Definition Of An Amorphous Solid

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Definition Of An Amorphous Solid Crystalline solids have long-range order, meaning that, the atoms are arranged in a regular way by

repeating the "elementary cell" in the three directions in space. Bravais has shown that there are 14

possible configurations of the elementary cell. For metals, the most common configurations are 3:

ccc, cfc, and e.c.

Solid alloys with an atomic arrangement like a liquid are called either amorphous metal glasses:

a glass is, literally, a liquid that has been frozen in a "solid" state without crystallizing, while a

material having the same structure obtained with some process other than simple cooling is called

amorphous.

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In order to obtain a "glass" it is necessary to lower the temperature of the molten metal to the glass

transition temperature (Tv), temperature at which the atoms are no longer able to move with large

displacements but they can only carry small movements around their position due to the high viscosity.

The problem was that this temperature (TV) was below the temperature of solidification and therefore

before the liquid could reach the metal solidified with crystalline structure. It has been shown that in

the solids there is a more stable crystalline phase than the amorphous one; the crystalline state has a

lower free energy and is favored by the thermodynamic point of view. Even in the case of the common

glass (silicate-based), the spontaneous tendency of the material to the crystalline structure is dealt with,

but the time required for processing is very long (this is why glasses that date centuries tend to break).

To overcome the problem then we had to act on the transformation kinetics. To form a crystalline

solid, reaching solidification T, it needs some time: first clusters, nuclei of a few atoms in crystalline

configuration, are formed that act as aggregation centers for other atoms; solidification takes place for

the subsequent growth of crystalline solid around these clusters. If you then solidify a molten metal

with such a rate to reach the TV before cluster formation then you get a frozen liquid in an amorphous

structure.

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Methods of obtain Amorphous materials

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Amorphous materials can be obtained starting from a solid a liquid or a gas.

Starting from the liquid:

1) Melt spinning:

a melt casting is projected against a cooled, Cu wheel rotating at a speed of about 200 m/s; the

metal undergoes a cooling that can reach a million degrees for second; a ribbon of amorphous

material is obtained.

Melt spinning

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2) Piston-and-anvil method:

Metal in drops is released, these are locked between two plates that are quickly clashed. You

get a disk of amorphous material.

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3) Twin- roller method:

Casting a molten metal into the space between two rotating wheels in the opposite direction.

A ribbon of amorphous material is obtained.

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4) Atomization:

Molten metal is hit by inert gas under pressure or low temperature liquid that solidifies

quickly the metal droplets forming an amorphous structure. Thus, a powder of amorphous

material is obtained.

5) Rotating water spinning process:

Molten metal is injected into a rotating water to get a wire of amorphous material.

With all these methods it is necessary to extract heat quickly from the molten metal: in the

case of amorphous ribbons or sheets the typical thickness is 20-50 m; the wires have a

diameter of 50-100 m, the powders have a diameter of about 20-100 m. Cooling rates

vary with melt size, cooling methods (wheel rotation speed) etc , of about 104-107 Ks-1

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Starting from a solid:

1) Laser treatment:

Through a laser beam focused on a small surface of a solid metal, the affected area is melted

Underlying solid metal removes heat from the melted area at high speed.•

2) Electrolyte Deposition:

In 1950 a layer of amorphous material (Ni-P with 10% P) was deposited for the first time to

get ultra-hard coating. Co-W-B alloys are also suitable for use as amorphous coatings. An

electrically-diffused amorphous Cr coating was also obtained, with very high hardness,

starting from a chromic acid solution with addition of additives. Certain organic materials

such as polyacetylene can be used as a catalyst for the electrode distribution of amorphous

materials such as Ni-Co-B and Ni-Co-P.

3) Ion implantation:

a large number of amorphous phases were obtained by high-energy ion implantation of

solute ions in metallic surfaces. For example, an amorphous, wear-resistant layer was

obtained by implanting Ti and C ions on Fe surfaces.

