• Corresponding author: Ritam Chatterjee,E-mail address: ritam.chatterjee@iitgn.ac.in

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.

References [1] Greer, A. L. (2009). Metallic glasses...on the threshold.

Materials Today, 12(1-2), 14-22. doi:10.1016/S1369-

7021(09)70037-9

[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

[10] Schroers, J. (2010). Processing of bulk metallic glass.

Advanced Materials, 22(14), 1566-1597

[11] http://www.indo-mim.com/howMimWorks.html

[12] http://www.azom.com/article.aspx?ArticleID=1080