductility also makes them brittle: when the material fails, then suddenly and by breaking, not by bending, as with a metal.
The corrosion resistance is usually higher than for metals of comparable chemical composition. This is because corrosion usually attacks at grain boundaries between the single crystallites of a metal, which does not exist in amorphous materials.

There are magnetic and non-magnetic amorphous metals. Some of them are (essentially 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, usually (i.e., in the case of non-dominance of cobalt) soft magnetic, i. H. with low coercivity, and have at the same time
a high electrical resistance (usually the conductivity is metallic, but of the same order of magnitude as molten metals just above the melting point). This leads to low electrical eddy current losses, which makes the materials of transformers interesting (see below).

Conventional metals typically contract suddenly on solidification. Since solidification as a glass is not a first-order phase transition, this volume jump does not take place here. When the melt of a metallic glass fills a mold, it keeps it on solidification. This is a behavior that is familiar, for example, from polymers and that offers great advantages in processing (eg injection molding). The highest hopes for the future significance of amorphous metals are thus placed in this property.

Thermal conductivity of amorphous materials is lower than that of crystalline metal. As formation of amorphous structure relies on fast cooling, this limits the maximum achievable thickness of amorphous structures.

To achieve formation of amorphous structure even during slower cooling, the alloy has to be made of three or more components, leading to complex crystal units with higher potential energy and lower chance of formation. The atomic radius of the components has to be significantly different (over 12%), to achieve high packing density and low free volume. The combination of components should have negative heat of mixing, inhibiting crystal nucleation and prolonging the time the molten metal stays in supercooled state.

The alloys of boron, silicon, phosphorus, and other glass formers with magnetic metals (iron, cobalt, nickel) have high magnetic susceptibility, with low coercivity and high electrical resistance. Usually the conductivity of a metallic glass is of the same low order of magnitude as of a molten metal just above the melting point. The high resistance leads to low losses by eddy currents when subjected to alternating magnetic fields, a property useful for e.g. transformer magnetic cores. Their low coercivity also contributes to low loss.

Amorphous metals have higher tensile yield strengths and higher elastic strain limits than polycrystalline metal alloys, but their ductilities and fatigue strengths are lower. Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible (“elastic”) deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys. One modern amorphous metal, known as Vitreloy, has a tensile strength that is almost twice that of high-grade titanium. However, metallic glasses at room temperature are not ductile and tend to fail suddenly when loaded in tension, which limits the material applicability in reliability-critical applications, as the impending failure is not evident. Therefore, there is considerable interest in producing metal matrix composites consisting of a metallic glass matrix containing dendritic particles or fibers of a ductile crystalline metal.

Perhaps the most useful property of bulk amorphous alloys is that they are true glasses, which means that they soften and flow upon heating. This allows for easy processing, such as by injection molding, in much the same way as polymers. As a result, amorphous alloys have been commercialized for use in sports equipment, medical devices, and as cases for electronic equipment.

Thin films of amorphous metals can be deposited via high velocity oxygen fuel technique as protective coatings.

Production from the melt

Thermodynamic principle
As in the case of silica glass, the molten alloy, cooled to the solid state, will be amorphous only if the melting temperature T f has passed sufficiently fast that the constituent atoms of the alloy have not the time to organize according to a crystalline structure. That is, the liquid must be cooled at a speed above a critical speed R c such that the temperatures below T f are reached without the liquid having solidified.

This results in the continuity of the variation of a thermodynamic quantity such as the volume occupied by this phase (by maintaining the constant pressure) or one of the molarenergy thermodynamic functions, such as the enthalpy H, for example, without any change in slope at point T f. A crystallization would have led to a discontinuity for these quantities, and a change of their slope on a diagram (V, T) or (H, T).

After the passage of T f, the material is in a metastable state called supercooled liquid; it is still liquid, but its viscosity increases rapidly with the lowering of its temperature.

By continuing to lower the temperature, the liquid freezes into an amorphous solid where the atoms have a disordered organization similar to that which they had in the supercooled liquid.

The passage of supercooled liquid to amorphous solid results in a diagram (V, T) or (H, T) by breaking the slope of the curve at the point T g (glass transition temperature), without discontinuity of the specific volume or of enthalpy. If, left at constant temperature, the supercooled liquid can crystallize in observable times, this is no longer the case of the amorphous solid.

All this makes the similarity between metallic glass and silica glass. The major difference between these two types of materials from the point of view of their obtaining is the critical quenching speed R c which depends on the composition of the liquid to be cooled. If for silica glass, R c is low enough to work and shape the glass paste for a long time, the metals have a very high propensity to crystallize and the first amorphous alloys obtained for the Au 80 Si 20 binary required a hyperemperature at 10 6 K / s.

This difference in critical quenching speed means that the methods used and the parts obtained for these two materials are radically different.