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4) Irradiation:

a number of intermetallic compounds have been "amorphized" by irradiation with high energy electrons

(2MeV) or heavy ions such as Ni+. This results in an incomplete destruction of the long-range

crystallographic order.

5) Ball milling:

is a process that combines deformation with mixing. A small amount of powder is put into a rotating or

vibrating container with hardened steel balls. This technique is also used to mechanically bond two

metals. In the case of Ni and Ti the amorphization reaction is directly between the Ni and Ti zones that

come into contact during grinding. Disorder is mechanically induced.

6) Interdiffusion and Reactions:

It was discovered in 1983 by Schwarz and Johnson that an amorphous alloy could be formed by the

inter-diffusion between two pure polycrystalline metals. They deposited Au and La layers of 10-50 nm

thick and treatment heat (50-100 °C) was made: the final composition of the mixed phases depends on

the relative thickness of the two superimposed films. The phenomenon depends on the different

diffusion rate of one element in the other: Au diffusion is faster in La (several orders of magnitude

faster than La self diffusion).

To better understand the phenomenon, a study (1990 by Greer et al.) has been developed on Ni-Zr

systems. The diffusion of Zr in an amorphous (a-Ni65Zr35) was studied at various temperatures: at 573K

the diffusion of Zr in amorphous solid is about 106 times smaller than that of Ni. This difference was

attributed to the different magnitude of the Ni and Zr atoms. A consequence of a very fast diffusion of a

constituent into an amorphous layer is the formation of vacancy in the layer. Many Zr-M alloys (with M

= Mn, Fe, Cr, Co, Ni, Cu, Be) are glass-formers because they show very rapid diffusion due to the fact

that the solute is much smaller than the solvent.

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Starting from the gaseous phase

1) Rapidly cooling the gas phase through: thermal evaporation, sputtering, Chemical

Vapor Decomposition.

Condensation of a metal on a cold substrate is equivalent to an ultra fast-quenching from the

melt. In the 1930 a physicist (Kramer) said he had generated an amorphous Sb using the

evaporation technique. Later (Buckel and Hilssch) other metals such as Bi, Ga and Sn-

Cu alloys were evaporated on substrate kept 4 K . The discussion continued over the

years if the results of the various experiments were materials with ultrafine grain or they

were really amorphous. Finally, in the 1980, measurements with a differential scanning

calorimeter were able to distinguish between microcrystalline materials and the

amorphous ones.

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Main criteria for obtaining metallic amorphous

The most favorable combinations of elements for metallic amorphous are summarized:

Five main categories can be distinguished: Class 1 includes glass consisting of metal +

metal halide (the first); they are easy to obtain by rapid solidification; small atoms in large

metal atoms, the concentration of metalloids is generally around 20%: Pb40Ni40 P20 and

Pb77.5Cu6Si16.5 . They can also be obtained at low speeds of 1K/s if one can avoid

heterogeneous nucleation on a surface by appropriate methods.

Class 3 and 5 are a minority interest. Instead, class 4 (Be-bearing glasses) is interesting for

the low density, high strength but difficult to obtain for the presence of Be. Recently, a new

class of glass Al base was discovered. These have a high strength combined with a good

toughness and low density. The typical composition is 80 at% Al 10 at% transition metal

(Ni, Co or Fe) and the rest of rare earths (Y, Ce and La) for example: Al69Cu17Fe10Mo1Si3

As regards the composition range to obtain amorphous structures starting from two

components with different techniques, the following figure shows the Co-Zr system: for

example, melt spinning (m.s) favors the formation of amorphous phases near the eutectic

composition. In a given alloy, the composition range, in which a glass can be obtained,

depends on the cooling rate in the case of rapid solidification.

There is a critical cooling rate that

needs to avoid nucleation of the

crystalline phase and the nose of the

T.T.T. curves. so it is important to

choose the right method for each

alloys. Another parameter is the

atomic size difference between A and

B, it has been found that CBmin (vB-

vA) 0.1, with CBmin the minimum

solute concentration B in A and v is

the respective atomic volumes.

