A new type of metal may soon find its way into your home–one that
is up to twice as strong as typical titanium alloys and much tougher
than ceramics, yet is elastic and springy, with very high wear and corrosion
resistance characteristics. It can even be cast directly and inexpensively
into useful shapes that take advantage of these properties. This extraordinary
material, called bulk metallic glass, relies on a new technique to order
the atomic structure of solid metals, and is currently the subject of
intense research among materials scientists. Due to its useful properties,
bulk metallic glass is already being used in a number of practical applications
with many more planned for the future.
All metallic glasses are made up of metal atoms, such as copper, nickel,
and titanium. What makes metallic glass different from ordinary metal
is the way in which the atoms are arranged. In a conventional metal
structure, the atoms are arranged in an organized, periodic structure
called a crystal. In a glass, including window glass and metallic glass,
the atoms are disordered and arranged randomly. The very name, “metallic
glass”, signifies a material that is made out of metal atoms that
exist in a disordered, glassy state. The unique structure of metallic
glass gives it interesting and useful properties.
Metallic glass looks much like its regular metal cousins. It is gray
and opaque, with a surface that is very shiny. The surface, though,
is so smooth that paint has difficulty sticking to it. Otherwise, it
is difficult to distinguish metallic glasses from ordinary metals simply
by looking at them, but this similarity hides the great differences
at the atomic level that result in very different properties.
One company that has already capitalized on the many desirable qualities
of bulk metallic glass is called Liquidmetal Technologies®, located
in Tampa, Florida. Their Liquidmetal alloy is a particular type of bulk
metallic glass that was discovered by Prof. William Johnson, a materials
scientist at Caltech. Among the company’s most successful products
are golf clubs with heads made of the alloy, one of the first mainstream
applications of bulk metallic glass. These clubs have superior characteristics
to those made from other material, such as titanium, allowing the player
to hit the ball farther. Many other uses, from satellite instruments
to armor piercing weapons, are currently either available or are planned.
The first form of metallic glass was discovered over forty years ago.
In 1960, Pol Duwez, another pioneering materials scientist at Caltech,
found with his team that they could force molten metal to form a glassy
instead of crystalline state by cooling the atoms very rapidly. Using
the same general process used to make window glass, Duwez used techniques
that had never before been successful with a metal because of how quickly
the molten metal needed to be cooled to form the glassy state. Cooling
rates of about a million degrees Celsius per second were required by
the early metallic glasses, rates that are very difficult to achieve.
By comparison, these metallic glasses required a far greater cooling
speed than the cooling rate used to harden steel by quenching, which
typically requires only several hundred degrees of temperature decrease
per second. Also, metallic glass conducted heat poorly, so only very
thin ribbons of less than 1 millimeter thick could be made, which limited
In the early 1990s, Akihisa Inoue and his colleagues at Tohoku University
in Japan, and shortly afterward, William Johnson and Atakan Peker at
Caltech, discovered new metal alloys that allowed formation of metallic
glasses at much lower, and hence, easier to obtain, cooling rates; these
typically range from 1 to 100 degrees per second and are similar to
the conditions used to make window glass. These cooling rates allow
much larger samples of the resulting metallic glass to be made, although
there is still a limit of about four inches in depth for the best quality
alloys to form, which thus limits possible applications.
This newer type of metallic glass, made using low cooling rates that
permit a greater ease of manufacturing larger quantities, is called
bulk metallic glass. Although early metallic glasses did have some industrial
applications, it was the discovery of bulk metallic glass that has really
allowed materials scientists to dream up a wide range of practical uses.
Current and Future Applications of Bulk Metallic Glass
The early forms of metallic glasses have been used in a number of technical
applications for many years, including as a protective coating sprayed
onto drilling machinery and as thin ribbons used in electrical transformers,
but since the discovery and manufacture of bulk metallic glasses has
a very recent development, possible applications are just beginning
to be explored. Liquidmetal Technologies, working with Howmet Metal
Mold, a company that manufactures and processes the material, is currently
the only company developing and selling commercial bulk metallic glass
products. They market a material called Liquidmetal alloy, also known
as VitreloyTM, which ranges in current and future applications from
golf clubs to armor piercing tank rounds.
Bulk metallic glass is a particular type of metallic glass, just as
tempered steel is a particular type of steel. In both cases, the subcategory
has different and often superior qualities to the general material type:
tempered steel is harder and stronger than most other types of steel,
and bulk metallic glass can be made in much greater volume, or bulk,
than other types of metallic glass. Just as scientists and engineers
distinguish between particular tempered steels, there are even sub-classes
of bulk metallic glasses, each with different compositions and slightly
different properties. The distinction between bulk metallic glass and
general forms of metallic glass allows materials scientists to emphasize
and explore the differences that allow bulk metallic glass to have far
more possible applications than other types.