Elaboration of the alloy
For a given alloy composition, the critical quenching rate R c is set; it varies from one alloy to another. For many compositions, no current method makes it possible to obtain an amorphous solid from the molten state. However, rules Empirical were stated by Akihisa Inoue that provide criteria to check for a better ability to form an amorphous solid. These rules say that:

the alloy must consist of several components (at least three elements and very often five or more);
the main elements of the alloy must have a difference in atomic sizes of at least 12%;
the binary and ternary phase diagrams of the constituent elements must have deep eutectics, which indicates slower atomic motions in the alloy;
the mixing energy between the main elements must be negative.

These rules are the result of experimental observations of trends and are however to be considered with caution: indeed, a slight change in the composition of the alloy, not altering the respect of the rules of Inoue can change the ability to form amorphous solids significantly.

The capacity to form an amorphous solid can be evaluated for example by the amplitude of the temperature range of the supercooled liquid zone. As it increases, the critical quenching rate R c decreases, which makes it possible to manufacture an amorphous solid with less rapid quenching, under less severe conditions and with a greater thickness. Since quenching techniques are difficult to improve and the quenching rate is always limited by the heat diffusion in the sample itself, the sometimes systematic exploration of alloy compositions for great abilities to form amorphous solids is a very active area of research.

Quenching methods
Once the alloy is developed, the quenching method greatly conditions the final shape of the objects produced: the liquid solidifies during quenching and the machining of these fragile materials is difficult. However, the amorphous material once solidified, if it has a large zone of supercooled liquid, can be heated up to these temperatures and then has plastic properties interesting for shaping.

Quenching on wheel
The melt spinning is a method used since the beginning of the amorphous metal alloys. It makes it possible to obtain very high quenching speeds by contact with a cooled metal drum, and by producing thin samples (approximately 10 μm thick). This gives a hypertrempe (10 ^ 6 K / s). Thus, long ribbons can be produced in an industrial manner which, if annealed and rolled up, find application as a ferromagnetic core for transformers.

Pouring into a cooled mold
It is simply a question of injecting or letting the liquid metal flow in a metal mold which is a good conductor of cooled heat, for example by a water cooling circuit. This makes it possible to produce solid metal glass samples, provided that the size of the desired sample is in agreement with the composition of the alloy employed.

Given the difficulty of machining pieces of metal glass because of their great fragility, the shape of the mold will be that of the final sample. The forms used are generally bars or plates.

Quenching with water
The liquid can also be released into a cold liquid tank, such as cold water. Amorphous solid beads are then obtained.

Other production methods

Mechanical properties
At room temperature, metallic glasses have very high breaking forces (up to more than 2 GPa for Zr- based glasses) associated with particularly important elastic deformations (of the order of 2%). Macroscopically, they show a generally fragile behavior (rupture without prior plastic deformation) but one notes the presence of shear bands, characteristic of a local plastic activity: thus, this mode of deformation is called heterogeneous mode. This local plastic deformation capacity is the reason why these alloys conventionally have good resistance to shocks and cracking. Unlike their crystalline counterparts, the plasticity of amorphous metal alloys is sensitive to pressure: in crystalline metals as amorphous, plasticity is induced by shear, but in crystals isostatic pressure does not influence plasticitywhile in amorphous, it reduces.

At high temperature (T> 0.8T g) the material can follow a homogeneous mode of deformation, for which the shear bands completely disappear, and all the material participates in the deformation. The glass can undergo in this mode there deformations up to more than 10 000% in traction.

Diffusion in metallic glasses
Crystalline materials have two main modes of diffusion: gap- mode diffusion, which occurs for atoms on network sites; and interstitial diffusion, in this case small atoms located between the sites of the crystal lattice can migrate by jump between the atoms of the lattice. In the case of amorphous materials, the situation is less clear because of the absence of a crystal lattice.

Experimentally, with regard to the metallic glasses, a change of slope in the diffusion regime is observed during the transition of the glass transition, it results in a smaller dependence of the diffusion coefficient on the vitreous temperature, the coefficient thus becoming higher than would be predicted by the extrapolation of supercooled liquid values.

Structural relaxation
When a glass is maintained at a temperature T <Tg, it exhibits the phenomenon of structural relaxation. The glass undergoes atomic rearrangements tending to bring the fictitious temperature T f closer to the isothermal treatment temperature. Thus, the density of the glass will tend to increase. Russew and Sommer have shown that in the case of Pd-based glasses, this density variation can reach about 0.2%.

This density variation was confirmed by positron life time (PAS Positron Annihilation Spectroscopy) measurements on Zr base grades. Structural relaxation can be followed by X-ray diffraction experiments that show the existence of two mechanisms associated with structural relaxation: radial atomic movements that tend to increase glass density (topological short-range ordering or TSRO) and local movements that increase the chemical shortrange ordering (CSRO) but leave unchanged the density.

The increase in density is accompanied by an increase in Young’s modulus 21 which can reach 10% in the case of Pd-based amorphous ribbons. Structural relaxation results in a variation of the enthalpy associated with the glass transition measured by DSC, directly proportional to the density variation.

