• Corresponding author: Ritam Chatterjee,E-mail address: [email protected]
Doi: http://dx.doi.org/10.11127/ijammc2017.04.05 Copyright@GRIET Publications. All rights reserved
Advanced Materials Manufacturing & Characterization Vol 7 Issue 1 (2017)
Advanced Materials Manufacturing & Characterization
journal home page: www.ijammc-griet.com
Manufacturing of Metallic Glasses
Ritam Chatterjeea
aDiscipline of Mechanical Engineering, Indian Institute of Technology, Gandhinagar, India
Abstract The present work reviews the different techniques that are currently
being used in synthesis of metallic glasses viz. solid state, liquid state and vapor state processes. The advantages and disadvantages of each
technique is discussed in detail and the various processes are compared
to each other to determine the most economically and technologically feasible technique to be used according to desired product
characteristics. Finally a section of the paper is devoted to a discussion
of the manufacturing techniques being used for producing bulk metallic glassesMetallic glasses are a novel class of materials having
extraordinary strength,toughness and elasticity.
Keywords:Bulk Metallic glasses; Synthesis; Manufacturing
Introduction
1.1. Metallic Glasses
Glass is an amorphous material that is usually formed by
rapid cooling of molten material to below the glass transition
temperature i.e. the temperature above which the amorphous
‘glassy’ material changes to a viscous, rubber type material.
Metallic glasses are prepared by cooling a metallic liquid so
rapidly such that crystallization is avoided and the atoms have
no time to arrange themselves into a crystalline lattice. The
formation of the first metallic glass Au75Si25 was reported by
Prof. Pol Duwez at Caltech laboratory, USA, in 1959 [1]. They
developed rapid quenching techniques viz. splat quenching for
chilling metallic liquids at very high rates of around 105–106 K/s.
Compared to conventional metals and metal alloys, metallic
glasses have extra-ordinary mechanical properties. This mainly
stems from the lack of crystalline structure due to which there
are no crystal defects i.e. point defects, linedefects etc. resulting
in very high strength, toughness and elasticity. Also, there are no
grain boundaries due to which the corrosion resistance and wear
resistance is superior as compared to conventional metals, alloys
[2].
1.2. Bulk Metallic Glasses (BMG)
Metallic glasses having a diameter or section thickness of at
least 1mm are classified as ‘bulk’ metallic glasses [3]. They have
the following important characteristics:
• They are generally multi-component systems i.e. there
are three or more sub-components. All the components
are in non-crystalline phase.
• They can be produced at low solidification rates ~ 103
K/s or less.
• There is a large difference between the glass transition
temperature and the crystalline temperature hereby
resulting in a large supercooled liquid region as shown
in Fig 1.
Bulk Metallic glasses are about three to four times as strong
as the strongest steels and also, have excellent elasticity and
wear resistance. The major limitation is the difficulty of difficult
to use BMG’s for making complex shaped components.
24
25
Figure 1. A time–temperature-transformation diagram for
processing as well as the brittle nature due to which it is the
primary crystallization of a bulk metallic glass [2]
2. Synthesis of metallic glasses
2.1. Vapor State Processes
2.1.1. Physical Vapor Deposition (PVD)
As displayed in Fig 2, in this method, the material to be
deposited is vaporized by physically heating or by sputtering and
then deposited on to a surface [4].
Figure 2. Process Flow Diagram for PVD [4]
A few examples of PVD are:
(a) Evaporative Deposition: The material to be
deposited is heated to a high vapor pressure by
electrical resistance heating in a vacuum chamber.
(b) Sputter Deposition: Here the material is bombarded
with ‘glow’ plasma which takes away the material as
vapor and then deposited on the target.
(c) Pulsed Laser Deposition: Here, high powered laser is
used to vaporize the material.
(d) Cathode Arc Deposition: A high powered electric arc
is discharged at the target material and deposited.
(e) Electron Beam PVD: Material to be deposited is
vaporized using a high energy electron beam in
vacuum environment and then transported via
diffusion to be deposited on the target.
2.1.2. Chemical Vapor Deposition (CVD)
In this method, the target material is exposed to some
chemical reagent which decomposes on/reacts with it and
produces the desired coating on it. CVD can be classified
according to the operating pressure viz. atmospheric pressure or
low pressure or Ultra-high vacuum pressure [5]. The latter two
are more prevalent nowadays. A few important examples for
CVD:
(a) Plasma Enhanced CVD: Plasma is used to increase the
affinity of the chemical precursor for the target. It
allows for low temperature deposition and is useful
especially for the semi-conductor industry.
(b) Aerosol Assisted CVD: The precursors are transported
to the target via an aerosol gas which can be generated
ultrasonically.
(c) Rapid Thermal CVD: A heating lamp is used to heat the
target to allow easy deposition.
