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.
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.
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.
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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."
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