Various physical properties
Metallic lenses have an exceptional set of properties: resistance to corrosion and abrasion, exceptionally soft ferromagnetism, very high yield strength, possibility of formatting, biocompatibilityetc. Their commercialization began in the last few decades in the form of ribbon, for transformers or as reinforcements of reinforced concrete, and in recent years for massive glasses, as sporting goods (tennis rackets, golf clubs, baseball bat), elements for high fidelity electronic components, etc. These materials are however expensive, they mainly target sectors with high added value (medical, military, luxury…) or the sector of micro-mechanics for which the price of the material becomes negligible compared to the cost of the manufacturing process.

Currently the most important application is due to the special magnetic properties of some ferromagnetic metallic glasses. The low magnetization loss is used in high efficiency transformers (amorphous metal transformer) at line frequency and some higher frequency transformers. Amorphous steel is a very brittle material which makes it difficult to punch into motor laminations. Also electronic article surveillance (such as theft control passive ID tags,) often uses metallic glasses because of these magnetic properties.

Amorphous metals exhibit unique softening behavior above their glass transition and this softening has been increasingly explored for thermoplastic forming of metallic glasses. Such low softening temperature allows for developing simple methods for making composites of nanoparticles (e.g. carbon nanotubes) and BMGs. It has been shown that metallic glasses can be patterned on extremely small length scales ranging from 10 nm to several millimeters. This may solve the problems of nanoimprint lithography where expensive nano-molds made of silicon break easily. Nano-molds made from metallic glasses are easy to fabricate and more durable than silicon molds. The superior electronic, thermal and mechanical properties of BMGs compared to polymers make them a good option for developing nanocomposites for electronic application such as field electron emission devices.

Conventional metallic glasses, which can be produced relatively inexpensively as thin tapes, have been used since the 1980s mainly in the following fields of application of electrical engineering because of 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 used primarily in the USA.
in harmonic and acoustomagnetic security tags.

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

Ti40Cu36Pd14Zr10 is believed to be noncarcinogenic, is about three times stronger than titanium, and its elastic modulus nearly matches bones. It has a high wear resistance and does not produce abrasion powder. The alloy does not undergo shrinkage on solidification. A surface structure can be generated that is biologically attachable by surface modification using laser pulses, allowing better joining with bone.

Mg60Zn35Ca5, rapidly cooled to achieve amorphous structure, is being investigated, at Lehigh University, as a biomaterial for implantation into bones as screws, pins, or plates, to fix fractures. Unlike traditional steel or titanium, this material dissolves in organisms at a rate of roughly 1 millimeter per month and is replaced with bone tissue. This speed can be adjusted by varying the content of zinc.

With high material prices unimportant in these areas because of the generally high cost and safety priority, metallic glass is considered everywhere where its unique properties could play a role. Parts of the Genesis probe solar wind collectors were made of amorphous metal.

Material finishing 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).

Already available are (especially ophthalmic) scalpels made of amorphous metal, which are because of the great hardness sharper than those made of stainless steel and retain their sharpness even longer. Due to its biocompatibility, high strength and relatively low weight and resistance to wear, surgical implants are being considered.

Numerous development projects, in particular those of the US Department of Defense, are testing the use of amorphous metals for various applications. For example, tungsten- based metallic glasses are expected to replace conventional tungsten alloys and depleted uranium in armor-piercing balancing bullets because of their high hardness and self-sharpening behavior. In military aviation, amorphous metal coatings are said to increase the hardness and corrosion resistance of lighter metals such as aluminum and titanium.

Some metallic glasses are made of precious metals (eg platinum), but are much harder than these and therefore do not scratch. In addition, the special processing options allow the production of shapes that are difficult to achieve with conventional metals.

Sports and leisure articles
Golf clubs were one of the first commercial amorphous metal products in 1998 and were used by the company Liquidmetal to launch the material in large-scale advertising campaigns (including PGA Tour professional golfer Paul Azinger). Golf clubs benefit above all from the unrivaled elasticity of amorphous metals. In development (although not yet commercialized) are tennis and baseball bats, fishing equipment, skis, snowboards, bicycles and sports rifles.

Consumer electronics
The smooth, shimmering and scratch-resistant surface of metallic lenses has led to the use of the cases of exclusive mobile phones, MP3 players and USB sticks. The high strength (better than titanium) allows thinner wall thickness, thus even lower weight and even more miniaturization. Injection molding allows more freedom in design and cheaper processing than stainless steel or titanium that needs to be forged. Dainty mobile phone hinges, where large forces attack the smallest components, benefit from the superior mechanical properties of metallic glasses.
High expectations are placed on amorphous steels should they become 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 suitable for larger components. Should the existing technical problems be solved and amorphous steels become ready for the market, they would, in particular, compete with titanium and stainless steel and score points with their higher corrosion resistance and better processability.

Modeling and theory
Bulk metallic glasses (BMGs) have now been modeled using atomic scale simulations (within the density functional theory framework) in a similar manner to high entropy alloys. This has allowed predictions to be made about their behavior, stability and many more properties. As such, new BMG systems can be tested, and tailored systems; fit for a specific purpose (e.g. bone replacement or aero-engine component) without as much empirical searching of the phase space and experimental trial and error.

Source from Wikipedia