(d) Combustion CVD: A flame is used in open atmosphere
to deposit thin film coatings.
2.1.3. Ion Implantation
Figure 3. Ion Implantation Setup [6]
In this method, the ions of a material are deposited on to a
substrate by using an electric field. The process is used to change
the physical, chemical and electrical properties of the target
material. Is extensively used in the semiconductor industry for
fabricating micro-chips. As shown in Fig 3, Ion implantation
equipment typically consists of an ion source, where ions of the
desired element are produced, an accelerator, where the ions are
electrostatically accelerated to a high energy, and a target
chamber, where the ions impinge on a target, which is the
26
material to be implanted. Thus ion implantation is a special case
of particle radiation [6].
2.1.4. Drawbacks of different vapor state processes
S.No. PVD CVD Ion
Implantation
1. Environment
friendly
process.
Imparts high
corrosion
resistance,
impact strength
and abrasion
resistance.
Carbide, nitride
coatings impart
hardness, wear
resistance to the
substrates.
Polymerized
thin coatings
impart lubricity,
hydrophobicity
and weather
resistance.
Products are
very resistant to
chemical
corrosion and
wear due to
friction. Can
amorphize the
target due to
crystallographic
damage which
imparts
excellent
mechanical
properties.
2. A few
limitations are
requirement of
high operating
temperatures,
cooling system
to dissipate
high heat.
Sometimes it is
difficult to fully
cover complex
geometries due
to line of sight
transfer of the
vapor [4].
Sputtering has
a big advantage
over CVD in
that it covers a
wide range of
substrate
materials due
to non-
requirement of
special
precursor
materials.
Require higher
operating
temperatures
than PVD (~300
to 900oc) [7] and
hereby a robust
cooling system.
A few by-
products are
toxic and hence
need to be
handled with
extreme care.
Laser based CVD
are superior to
PVD methods
such as
sputtering in
terms of
composition,
uniformity of the
thin film
deposited. But
they are more
expensive.
A few
limitations are
the generation
of point defects
such as
vacancies &
interstitials in
the target
material. To
redress this
issue, thermal
annealing is
carried out
which increases
cost and process
time. This
process slowly
etches the target
due to
sputtering
effect. This is
appreciable only
for very large
doses [6].
2.2. Liquid State Processes
2.2.1. Rapid Solidification Processing (RSP)
It is a technique which involves rapid solidification of the
molten ‘glassy’ melt at cooling rates ~ 106K/s to attain a metallic
glass structure. This requires that the heat be removed from the
melt at a very high rate due to which the section thickness of the
final product is limited to micron ranges [3]. The traditional
methods of achieving such high cooling rates are:
2.2.1.1. Droplet Methods
Here, a molten metal is atomized into small droplets and then
these droplets are solidified either by being exposed to a stream
of cold air or an inert gas. Another method of solidification is by
impinging these droplets i.e. splatting on a good heat conducting
surface.
2.2.1.2. Jet Method
In this method, a flowing stream of molten metal is
continuously solidified by moving it in contact with a moving
chill surface. The metallic glasses that are formed are in shape of
ribbons or sheets or wires.
2.2.1.3. Surface Melting Techniques
This technique involves rapid melting at the surface of a bulk
melt and then solidification via rapid heat extraction into the
unmelted region. A way to do this is via laser treatment.
RSP’s have been one of the most popular methods of
producing metal glasses. They have been finding applications in
technologically diverse areas such as fuel cell technology,
medical implants and dental amalgams, powder metallurgy tool
steels and superalloys etc. [3]. The products formed generally
have very thin cross sections and hence it is difficult to find direct
applications. Such metal glasses are used to produce BMG’S of
larger cross sections which can then be used for a multitude of
applications.
2.2.2. Splat Quenching
Splat quenching typically involves rapid solidification of
molten metal in between two rollers which are cooled
continuously to remove heat. A thin sheet of metallic glass is
formed with low volume to area ratio. It is basically a liquid
rolling technique. A better technique is Duwez and Willen’s gun
technique [8]. Here, the molten metal is thrown towards a
quencher plate due to which its area rapidly increases and it
rapidly cools to form a thin sheet. A wider range of near
amorphous metallic glasses can be produced.
The two important process parameters for splat quenching
are the velocity of the droplets and their volume. If the volume is
too large or the velocity is too low, the droplet doesn’t completely
27
solidify. Hence, it is experimentally determined as to what is the
optimum droplet size and velocity that is suitable for forming a
thin sheet having uniform thickness, composition and good
mechanical properties. Products which are produced using splat
quenching generally have ‘near amorphous’ structure and
excellent paramagnetism due to which this technique is useful
for applications related to magnetic shielding etc. [8].