The first commercial use of bulk metallic glass involved the manufacture
of golf club heads made of Vitreloy. Sold under the subsidiary Liquidmetal
Golf, these golf clubs were introduced in 1998. Attached to a conventional
graphite shaft, the Vitreloy head gives these golf clubs better performance
than the best titanium or steel clubs on the market. The new technology
of bulk metallic glass allows players to hit the ball farther and with
better accuracy. These clubs have been well received by both amateur
and professional golfers.
A high tech material like bulk metallic glass would be first used to
make golf clubs, rather than in more exotic applications such as aircraft
or spacecraft, because its unique properties resulting from its structure
make Liquidmetal ideal for this use. Liquidmetal is much stronger than
steel or titanium and has a density between the two, enabling different
shapes and sizes of heads can be made out of Liquidmetal, since golf
club heads are restricted in weight instead of size. Its greatest advantage
is that Liquidmetal is much more elastic than conventional metal, meaning
it can store and release energy like a spring. Thus, when the ball is
struck, more energy is transferred to the ball, resulting in longer
drives and giving even average players a powerful edge (Larsen 1999).
Liquidmetal Technologies demonstrates this elastic property in test
involving metal balls that they bounce in tubes onto metallic glass
in contrast with conventional metal surfaces such as steel. Every time
the balls bounce, each impact with the metal or metallic glass plate
transfers some energy from the ball to the plate so that the apex of
their rebound reaches a lower and lower height. The ball bouncing on
the metallic glass plate, however, loses less energy per bounce than
the others and continues to bounce long after the others have stopped.
On the Liquidmetal Technologies website, a video clip of this demonstration
shows that balls bouncing on steel and titanium stop bouncing in under
30 seconds, while the ball bouncing on the Liquidmetal alloy keeps bouncing
for more than one minute. Liquidmetal’s ability to store and release
energy is seen in these kind of simple demonstrations.
Bulk metallic was also used onboard the Genesis spacecraft, which was
launched in August 2001 to collect samples of the solar wind. The probe’s
solar wind collector was made of a bulk metallic glass prepared by Charles
Hays at Caltech and by Howmet, the company that manufactures products
for Liquidmetal. The bulk metallic glass plate on Genesis is designed
to absorb ions coming from the sun, which will eventually be returned
to earth for analysis. Metallic glass was used because its surface dissolves
evenly, alloying the trapped ions to be released in precise layers.
Although unrelated to Liquidmetal’s elasticity that results in
great golf clubs, this property of its surface is also a result of the
microscopic structure of metallic glass.
Liquidmetal Technologies is working to develop a number of future applications
of Vitreloy. It will soon be used in consumer electronics casings, such
as in LCD screen frames for cell phones and it may also be used in the
medical field for surgical instruments such as scalpels or for implants.
Military applications are also being considered, especially the possibility
of using bulk metallic glass in rounds fired from tanks that pierce
heavy armor, and as armor itself. As bulk metallic glasses are further
studied and better understood, more applications will become available.
Unique Properties of Bulk Metallic Glass
The mechanical properties of a material are among the most important
characteristics considered when scientists and engineers choose the
best material for a particular use. For instance, steel is used to build
skyscrapers in large part due to its high strength. Likewise, bulk metallic
glass is used for certain applications, such as golf clubs, due to its
properties, such as high elasticity.
Vitreloy 1, Liquidmetal’s first alloy, is the most widely available
and studied type of bulk metallic glass. Although the properties of
various bulk metallic glasses differ somewhat, Vitreloy’s mechanical
properties roughly represent the properties of bulk metallic glass in
general. These properties are generally superior to those of conventional
metals, making bulk metallic glasses attractive replacements for other
metals in many applications.
Toughness (MPa m1/2)
Table 1: A table comparing the mechanical
properties of Vitreloy 1 to those of conventional metals. Higher yield
strength, elastic strain limit, fracture toughness, and specific strength
results in a more useful material. Vitreloy’s properties are generally
superior to those of other metals. Data from Ashby and Jones, 1998,
Aydiner et al, 2003, and Liquidmetal Technologies
Vitreloy 1 has a density between that of titanium and steel, while
its strength is two or three times higher than typical steel or titanium
alloys. Yield strength, a measurement of how much stress is needed to
permanently deform the material, directly translates to resistance to
bending, breaking, and crumpling. Vitreloy’s combination of high
strength and moderate density gives the bulk metallic glass a very good
specific strength, which is yield strength divided by density. This
property is important in applications where weight must be minimized,
such as aerospace or automotive design. Aluminum’s relatively
high specific strength is the major reason why airplanes are made out
of aluminum instead of steel, making Vitreloy 1 a good replacement for
aluminum for some applications.