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Diffusion is very important process in solid state amorphous mechanisms. The graph below

shows:

(a) The dependence of diffusivity of different metals in two metallic glasses on atomic

radius of the diddusing species

(b) shows the diffusivities (D) of some metals in a Ni-Zr amorphous alloy with a Ni content

of 50-65% as a function of atomic volume of the diffusion species.

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Main properties In crystalline metallic materials, long-range order determines a directionality or anisotropy

of mechanical, electrical, and magnetic properties, while metallic glasses exhibit some

isotropy because they can be considered substantially homogeneous even on a large scale;

Chemical properties: Corrosion resistance:

Iron-based Amorphous materials containing

metalloid addition in various acid and

sodium chloride solution. Corrosion resistance

is enhanced by the addition of metallic solids

such as Cr and Mo. For example, a-Fe72Cr8P13C7

spontaneously passivates in 2N HCl at room T.

Selective oxidation of, or the absorption of

hydrogen in, glasses such as Ni-Zr, Cu-Zr, Pd-Zr

modifies their surface: hydrogen absorption on Cu-Zr generates a Cu – enriched surface

layers containing Cu microcrystal.

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High strength and toughness combined with good stiffness: used as reinforcement

in composites and cutting utensils (razor blades, glass needle for eye surgery).

Some metal glasse have high bend fatigue resistance (glass ribbons for springs).

Tire reinforcement material: wires of 0.1 mm diameter with amorphous Fe-Si-B were

obtained by melt-spinning: high strength, good adhesion to rubber, excellent fatigue

and corrosion resistance.

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Mechanical properties

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The following table compares the main mechanical properties of some reinforcing fibers

and ribbons in function on density compensated strength and stiffness

. Al base glasses are twice as strong as the strongest commercial crystalline Al base alloys

and the corrosion resistance is very much better than Al base alloys .

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Transformer laminations: the growing use is based on two properties, a more slender

magnetization (Hysteresis) loop than grain-oriented Fe-Si sheet and higher electrical

resistivity which reduces induced eddy current in comparison to the crystalline Fe-Si alloys.

An example is the «Metglas 2605 SC» (Fe81B13.5Si3.5C2).

Another property concerns that they do not have Weiss domains, so they almost instantly

lose any previously acquired magnetization. For this reason they find application in the field

of computer science.

Applications

Brazing foils: Amorphous brazing alloys can be obtained by melt-quenching in the form of

sheets. For example, Cu-P, Ni-Si (B, P), Co-Si-P, Cu-Ti-Ni. The sheets obtained are ductile.

Alloy compositions are chosen by criteria such: low melting point, low surface tension, and

costituents which reduce surface oxides on the components to be joined.

Coating material (W0.6Re0.4) 76B24 to improve the wear resistance of steels.

Electromagnetic filters: Fe-Cr-P-C ribbons are used as active elements in electromagnetic

filters to eliminate rust in water.

Diffusion barriers: in the manufacture of complex circuits, diffusion barriers are necessary

between Si and Al metallization applied to make interconnections between circuiti elements,

because Si and Al reacts at high Temperatures, either during the later stages of circuits

manifacture. An example is Ta-Ir used between the substrate of Si and Y-Ba-Cu-O

superconductor layers.

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Diffusion barriers

An example is shown in the figure: Au diffuses much more slowly in a-Ni55Nb45 than it does

in the same crystalline alloy, at relative low temperature because in this temperature range

diffusion in polycrystals is dominated by grain-boundary transport which is excluded in the

amorphous. It can be seen at 400 ° C that the diffusivity difference is of 7 orders of

magnitude.

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Metallic precursors for

nano-crystalline materials

When a metallic glass is heated it will tend to crystallize, forming a combination of

intermetallic compounds and metallic solid solutions. The crystallization mechanisms are

mainly divided into 3 categories:

1) In polymorphous crystallization a single intermetallic compound cristallizes without

change in composition (Fe75B25)

1) In eutectic crystallization the amorphous trasforms (Fe80B20) to two phases growing in a

cloesed coupled form (example Fe and Fe3B).