2.3. Solid State Processes
2.3.1. Mechanical Alloying
Figure 4. Mechanical Alloying [9]
Mechanical alloying is a powder metallurgy technique that
was developed by John Benjamin at INCO in the mid 1960’s [3].
The different steps involved in mechanical alloying are:
1. The blended elemental powder particles and the
grinding medium (Tungsten carbide or stainless steel
balls) are kept in a container.
2. The container is agitated at high speed for some pre-
determined time duration.
3. The soft powders of each metal get crushed and assume
a flat shape with thin cross section. These flat structures
have a layered arrangement.
4. Due to heavy plastic deformation, crystal defects such as
dislocations, vacancies, grain boundaries etc. are
introduced in the glass. Temperature also rises.
5. Due to rise in temperature, diffusion is facilitated
resulting in mixing of the metallic powders to form
alloys.
6. These alloys can then be molded to desired shape using
techniques such as Hot Isostatic Processing, hot
extrusion, vacuum hot pressing etc.
Mechanical alloying can also be used to synthesize a variety of
non-equilibrium phases such as supersaturated solid solutions
(SSSS), metastable phases etc. [3].
3. Manufacturing of bulk metallic glasses
3.1. Casting
Methods such as die casting have been used for near net shape
processing of Bulk Metallic Glasses. As observed in Fig 5, the
cooling rate for casting process is such that crystallization of the
glass is narrowly avoided. The advantages of casting [10]:
1) Reduced tool cost.
2) Reduced wear as compared to techniques such as
mechanical alloying etc.
3) Lower energy consumption.
4) Shorter cycle times since it is a one step process.
5) Melting temperatures of a few BMG’s are quite low
which is useful in casting process.
6) Homogeneous microstructure is achieved.
7) Less solidification shrinkage due to absence of a first
order phase transition during solidification. Hence,
dimensional accuracy is high.
The disadvantages are:
1) High viscosity of BMG’s results in low fluidity which
makes casting difficult.
2) Internal stresses are developed due to rapid cooling
that is required to form BMG’s.
3) BMG’s react with atmospheric gases hence a vacuum
environment is necessary.
4) A careful balance of the cooling rate is required to avoid
the crystallization of the glass and also to aid the filling
of the mold.
3.2. Thermoplastic Forming
Figure 5. (1) Casting (2) Thermoplastic Forming [10]
Thermoplastic forming is a technique which takes advantage of
the drastic softening of the BMG on heating above the glass
transition temperature in order to form the glass into complex
shapes. The process is also known as hot forming, superplastic
forming, viscous flow working and viscous flow forming. [10]
The glass is kept in the supercooled liquid region where it exists
in metastable state and then it is crystallized.
The two important parameters to maximize the formability of
the glass in the supercooled liquid region are the viscosity and
the processing time. The viscosity has to be low and the
processing time has to be high. The extent to which a BMG can
be formed in its supercooled liquid state is dependent on the
variation of viscosity with temperature and the crystallization
28
time [10]. It has been concluded by Schroers [10] that the
properties that indicate good formability of a BMG are: large
Poisson’s ratio and low glass transition temperature. The
advantages of Thermoplastic forming methods are [10]:
1. Decoupling of forming and fast cooling due to which a wide range of complex shapes can be produced.
2. Higher dimensional accuracy as compared to other techniques due to very low solidification shrinkage.
3. Can be processed in air. Unaffected by heterogeneous influences.
4. Low processing temperature and pressure due to which no significant investment required. Hence, commercially viable.
The disadvantages are:
1. More number of steps as compared to casting due to decoupling of forming and rapid cooling.
2. Requires skilled manpower due to novel methods of processing.
3.2.1. Fabrication techniques based on Thermoplastic
Forming
3.2.1.1. Compression and Injection Molding
Figure 6. Compression molding [10]
In compression molding, the feedstock material is fed into a
mold cavity and then heated to supercooled liquid region.
Pressure is applied such that it exceeds the flow stress of the
BMG while avoiding crystallization. The schematic diagram is
shown in Fig 6. Two varieties are described below:
3.2.1.1.1. Metal Injection Molding (MIM)
The steps involved in this process are [11]:
1. Compounding
Fine metal powder (size <20µm) is mixed with thermoplastic
binders in a pre-determined amount and kept in a special mixing
apparatus. The mixture is heated up to melt the binder. The mass
is stirred to achieve uniform mixing of the powder and the
binder. The mass is then cooled and broken into small pellets to
be fed to the injection molding machine.
2. Molding
The pellet feed is heated and fed into a die cavity at high
pressure. The temperature is kept at around 200oc and the
binders melt and carry the powder with them. The feed is then
cooled and ejected from the cavity.
3. De-binding
In this step, the binder is removed from the molded component.