One of Vitreloy 1’s best properties is its very high elastic
strain limit, giving it the ability to “bounce back” when
deformed. When a material is placed under stress, it will deform elastically,
meaning it will return to its original shape, up to a certain point
called the elastic strain limit. Up to a point, all materials behave
in this way, but the property can be most easily seen in a rubber band’s
ability to return to its original shape when stretched. Springs of ordinary
metals also demonstrate this property. Past the elastic strain limit,
a material will become permanently deformed and eventually fracture,
just as a rubber band will break when it is stretched too far. Vitreloy’s
ability to deform elastically at high strains is a major reason why
it is used in golf club heads and possibly other sports products in
Ease of Manufacture
Although bulk metallic glasses such as Vitreloy 1 have useful mechanical
properties, possibly their greatest strength is ease of manufacture
and processing compared to that of conventional metals. Along with their
physical properties, this ease of manufacture is also a result of the
unique structure of bulk metallic glasses.
Making conventional metal parts or products often requires a lengthy
and expensive series of steps. A new ceramic mold must usually be made
then destroyed for each individual piece cast. When a conventional metal
is cast, it shrinks as it freezes into solid form, which, along with
a resulting tendency to create surface roughness, often necessitates
further processing and finishing and additional steps for grinding and
polishing. All of these steps add further cost to the final metal product.
Plastics products are much easier and cheaper to make than those made
from metal. Plastics became common largely because they soften when
heated and can be formed very cheaply through molding techniques that
directly result in the finished product without the need for further
processing. Along with much lower raw material costs, the ease and low
cost of forming plastics into desired shapes is a major reason for their
Bulk metallic glass offers manufacturers the best of both worlds, combining
the strength of metals with the ease of manufacture of plastics. Vitreloy
1 has physical properties, such as strength, that are as good or better
than metals, but it can be shaped and processed like plastics. Before
being cooled into a solid shape, Liquidmetal is a thick, viscous liquid
like heated plastic, in contrast to the flowing nature of molten metals,
and because of this viscosity it can be more easily shaped. As Vitreloy
1 cools in a mold, it shrinks far less than conventional metals, so
the resulting piece can also meet much higher dimensional tolerances,
for example, allowing casting of scalpels that emerge from the mold
already sharp, whereas production of a steel blade would require an
additional step for sharpening. In addition, the molded piece emerges
from the mold with a highly polished and shiny surface finish. Vitreloy
1 can thus be molded into almost-finished products that require very
little further processing, making the manufacturing process much cheaper
than that for conventional metals.
Vitreloy 1 does have a few practical drawbacks, however. It cannot
be used at temperatures above 260� C because, under such conditions,
it once again becomes soft and loses its strength. Although this low
melting point makes it easier to mold, it also limits possible high
temperature applications; it would melt if used to make the turbines
of a jet engine, for example. Also, since the metal alloy must be cooled
at a fast rate to form a glass, one dimension of bulk metallic glass
products must be less than four inches. Thus a plate can be made fifty
feet long and wide, but only four inches thick, preventing Vitreloy’s
use in making pieces that are both wide and deep. Vitreloy 1 is also
expensive, costing about $15 per pound, which is similar to the price
of titanium, but much more expensive than aluminum or stainless steel,
both of which cost about $3 to $5 a pound. As with titanium, this high
price prevents Vitreloy’s use in products such as car chassis.
The Science of Metallic Glass
The properties that make bulk metallic glasses such as Vitreloy 1 useful
for many applications are a direct result of the compounds’ unique
atomic structure. Although much about metallic glasses remains unknown
and some of their properties are still poorly understood, their microscopic
structure explains many properties such as their strength and springiness.
The fundamental difference between bulk metallic glasses and traditional
metals is that their atoms are arranged in very different microscopic
Traditional solid metals, including steel, aluminum, and titanium,
are composed of atoms arranged in a highly ordered, periodic crystal
structure that is made up of a large number of repeating unit cells.
Metals are usually not used as single crystals, however. Instead, a
piece of any metal consists of a large number of very small crystals,
called grains. Where these grains meet a weak spot in the metal known
as a grain boundary occurs, an area of less than optimal atomic packing,
where a plane of atoms meets another at an angle, which weakens the
metal. When a metal is placed under sufficient stress and high temperature,
grains may move and slide past each other, causing the metal to deform.