1) In primary crystallization (Fe86B14) a primary phase -Fe crystallizes out first, and then

crystallizes a Fe3B compound.

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The figure shows the time for the start of crystallization of a range of metallic glasses as a

function on temperature. Curves are obtained with isothermal anneals at a range

temperatures. Among these are W65Ru35, W50Re50 both with high crystallization temperature

about 800°C and Ta55Ir45 with crystallization temperature about 900°C.

It is therefore thought to use amorphous alloys to obtain nanocrystalline materials (Fe-Cu-

Nb-Si-B). The formation of nanocrystals in these alloys comes presumably from the

homogenous nucleation that occurs during crystallization.

Alloys developed by Das et al. are Ni-Mo-B and Ni-Al-Ti-X-B and later Ni-Mo-B with the

addition of Cr. These alloys were made by melt quenching. During the processing ordered

phases (Ni4Mo; Ni3Al or Ti) are precipitated from the crystallized matrix together with

stable boride precipitates. This family of alloys is manufactured commercially under trade

name «Devitrium» ; the best of these alloys have very high temperature properties exceeding

high grade tool steels.

The devitrification process has been used

to make nanocrystalline soft-magnetic alloys

that are Fe-Cu-Nb-Si-B.

The nanocrystallinity is presumably derived

from homogenous nucleation

during crystallization.

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Example: Liquidmetal

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Early amorphous metals were limited to thin ribbons because of the high cooling rates required to form the non-crystalline

structure, Nevertheless, low-cost commercial sheet fabrication of these thin ribbon materials lead to a very successful industry.

Amorphous metal ribbons have been wound and used as transformer coils and anti-theft I.D. tags due to their magnetic properties.

What makes Liquidmetal a fundamentally different material than all of its crystalline counterparts are its truly unique combination

of processing and mechanical properties. Much like aluminum, magnesium and zinc alloys, Liquidmetal can be readily cast from

the liquid into extremely complex, net-shaped (i.e., require little or no post-processing operations) parts. Unlike those alloys

however, cast Liquidmetal parts are hard, high strength and can have a lustrous surface finish directly out of the mold.

The applications for Liquidmetal alloys are growing significantly and this first blog post represents our Company’s commitment to

advance the Commercial applications of Amorphous Alloys in the Global marketplace.

Amorphous Block Cores

(AM-R-XXX series)

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Recently there have been compositions for which only one to two hundred degrees per second are enough. These velocities

allow to obtain bulk metallic glasses. An example is shown in the figure.

Part produced from a Zr-Ti-Cu-Ni-Al alloy cast in a copper mold. From a engineering point of view glassy metals are very

interesting for their unique characteristics. For example, they have high mechanical strength, high elasticity, high fracture

toughness.

The main problem of amorphous materials is that they have a glass transition temperature. This means that the operating

temperature must be carefully controlled in order to avoid overheating that would affect mechanical properties, but also

means that over some temperatures the material can easily be formed even in complex geometries.

In 1993 Peker and Johnson designed Zr41,2Ti13,8Cu12,5Ni10Be22,5 or (Zr3Ti) 55 (Be9Cu5Ni4) 45, commonly called Vitreloy1

(Vit1), having a critical thickness of a few centimeters. This work can be considered as the starting point for the use of

amorphous materials in structural applications. Over the next 10 years, Vit1's properties were intensively studied. In 1997,

Inoue revisited Pd40Ni40P20 replacing 30% Ni with Cu. The result was a material that could produce objects of 72 mm in

diameter. Today, the Pd-Cu-Ni-P family is the metal system that has the best castability.

The figure shows the critical thickness as the function of the year in which the alloy was developed. Starting from the first

glass produced by Duwez, there was an increase of three orders of magnitude in 40 years.

By comparing with steel and titanium alloys it is noted that metal glass have a similar density, but more elastic limit (~ 2%)

and tensile resistance (~ 1,9 GPa). This leads to a high strength / weight ratio, which makes Al-alloys substitutable metallic

glasses. In addition, amorphous alloys exhibit very high fracture toughness (KIc).