Only a little binder is left which can then escape during the
subsequent sintering step.
4. Sintering
The component is kept on a ceramic heating platform and then
slowly heated to weed out the remaining binder. Once the binder
has evaporated, the component is heated to a very high
temperature to make the particles bond to each other and fill the
voids left by the binders. The component becomes compact.
5. Finishing
Heat treatment is carried out to improve the physical properties.
Machining is carried out to achieve desired dimensional
accuracy.
3.2.1.1.2. Powder Injection Molding (PIM)
The technique is derived from polymer injection molding. It is an
efficient method for high volume production of components
having complex shapes.
Figure 7. Powder Injection Molding [12]
As observed in Fig 7, the metal powder is mixed with polymeric
binder. This mixture is heated and then fed under pressure into
a cavity where it is cooled and then ejected. In the market, PIM is
competitive mainly for producing small sized components of
complex shapes. The technique is being used for commercially
producing components for the electronics, computer, medical
industries etc. U.S holds about 40% of the global market share
now and the PIM industry is predicting a growth of 20 to 40% on
current global total sales of $200 million [12].
3.2.1.2. Miniature fabrication
Used in the fabrication of small sized devices used in micro-
electromechanical systems (MEMS), medical devices etc. The
range of length of products made lies between 10µm to 1mm.
The aspect ratio is usually very high i.e. the width of cross section
29
is very low as compared to the length. Generally, thermoplastic
forming methods are used to replicate structural features of the
mold to the surface of the BMG [10].
Figure 8. Miniature Fabrication process [10]
3.2.1.3. Blow Forming
For producing parts of complex 3D profile by techniques such as
compression molding, often impractically high pressures are
required. This is because of high frictional force between the die
wall and the metallic glass particles. This is usually taken care of
by using lubricants but this deteriorates the surface finish of the
final component. Hence, a good strategy to reduce the operating
pressures is to reduce the area of contact between the die and
feedstock. Blow forming is a technique used for forming such 3-
D parts with complex profiles using comparatively low
pressures as compared to injection molding. This is because, in
blow molding the part touches the die only when it is fully
formed i.e. friction is completely eliminated[10].
Figure 9. A simulation of a disk blow molded into a spherical
container [10]
Blow molding is similar to superplastic gas forming process and
here, the required gas pressure difference is created by using
vacuum on one side. With respect to this technique, the major
difference between BMG’s and plastics is the high values of strain
rate sensitivities for BMG’s (~50) which signifies that the BMG
acts as a fragile liquid. Hence, its formability is lesser as
compared to plastics. Flow stresses in BMG’s however, are less
sensitive to temperature changes as compared to those in
polymers. This is an advantage of BMG’s over polymers.
Altogether, it can be concluded that blow molding is the most
effective technique for producing complex 3D parts of BMG’s
having excellent surface finish, dimensional accuracy and
involving minimum possible production expenses [10].
4. Conclusions
1. Bulk metal glasses have exceptional mechanical
properties as compared to metals. The major limitation
is the brittle nature and low formability due to which it
is difficult to form complex shaped components.
2. Among the vapor phase processes, physical vapor
deposition is generally the most economical and
environment friendly technique. Each technique has its
advantages depending on the product requirements.
3. While forming Bulk metallic glasses, vapor phase
processes have the highest cooling rate, followed by
liquid phase and then solid phase processes.
4. Vapor phase processes are the most expensive,
followed by liquid and then solid phase.
5. Generally to form bulk metallic glasses, thermoplastic
methods are preferred over casting techniques due to
overall superior quality of products formed.
6. For mass production, injection molding techniques are
most suitable.
7. For making BMG products with specific and intricate
requirements, techniques such as miniature
fabrication, extrusion etc. are used.
8. To produce three dimensional components from
powder metal, blow molding is the most economical,
environment friendly and efficient process.
Acknowledgement
I hereby acknowledge the invaluable contribution of my mentor
Dr.T.R.Ramachandran of IIT Gandhinagar whose inputs were
immensely helpful in shaping this article.
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[2] Mivsek, K. (2006). Metallic Glass. (November), 1-19.
[3] C. Suryanarayana, A. I. (2010). Bulk Metallic Glasses. CRC
Press.
[4] https://en.wikipedia.org/wiki/Physical_vapor_deposition
[5] https://en.wikipedia.org/wiki/Chemical_vapor_deposition
[6] https://en.wikipedia.org/wiki/Ion_implantation
[7]http://documents.indium.com/qdynamo/download.php?do
cid=1957
[8] https://en.wikipedia.org/wiki/Splat_quenching
[9] https://en.wikipedia.org/wiki/Mechanical_alloying
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[11] http://www.indo-mim.com/howMimWorks.html
[12] http://www.azom.com/article.aspx?ArticleID=1080