In addition to grain boundaries, the structures of metals always include
a large number of defects, such as dislocations. Dislocations are lines
in grains where the crystal structure is distorted by extra atoms that
are not arranged properly. Dislocations move through the crystals easily
when stress is applied, rearranging atoms as they go and causing the
metal to deform. The classic analogy is of a large carpet, which is
easy to move when a large bulge or bubble is introduced under it. Dislocations
allow one plane of atoms to move at a time, just like a bulge in a piece
of carpet allows a small part of the carpet to be moved, progressively
moving the entire carpet. The metal is greatly weakened since dislocations
allow the metal atoms to move around easily. Although individual dislocations
have a small effect, metals have a very large number of them: a cubic
centimeter of metal can often contain billions of dislocations.
Because of grain boundaries and especially dislocations, metals have
much lower strength than the theoretical maximum that would be achieved
if they were single flawless crystals. Figure 6 shows one view of a
dislocation, an extra plane of atoms that distort the crystal structure.
Unlike in other metals, the atoms in a metallic glass are not arranged
in a crystalline structure. Instead, they are randomly distributed within
the material and lack any kind of order or structure (Figure 8). This
amorphous (noncrystalline), or glassy, state is just like the pattern
of atoms found in window glass and can loosely be described as a “frozen”
liquid. Because these disorganized atoms do not form crystals, metallic
glasses lack the grain boundaries and dislocations that weaken conventional
metals, giving metallic glasses very high strength and elasticity.
Metallic glasses consist of metal atoms, such as zirconium and titanium
in the case of Vitreloy 1. If cooled improperly, the liquid molten alloy
will crystallize, forming a conventional solid metal with all of the
normal associated weaknesses. However, if the hot melt is cooled quickly
enough, crystallization can be avoided, and the atoms become frozen
in a glassy state. Although the crystal structure is the preferred,
lowest energy state, if the atoms are cooled fast enough they will not
have the necessary time or energy to move and rearrange into an orderly
crystal. They will become effectively frozen in an amorphous, liquid-like
arrangement. This structure is the key to bulk metallic glasses’
The Future of Bulk Metallic Glass
The discovery of bulk metallic glass, little more than ten years ago
was one of the greatest recent breakthroughs in materials science. With
properties such as strength and elasticity that are far superior to
those of conventional metals, combined with costs for cheap fabrication
that are similar to those of plastics, bulk metallic glasses have tremendous
potential. Although current uses are limited to relatively mundane applications
like golf clubs or cell phone casings, the greatest benefit offered
by a new material is the possibility of new applications that would
not be possible without it; computer chips, for example, would not be
possible without very pure silicon. Bulk metallic glass is likely to
see its greatest successes not in currently available commercial products,
but in future projects where it is the only material that can get the
Liquidmetal Technologies, the leading developer of commercial bulk
metallic glass products, is so confident in this material that it predicts
bulk metallic glass will become just as important and widely used as
plastics and steel. Whether bulk metallic glass can achieve this lofty
goal remains to be seen. However, because of its useful properties that
directly result from its unique structure, bulk metallic glass certainly
has the potential to achieve equally widespread use.
Chen H. S. 1980. Glassy Metals. Rep. Prog. Phys. 43: 353-432
Department of Materials Science and Engineering, Stanford. Mechanical
Behavior of Bulk Metallic Glass.
Accessed 2003 Mar 6.
Division of Chemistry Education, Purdue University. More About Materials
topicreview/bp/materials/defects3.html#top> Accessed 2003 Mar
Hufnagel T. C. Department of Materials Science and Engineering, Johns
University. Metallic Glass Research.
Accessed 2003 Mar 6.
Johnson T. 2001 Nov 30. Liquidmetal Golf moves in a new direction. Golfweb.
Accessed 2003 Mar 6.
Johnson W. L. and Peker A. 1993. A highly processable metallic glass:
Zr41.2Ti13.8Cu--12.5Ni10.0Be22.5. Appl. Phys. Lett. 63(17): 2342-2344
JPL Press release. 2000. New bulk metallic glass to catch pieces of
the solar wind.
Larsen D. 1999. Vitreloy–a metal that thinks it is glass. International
Review 4: 15-17
Louzguine D. V. and Inoue A. 2003. Structural and thermal investigations
high strength Cu-Zr-Ti-Co bulk metallic glass. Philosophical Magazine
Letters 83(3): 191-196
Oak Ridge National Laboratory Review. ORNL Breaks into Metallic Glass
Accessed 2003 Mar 6.
Qiu K. Q., Wang A. M., Zhang H. F., Ding B. Z. and Hu Z. Q. 2002. Mechanical
properties of tungsten fiber reinforced ZrAlNiCuSi metallic glass matrix
composite. Intermetallics 10(11-12): 1283-1288Üstündag E.
Department of Materials Science, Caltech. 2003. Internal stresses in
bulk metallic glasses.
Accessed 2003 Mar 6.