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 possible applications.

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.

Properties

Vitreloy 1

Aluminum Alloys

Titanium Alloys

Steel Alloys

Density (g/cm3)

6.0

2.6-2.9

4.3-5.1

7.8

Yield Strength (GPa)

1.90

0.10-0.63

0.18-1.32

0.50-1.60

Elastic Strain Limit

2%

~ 0.5%

~ 0.5%

~ 0.5%

Fracture Toughness (MPa m1/2)

20-140

23-45

55-115

50-154

Specific Strength (GPa/g/cm3)

0.32

< 0.24

< 0.31

< 0.21

 

 

 

 

 

 

 

 

 

 

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 the future.

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 widespread use.

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 structures.

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’ useful properties.

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 job done.

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.


References
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.
<http://easy.stanford.edu/dauskardt/kathy_flores/BMG/bmg.html>
Accessed 2003 Mar 6.
Division of Chemistry Education, Purdue University. More About Materials
Science. <http://chemed.chem.purdue.edu/genchem/
topicreview/bp/materials/defects3.html#top
> Accessed 2003 Mar 6.
Hufnagel T. C. Department of Materials Science and Engineering, Johns Hopkins
University. Metallic Glass Research.
<http://www.jhu.edu/~matsci/people/faculty/hufnagel/background.html>
Accessed 2003 Mar 6.
Johnson T. 2001 Nov 30. Liquidmetal Golf moves in a new direction. Golfweb.
<http://www.golfweb.com/u/ce/multi/0,1977,4616391,00.html> 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.
#2000-099 <http://www.jpl.nasa.gov/releases/2000/genesiscollector.html>
Larsen D. 1999. Vitreloy–a metal that thinks it is glass. International Glass
Review 4: 15-17
Louzguine D. V. and Inoue A. 2003. Structural and thermal investigations of a
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 Field.
<http://www.ornl.gov/ORNLReview/v35_3_02/metallic_glass.shtml>
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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.
<http://www.its.caltech.edu/~acers/MS15b/Thermal_temper.pdf>
Accessed 2003 Mar 6.


 

   

 

Figure 1:
Photograph of Liquidmetal alloy, a bulk metallic glass, showing the shiny surface of the material. Taken from Liquidmetal Technologies website:
<http://www.liquidmetal.com
/technology/default.asp
>

Figure 2:
Dr. William Johnson demonstrates the use of a Liquidmetal golf club in front of Keck Laboratory at Caltech. His pioneering research into bulk metallic glass has allowed the material to be used in applications such as these clubs.
<http://www.its.caltech.edu/
~matsci/wlj/Johnson.html
>

Figure 3:
Tunneling electron microscope image of atoms (white spots) in a zirconium alloy crystal, showing the highly organized crystal structure that exists in all conventional solid metals.
(Copyright 1999, Todd Hufnagel)
<http://www.jhu.edu/~matsci/people
/faculty/hufnagel/background.html
>

Figure 4:
Image of atoms in a zirconium alloy metallic glass, showing the random arrangement of atoms that gives metallic glass its properties. This structure is similar to that in window glass and in liquids.
(Copyright 1999, Todd Hufnagel)
<http://www.jhu.edu/~matsci/people/
faculty/hufnagel/background.html
>

Figure 5:
A photograph of Liquidmetal Golf club heads, the first commercial use of bulk metallic glass. Although these look similar to heads made of other metals, they perform much better than competing products.
(Copyright The American Society of Mechanical Engineers 1998)
<http://www.memagazine.org/backissues/
june98/features/metallic/metallic.html
>

 
   

Figure 6:
A diagram of a dislocation in a metallic crystal. The extra atoms above point D distort the otherwise orderly structure. The defect line is at point D and points into the page. Dislocations such as these greatly weaken metals, but are absent in metallic glasses.
(<http://chemed.chem.purdue.edu/genchem
/topicreview/bp/materials/defects3.html
>)

Figure 7:
Four grains, showing the ordered, crystalline structure of each and grain boundaries where they meet.
(<http://easy.stanford.edu/dauskardt
/kathy_flores/BMG/bmg.html
>)

Figure 8:
The random atomic arrangement in an amorphous (noncrystalline) solid, is identical to that in a liquid. (<http://easy.stanford.edu/dauskardt
/kathy_flores/BMG/bmg.html
>)