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Properties Vit1 Al-alloys Ti-alloys steels

Density [g/cm3] 6,1 2,6-2,9 4,3-5,1 7,8

Yield strength [GPa]

1,90 0,10-0,63 0,18-1,32 0,5-1,6

Specific strength [GPa cm3/g]

0,32 <0,24 <0,31 0,21

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Properties of Vitreloy1

Lega sf [MPa] E [GPa] H V [DPN]

Al85Y11Ni4 930 72,3 385

Al85Y10Ni5 950 72,4 380

Al85Y8Ni7 1150 82,2 375

Al85Y5Ni10 1050 71,5 380

Al85Y2Ni13 920 72,5 365

The figure shows a typical HR TEM micrograph

of an alloy Al85Y10Ni5 produced by centrifugal casting

at a speed of 47 m/s.

Vitreloy1 is used in some cases in watches to replace the Ni to avoid any problems related to allergies. Vitreloy1 is

biocompatible, so it is used where high resistance to corrosion and wear is needed. For example, DePuy Orthopedics is

using it to produce knee prosthesis. In 2002, Surgical Specialties began producing blades in Vitreloy.

Liquidmetal has received many orders from the US Defense Agency for the development of military-grade materials that

are resistant, lightweight and resistant to high temperatures. For example, it is intended to replace the uranium used in anti-

tank rockets with reinforced glass composites with W as they have the same density and penetration behavior.

The company is also developing a new shell for fragmentation bombs used by the Navy. In August 2001, the Genesis probe

was launched, costing $ 200 million, with the aim of collecting solar wind fragments. It is expected that the probe will

capture 10-20 mg of particles using 5 collectors with 1 m of diameter. Each collector consists of 55 10-inch hexagonal

plates coated with a layer of Zr-Nb-Cu-Ni-Al (designed by Caltech Hays) that absorbs and holds He and Ne . Once returned

to Earth, collectors will be chemically etched with a special technique designed by the University of Zurich to gradually

dissolve the surface and allow the release of the ions caught and their subsequent analysis.

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The Vitreloy is also used for the production of some electronic products, being durable,

lightweight and easy to carry. In September 2002, Liquimetal began manufacturing

components for the liquid crystal displays of mobile phones and other critical

components, such as those shown in the figure.

Glassy metal against : Super-resilient material is the first to combine strength and toughness.

Zeeya Merali

A metal glass is the first material to be fabricated that is as strong and as tough as the toughest steel. The feat could

eventually see such materials replace steel in buildings, cars or bridges.

"The challenge has always been to achieve both high strength and toughness," says Marios Demetriou, a materials scientist

at the California Institute of Technology in Pasadena. "But until now we have always had to compromise between the two."

Demetriou and his colleagues have developed an alloy that combines the best features of both by turning to 'amorphous

metals'. Their work is published in Nature Materials today1.

Amorphous metals are stronger. They are made by rapidly cooling molten metal, so that its atoms are stuck in a disordered

arrangement — resembling the structure of glass. Unfortunately, for a long time these 'glassy metals' also seemed to be

inherently brittle.

"It's unique to see this combination of strength and toughness in a single material," says John Lewandowski, a materials

scientist at Case Western Reserve University in Cleveland, Ohio. It's now important to investigate whether adding more

elements to the mix — to create small crystalline regions in the material — could improve its toughness further, without

sacrificing strength, he adds.

Lewandowski also notes that because palladium is a precious metal, it will be too expensive to use the material widely.

Demetriou acknowledges that problem. "It will be most suited for making strong and tough dental and medical implants,

because their very high fabrication cost can often justify a high material cost."

However, the team plans to look for a cheaper version based on copper, iron or aluminium, says Demetriou. "If we find it,

that material will take over from steel in large-scale engineering forever."

References

Demetrious, M. D. et al. Nature Mater. advance online publication doi:10.1038/nmat2930 (2010).

Schroers, J. & Johnson, W. L. Phys. Rev. Lett. 93, 255506 (2004).

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